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Lecture Comments (18)

1 answer

Last reply by: Dr Carleen Eaton
Fri Jan 29, 2016 10:58 AM

Post by Saakshi Dhingra on January 21, 2016

Hello Dr. Eaton. What is the difference between endergonic and exergonic reactions in comparison to endothermic and exothermic reactions?

1 answer

Last reply by: Dr Carleen Eaton
Tue Nov 18, 2014 4:39 PM

Post by Anmol Chowdhary on November 16, 2014

Hi Dr. Eaton,

How do enzyme activation and enzyme inhibition factors affect enzymatic speed?  The ones discussed were enzyme cofactors, temperature, pH, and substrate concentration.

Thank you

1 answer

Last reply by: Dr Carleen Eaton
Tue Mar 25, 2014 3:12 PM

Post by Emmanuel Nunes on March 14, 2014

Thank you  Dr Eaton for this great lecture

0 answers

Post by manuel dihr on July 18, 2013

i think there might something wrong with the structure of ATP: there is no OH connecting ribose with triphosphate. otherwise oxygen would disobey the octett rule

1 answer

Last reply by: Dr Carleen Eaton
Tue Apr 9, 2013 1:58 PM

Post by Ikze Cho on April 9, 2013

all enzymes are proteins right?

1 answer

Last reply by: Dr Carleen Eaton
Wed May 9, 2012 3:58 PM

Post by Gayatri Arumugam on May 6, 2012

Would ATP be the product of an endergonic reaction?

1 answer

Last reply by: Dr Carleen Eaton
Mon Feb 27, 2012 11:07 PM

Post by Michael Mann on February 23, 2012


In the video (5:01)when you are talking about delta G being negative and spontaneous and does not require energy input is that because there is always energy there?


1 answer

Last reply by: Dr Carleen Eaton
Wed Jan 18, 2012 12:09 PM

Post by Gaby Vazquez on January 16, 2012

Thank you Dr. Eaton! Excellent lecture, really clears concepts!

2 answers

Last reply by: Dr Carleen Eaton
Mon Oct 24, 2011 4:04 PM

Post by felix michoutchenko on October 19, 2011

Isn't delta = 0 an equilibrium (neither spontaneous nor non-spontaneous)??


  • Δ G = ΔH – TΔS is the Gibbs free energy equation.
  • Reactions with a negative ΔG release energy; these are exergonic reactions.
  • Reactions with a positive ΔG absorb energy; these are endergonic reactions.
  • Enzymes increase the rate of a reaction by lowering the activation energy required for the reaction to occur. They do not change the overall ΔG of a reaction.
  • Each enzyme binds only to specific substrates and catalyzes a particular reaction.
  • Enzyme activity is affected by substrate concentration, temperature, pH and the presence of cofactors. Enzyme activity may also be decreased through the effects of competitive or noncompetitive inhibitors.
  • Competitive inhibitors are similar in shape to the substrate. They bind to the enzyme’s active site and prevent the binding of the substrate.
  • Allosteric regulators affect an enzyme’s activity by binding to sites outside of the active site. Binding to these sites causes a conformational change in the active site.


Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Law of Thermodynamics 0:08
    • Thermodynamics
    • The First Law of Thermodynamics
    • The Second Law of Thermodynamics
    • Entropy
  • The Gibbs Free Energy Equation 3:07
    • The Gibbs Free Energy Equation
  • ATP 8:23
    • Adenosine Triphosphate (ATP)
    • Cellular Respiration
    • Catabolic Pathways
    • Anabolic Pathways
  • Enzymes 14:31
    • Enzymes
    • Enzymes and Exergonic Reaction
    • Enzymes and Endergonic Reaction
  • Enzyme Specificity 21:29
    • Substrate
    • Induced Fit
  • Factors Affecting Enzyme Activity 25:55
    • Substrate Concentration
    • pH
    • Temperature
    • Presence of Cofactors
  • Regulation of Enzyme Activity 31:12
    • Competitive Inhibitors
    • Noncompetitive Inhibitors
    • Feedback Inhibition
  • Allosteric Interactions 36:56
    • Allosteric Regulators
  • Example 1: Is the Inhibitor Competitive or Noncompetitive? 40:49
  • Example 2: Thermophiles 44:18
  • Example 3: Exergonic or Endergonic 46:09
  • Example 4: Energy Vs. Reaction Progress Graph 48:47

Transcription: Enzymes

Welcome to

Today is the first in a series of lessons on cellular energetics, and we are going to start out by focusing on enzymes.0002

To understand enzymes, you need to first understand energy.0010

We are going to start out with the discussion of some of the laws of thermodynamics.0015

Thermodynamics is the field that studies the conversion of energy between heat and other forms of energy. It is concerned with the transfer of energy.0020

And we are actually going to focus on two of the laws of thermodynamics and then, discuss enzymes.0031

The first law of thermodynamics states that energy cannot be created or destroyed. It can only be changed from one form to another.0037

The total amount of energy in matter in the universe remains constant.0048

It can change from one form to another, but it cannot be created or destroyed.0054

For example, you take in energy in a form of food, and the energy stored in the chemical bonds of the food can be released; and you could use that energy to take a walk for example.0060

You are using the energy from the food and transferring it into another form - kinetic energy - when you walk.0074

That is allowed. It is energy conversion.0081

The second law of thermodynamics states that the process of energy conversion, that I just discussed or energy transfer, increases the entropy of the universe.0084

What is entropy? Entropy is a measure of disorder.0096

Therefore, when energy is transferred or converted into another form, things become more disordered, more random, more chaotic.0108

The transfer of energy increases disorder in the universe.0117

When energy is transferred, it is not all conserved as usable energy. Some of the energy is converted to heat.0123

I mentioned that you could eat something. You could eat a sandwich, and then, you could use that energy to go walk converting it to kinetic energy.0132

However, not every single calorie that you eat is going to be converted into kinetic energy.0142

Not every little bit of that nutrient is going to go into making your legs move.0151

Some of it will be lost as heat. Heat is not usable energy.0155

Overall, the universe becomes more and more random because that heat, that unusable energy, contributes to the disorder of the universe overall.0161

Now, an individual situation could become more ordered.0171

You can go and clean up your room.0174

That is contributing to the order of your room, but taken as a whole, the universe is overall becoming more random; and the usable energy is decreasing.0176

In biology, we are often concerned with the portion of energy in a system that is available to do work, and this is known as the Gibbs free energy or just free energy.0188

This equation here is describing the change in free energy for a reaction, and this is called the Gibbs free energy equation.0198

Let's look at each part of this equation and discuss what it means.0206

Overall, it tells you the change in free energy at a constant temperature and pressure, so ΔG is the free energy change.0209

If we are talking about a chemical reaction, this would be the difference between the free energy of the products and the reactants.0219

The G of the final state, which would be the free energy of the products minus the free energy or G of the initial state of the reactants equals ΔH, so H is enthalpy.0229

ΔH gives you the change in enthalpy.0246

With biological reactions, this can just be thought of as the heat of the reaction for our purposes, so this is about the same as the heat of the reaction.0251

ΔS: I just described entropy, and S is entropy.0265

The change in entropy, increased entropy would mean an increase in disorder.0269

T is the temperature given in Kelvins.0279

The important thing to know is that if the ΔG is negative, the reaction is spontaneous.0289

If the ΔG is 0 or positive, the reaction is not spontaneous.0304

Now, what does that mean?0311

Spontaneous means that a reaction can take place without the input of energy. It does not require energy input.0313

This does not tell me anything about the speed of the reaction, the rate of the reaction.0327

It only tells me that the reaction can take place without the input of the energy,0331

whereas, a positive or 0 ΔG tells that for this reaction to occur, energy would have to be put into the system, so it requires the input of energy.0335

Alright, how could we end up with a -ΔG?0352

Looking at this equation, there are a couple of possibilities.0359

H could decrease. In other words, heat will be lost or increased TΔS, so what we could do here is increase in ΔS.0362

Overall, what would be happening is the system is increasing the disorder in the universe, so increased disorder.0380

If ΔH decreases or TΔS increases, either way, you are going to end up with a -ΔG, and the reaction will be a spontaneous.0392

If the reaction causes either a loss of heat going from reactants to products or an increase in disorder, either of those will result in a -ΔG and a spontaneous reaction.0401

The name for reactions that have a -ΔG in biology are known as exergonic reactions.0417

Exergonic reactions are spontaneous, and they have a -ΔG. Those with a positive or 0 ΔG are endergonic.0427

Frequently in the biologic systems, exergonic reactions, the energy from those is harnessed to allow endergonic reactions to take place.0436

In an exergonic reaction, energy would be released, and then, that energy is used to power endergonic reactions.0444

Systems overall tend towards lower free energy.0453

And looking outside of reactions in chemistry, looking at something as simple as a ball sitting on top of a hill, it has a higher potential energy, a higher free energy. It is less stable.0458

In order for it to go downhill, you are not having to input energy, it ends up at the bottom of the hill. It is more stable, and it is a lower free energy state.0470

Systems tend to want to go towards that lower free energy state.0480

Alright, if this cell is going to couple exergonic reactions with endergonic reactions, it has to have a means to do so.0486

It has to have a means to take that energy released by the exergonic reaction, and use it to power the endergonic reaction; and that means is through ATP.0494

ATP or adenosine triphosphate is what is called the energy currency of the cell.0504

It provides the direct source of energy for cellular processes.0510

An energy released from a chemical reaction could be used to form ATP, and then, ATP is broken down to release energy to power reactions that require the input of energy.0515

Let's look at how ATP stores energy.0530

Looking at the structure, what you have right here is a ribosugar, and here is the nitrogenous base adenine; and here, we have three phosphate groups.0533

The energy in ATP is stored in these bonds between the phosphate groups, and in particular, this third bond right here is the highest energy bond.0556

And the reason for that, the reason that there actually is energy stored in these bonds is because, look at the charge in the phosphate groups,0571

the phosphate groups have negative charges, but these light charges repel each other.0578

They do not want to be near each other, yet, these chemical bonds are holding the phosphate groups near each other.0586

And that is creating a high energy bond because the negative charge are repelling each other.0593

And these phosphate groups actually want to get away from each other, but they are forced together.0599

When this bond is cleaved, that stored energy is released.0604

When the terminal phosphate bond is hydrolyzed, energy is released, and that release of energy is in the form of this reaction.0608

The hydrolysis of ATP to form ADP, we would just have two phosphate groups on it plus Pi, and this is inorganic phosphate.0618

The ΔG under standard conditions of this reaction, is -7.3kcal/mol.0630

Hydrolysis of one ATP gives the cell about 7.3kcal/mol to use for processes such as0639

transporting certain substances into and out of the cell, repairing cell structures, growth, making proteins from amino acids.0651

All of these or almost all cellular processes are powered by ATP.0663

OK, where does the ATP come from?0671

In order to form ATP, the cell needs to take ADP - the reverse reaction of this - plus inorganic phosphate and form ATP, and that is going to require the input of energy.0673

That is an endergonic reaction. It has a ΔG of +7.3kcal/mol.0684

Well, cellular respiration, which we will go into in extreme detail in the next few lectures, provides the energy to form ATP.0692

Let’s look at the overall reaction for the breakdown of glucose.0703

Glucose C6H12O6 plus six oxygens will form six CO2 molecules plus six molecules of water.0707

The ΔG for this is -686kcal/mol.0720

When one molecule of glucose is hydrolyzed by the cell, the release of the energy will be -686kcal/mol.0727

And therefore, what you are going to end up with is six CO2s, plus six waters, plus some ATP,0737

which is where this energy is going to be stored, then, that ATP can go on to power other reactions.0743

Reactions like this that break down nutrients are called catabolic reaction- catabolic pathways.0749

Metabolic pathways can be divided up into catabolic, in which we have a breakdown of nutrients or molecules, and these release energy.0758

Anabolic pathways are the opposite.0775

Here, molecule are being combined or built, and here, that would require the input of energy.0778

These use energy. These will be endergonic reactions.0791

These are exergonic reactions.0794

For example, a cell could break down a molecule of glucose, release this energy, use that to form ATP, and then, the ATP could power an anabolic reaction0798

such as the formation of protein from amino acid monomers or polysaccharides, the creation of polysaccharides from monosaccharides, repair, building cell wall.0808

All of these what you are building up would be anabolic process.0822

Now, where does the cell get the glucose?0828

Well, for photosynthetic organisms like plants, glucose is made - in which we are going to talk about in a later lecture - using energy from sunlight plus some basic building blocks.0830

And glucose is formed and then, broken down.0844

In animals and organisms that do not undergo photosynthesis, nutrients need to be ingested and broken down, and then, that glucose is intracellular respiration pathway to release energy.0847

Alright, now we talked about energy, and we are going to get to where enzymes come into play.0865

Enzymes increase the rate of a reaction by lowering the active evasion energy required for the reaction to occur.0873

Remember that a spontaneous reaction, one with a -ΔG, it has an overall lower free energy in its final state than in its initial state.0880

However, you still may need to input some energy just to get the reaction started, but the overall change right here, we have reactants.0896

Here is the G of the reactants starting out, the free energy state right here, and then, here is a free energy of the products.0909

And if I look at the difference here, I take the G of the final state minus the G of the initial state.0919

In this reaction right here, it is going to be negative because the G of the final is lower than the G of the initial.0928

This is showing me the graph of an exergonic reaction.0937

Why do we need enzymes?0947

This is spontaneous reaction. It does not require the input of energy.0948

It will occur without the input of energy, but it may occur very, very slowly.0951

And in a cell, if a cell needs to break down glucose to release that energy, it cannot wait hours, week, several months, however long it would take for a reaction to occur.0958

A -ΔG tells me the reaction is spontaneous, but it could be very slow.0970

In order to speed up reactions, cells use enzymes. What enzymes are, are catalysts.0977

They increase the rate of the reaction but are not, themselves, altered by the reaction nor do they change the overall reaction that occurs.0984

Most enzymes are proteins.0994

Alright, before we go on, just to complete what we were discussing about exergonic reaction, just to make this clear, what would an endergonic reaction look like?0997

Well, let's say we started out here with some products, and we had the activation energy,1007

and then, the - excuse me - reactants started here then, the activation energy to get this going, and then, we ended up here with products.1015

Notice that the free energy G here of the products is higher than the free energy of the reactants.1028

If I measured the difference from reactants and products, I went ahead and I took OK, the G of the products minus the G of the reactants,1035

I would get a +ΔG because the products, the G is larger than the reactants.1048

This represents an endergonic reaction, and enzymes can catalyze either an exergonic or endergonic reaction.1058

For simplicity, we are going to focus here on exergonic reaction.1065

Now, as I said, it takes some energy to get the reaction going, and why is that?1068

Well, think about what a reaction is. It is breaking of chemical bonds and formation of new bonds.1076

Maybe you are taking a molecule of glucose and breaking those bonds and forming CO2 and water.1081

It takes energy to break the bonds, and to get the bonds in a position where they can break more easily, requires energy.1087

This state right here at the peak is what is called the transition state.1099

The input of energy to get to this transition state is called the activation energy Ea, so Ea is the activation energy.1107

It is the difference in the free energy between the reactants and the transition state.1117

We started here, some input of energy was needed to get this reaction going, to twist, turn, pull these bonds into a position where they will be ready to break.1126

Then, decrease in energy as the bonds break, and then, the new bonds are formed; and then, finally we get the products.1139

You can think of this as analogous to, let's say you are going to ride a bike.1151

You get on the bike, and you have to push extra hard to get the pedals moving; but once you are going, you do not have to pedal as hard.1156

It is the same idea here. Just to get things going, you need some energy input, and then, once they are moving, less energy required.1163

The uphill portion of this graph is what is called the activation phase. You get to the transition state, and then, the downhill part bonds are breaking and then, reforming.1171

Endergonic reaction, same idea, again, activation energy required to get to the transition state, and then, we have a downhill phase to get to the products.1184

How do enzymes work? How do they speed up reaction?1196

They do so by lowering the activation energy required.1199

Therefore, a reaction that is catalyzed by an enzyme might look something like this.1205

Notice that the blue one is catalyzed by an enzyme.1220

In fact, let’s even make this a little more dramatic. Let's lower the activation energy even more.1227

OK, notice that the activation energy, if I look right here, this Ea is smaller than the activation energy required for the uncatalyzed reaction.1236

Therefore, less energy is going to be needed to get this going, and the reaction will occur more quickly.1251

The ΔG has not changed.1259

If I look at where the reactants were, as far as free energy and where products are, as far as free energy- same for both the catalyzed and uncatalyzed reaction.1262

If I measure this difference- same. ΔG has not changed.1273

There is no change in the overall reaction. There is just a decrease in the activation energy required.1279

How do enzymes do this?1286

Well, what the enzymes do is that they hold the substrates in place in such a way that the bonds can more easily be broken, so some terminology first.1290

A substrate is the reactant that an enzyme acts upon, and you see it as enzyme specificity; and that is because enzymes are very specific for the reactions that they catalyze.1304

They can only catalyze reactions for certain substrates. They can only bind to very specific substrates.1315

Let's take a look at what is happening right here.1325

Here, we have the substrates or the reactants, and they are floating around; and what they do is they bind to the enzyme, and you see them bound here and formed.1327

This is the enzyme. This is what is called the enzyme substrate complex.1341

Binding occurs at what is called the active site.1355

This enzyme will only bind to the substrates that it is meant to bind to because of the shape of the active site.1364

And actually, the way this is drawn, it is very schematic, and it shows that this active site is already in a shape that will bind the substrates; but it is a little bit more complex than that.1372

Binding usually occurs by what is called an induced fit.1385

Induced fit means that the active site undergoes a conformational change once it comes into contact with the substrate.1389

Once the substrate or substrates come into contact with this enzyme, the active site will actually conform to fit really well.1397

It might start out, if you looked at it looking like it would not quite be the same shape.1407

It is not just like a puzzle piece that fits into a pre-cut puzzle board.1415

There is some change in the shape to allow the substrates to fit into the active site.1422

Binding occurs, and what the active site does is it holds the substrate or substrates in such a way that the bonds can break more easily.1428

The transition state, it moves the substrates toward that transition state. It might do it by twisting or turning or pulling.1440

The other thing it does is if there is more than one substrate, let's say instead of breaking down a molecule of glucose, we are going to form a molecule of glucose.1449

Then, the reactants, by putting them right next to each other, they are ready to collide and break bonds and reform new bonds.1458

After the reaction has occurred, the enzyme will release the reactants, the products - excuse me - the products.1469

Notice that the enzyme has not been changed.1478

It started out, bound to the reactants. Reaction occurs in the active site of the enzyme.1482

Products are released, and this enzyme is ready to go ahead and catalyze another reaction.1487

Therefore, you will only need a small amount of enzyme to catalyze many reactions.1493

As you come across enzymes in this course and in other courses, you will notice that often, they end in ase, and that is one way to recognize them for example lactase.1500

The ase tells me that it is an enzyme, and lac tells me what the substrate is. The substrate is lactose.1511

Lactase catalyzes the breakdown of lactose into the monosaccharides glucose and galactose, and you might have heard of lactose intolerance- people who cannot tolerate dairy products.1518

When they ingest dairy products their stomach gets upset. They get different GI symptoms.1536

And the reason is they are deficient in the enzyme lactase, and their body cannot breakdown lactose properly.1541

OK, we talked about how enzymes work, what they do. Now, we are going to discuss some factors that affect the activity of an enzyme.1550

There are multiple factors including the concentration of the substrate, temperature, pH and the presence or absence of cofactors.1560

Starting out with substrate concentration, at first, when you add more substrate, you will see that the reaction rate increases.1568

Adding more substrate, it makes sense that the reaction rate would increase because there might have been some enzyme sitting empty.1577

You add more substrate. Those enzymes bind to the substrate.1582

Reaction occurs. However, that only occurs to a point.1587

At a certain point, you can add more and more substrate, and yet, you see nothing else is happening.1590

The reaction rate is not increasing. The reaction rate just maxes out and stays steady, and that is because saturation has occurred.1596

Reaction rate increases by increasing the concentration of substrate until saturation is reached.1604

If you already have a lot of substrate, and all the enzymes are bound to substrate;1617

and there is more waiting there ready to go, dumping in a bunch more is not going to make things any faster.1623

It is just going to be a back log, so that is substrate concentration.1626

Enzymes also have what is called optimal conditions for their activity, and under optimal conditions, the rate of the reaction would be as fast as it could be.1631

And these conditions include temperature, pH and presence of cofactors.1643

Looking first at pH, this graph here, this shows a reaction occurring versus different pH, and you will see at these lower pHs, the reaction rate is not that fast.1648

But as the pHs increase, the reaction rate increases and increases and increases until it peaks at right around 7, approximately 7, so the optimal pH for this reaction is 7.1662

After that, if you continue to increase pH, make a more an alkaline environment, the reaction actually slows down.1676

The reaction rate slows down, and the reason is enzymes have an optimal pH that they function at; and outside of that pH, the enzyme may denature.1685

Remember that most enzymes are proteins and that denaturing means unfolding.1698

Proteins have a 3-dimensional conformation, and if the protein unfolds, then, the active site is going to change shape; and it is not going to bind to the substrate as well.1702

At certain temperature and pH, they are in optimal conditions. Outside of that, the enzyme does not function as well.1713

In humans, most enzymes function optimally at a pH of about 7, and this makes sense because the pH in our blood is about 7. However, that is not always the case.1726

For example, if you look in the stomach, the stomach has a very low pH.1738

The pH in the stomach is about 2-4, and if you look at an enzyme such as pepsin, it is going to function optimally at a lower pH.1741

Alright, that is pH. Now, temperature- same idea.1755

Increase in temperature up to a point will increase the rate of the reaction because the molecules are going to be moving faster, and then, they are more likely to collide.1759

Bonds are more likely to break, but once you get past the peak temperature, the rate of the reaction quickly drops off.1768

And again, that is because the protein becomes denatured or essentially melts at high temperatures.1774

In humans, optimal function of enzyme is at 37°C. That is our body temperature.1780

Above that, proteins may become denatured.1787

Below that, although, the enzyme will not denatured, the molecules just start moving as fast, and so the bonds are not as likely to be broken.1789

The final factor that we are going to discuss that can affect enzyme activity is the presence of cofactors.1797

Cofactors are molecules that certain enzymes require in order to function.1803

In order for the reaction to take place, of course, there needs to be the reactants. The enzyme will speed it up, and cofactors may be required.1809

An example of cofactors could be inorganic elements such as iron or copper ions.1819

There are also organic cofactors. These are usually called coenzymes.1827

These are organic cofactors. A major example is vitamins.1833

The importance of vitamins is that they act as coenzymes, and later, we will discuss some specific coenzymes.1841

They have different functions and different reactions, but an example would be acting as an electron acceptor for a reaction.1848

In order for the reaction rate to be as fast as possible, an enzyme needs to be under optimal conditions.1856

High substrate concentration, optimal temperature, optimal pH and necessary cofactors need to be available.1865

Now, if enzyme activity were not regulated, there would just be complete craziness in the cell because the cell might breakdown all this glucose, and the energy is not needed yet.1873

Or it might use up all its energy to make proteins that are not needed. Things would just be chaotic.1886

Obviously, the regulation of the reactions that occur in the cell is extremely important, and these reactions can be regulated by regulating the activity of the enzymes.1892

One way in which this regulation occurs is through inhibition, and there are two general types of inhibitors: competitive and non-competitive.1903

This is the figure I showed before, where it showed the enzyme, and there is a substrate and then, the enzyme substrate complex, the reactions occurring here at the active site.1912

And then, we have the enzyme again with the products.1925

Competitive inhibitors bind at the active site and compete with the substrate for binding.1934

Let's say I had this inhibitor, and it is a competitive inhibitor.1940

It shaped such that it can bind at the active site, and binding of this inhibitor to the active site would prevent binding of the substrate.1945

In that way the reaction is inhibited. Substrate cannot bind.1954

One thing to consider, though, is that it would be possible to overcome this inhibition by increasing the concentration of the substrate.1959

Let's say I had one molecule of inhibitor for every five molecules of substrate.1967

Well, that is going to significantly decrease the rate of the reaction.1975

However, if I put in lots more inhibitor, and I ended up with a thousand molecules of substrate for every molecule of inhibitor,1979

the effect to the inhibitor is going to be very small because there is just so much substrate there that the chances are the enzyme is going to bind with the substrate, not the inhibitor.1988

Competitive inhibition can be overcome by increasing the concentration of the substrate, so this is substrate concentration.1999

Non-competitive inhibition works much differently.2023

Competitive inhibitors bind at the active site. Non-competitive inhibitors bind to a site outside the active site.2028

They are not competing directly with the substrate for the site. Instead, they are binding elsewhere.2031

Let's say it is going to bind right here. OK, let's say I have a non-competitive inhibitor, and it binds here.2039

What this binding of a non-competitive inhibitor can do is cause that conformational change in the active site.2045

They cause a conformational change in the active site. They do not act directly on the active site.2053

They act indirectly, and this conformational change may cause the active site to have lower affinity or be less likely to bind with the substrate.2064

Two ways that inhibition can occur here: competitive, where the inhibitor literally competes with the substrate for binding at the active site, can be overcome by increase in substrate concentration.2073

Non-competitive inhibition: you can increase a substrate concentration as much as you want, and is not going to help because this inhibitor is not binding at the active site.2105

It is binding at another site, and once that site is bound, this is going to decrease the chance that substrate can bind to the active site because of the conformational change.2115

One particular type of inhibition is called feedback inhibition.2124

Feedback inhibition is very common in biology, and this is when the product or an intermediate of a pathway goes back and inhibits the reaction that formed it.2128

Let’s say I have a reaction where the reactants are A and B, and there is a series of steps.2146

It forms C, then, it forms D, and eventually I get to the products E and F.2156

With feedback inhibition, let's say E could go back and act as an inhibitor.2162

If this were the enzyme that catalyzes this reaction, maybe E acts as a non-competitive inhibitor and binds here2172

and then, prevents A and B, then, from binding to the active site by changing the conformation of the active site.2180

This is a great way for the cell to self-regulate because once a lot of E builds up, it will shut down this reaction so that resources are not wasted making more E.2186

Once the concentration of E drops, then, less inhibition will occur. A and B will react, and more E will be formed.2198

And an intermediate such as C and D could also be an inhibitor. It just depends on the particular pathway that you are looking at.2208

There is another type of regulator for enzymes called allosteric regulators, and allosteric regulators, maybe inhibitors, or they may be activators.2216

And these are regulators that affect an enzyme's activity through binding its sites outside of the active site.2226

I know that it is similar to what we talked about with non-competitive inhibitors, but this is slightly different.2232

The focus here is more on multi-subunit enzymes, so let's go ahead and discuss that.2238

This can be either inhibitors or activators.2244

A multi-subunit enzyme, let’s say there is four subunits - 1, 2, 3, 4 - and these multi-subunit enzymes often exist in an active form and another form that is the inactive form- slightly different shape.2253

What allosteric regulators do is they bind, and they stabilize the enzyme either in its active form or inactive form.2282

Let's say I have an activator shaped like this.2292

What this can do is go ahead and bind here and stabilize this enzyme.2303

It stays in that form. It is active.2310

It catalyzes the reaction.2313

Conversely, an inhibitor - let's say it is shaped like this - and it goes ahead and it binds right here and stabilizes the enzymes in its inactive form.2315

This is binding outside the active site, but it is stabilizing the enzyme in its active form.2330

Binding occurs. The reaction occurs.2341

Here, the opposite, the inhibitor is binding and stabilizing the enzyme in its inactive form.2342

Multi-subunit enzymes may also bind their substrates cooperatively.2352

Again, looking at this enzyme, let's say it is in its active form, and it has multiple subunits; and each of those subunits may happen to bind to a substrate,2358

and if that is the case, what sometimes happen is cooperative binding.2373

In cooperative binding, binding of the substrate - let's say this is the substrate to the first subunit - will make it easier for the second subunit to bind.2377

And that will make it easier for the third subunit to bind, and that will make it easier for the fourth subunit to bind.2386

Binding of one subunit to a substrate increases the affinity of the next subunit for the substrate. Binding is easier and so on.2392

A classic example of such cooperative binding is with hemoglobin.2401

Hemoglobin is found in red blood cells, and the job of red blood cell is to transport oxygen.2406

In hemoglobin there are actually four subunits.2411

Each of those subunits carries an oxygen molecule, and it has been found that binding of oxygen to the first subunit increases binding of oxygen to the second subunit and so on.2414

Hemoglobin is not an enzyme. It is just a carrier of oxygen.2426

I just want to point out that allosteric interactions are not limited to enzymes.2429

There can be this type of cooperative binding in other molecules as well.2434

In the case of an enzyme though, an allosteric interaction would involve a substrate usually because that is what an enzyme is doing- binding a substrate, catalyzing a reaction.2437

Alright, now, we are going to go ahead and focus on some examples.2451

The figure below depicts the graph of the reaction rate versus substrate concentration.2455

The orange line represents the graph when an enzyme is present without an inhibitor.2462

The blue line represents the graph for an enzyme with an inhibitor. Is the inhibitor competitive or non-competitive?2469

Looking at this first line, here, we have a reaction catalyzed by an enzyme.2477

Here, we have a reaction catalyzed by an enzyme, but there is an inhibitor present.2482

I noticed that as substrate concentrations increase, the rate of the reaction increases, increases, increases and then, levels off.2488

This would be, this point where it levels off is saturation.2498

Adding more and more substrate does not help once you get to saturation point.2503

Then, I look at the blue graph, and I see that at lower substrate concentrations, the reaction rate of this reaction is significantly lower than when the inhibitor is not present.2508

At this particular substrate concentration, I see for the uninhibited reaction, the rate is up here at the same substrate concentration, so this is no inhibitor- non-inhibited.2520

This is inhibited. Much lower rate of reaction is being inhibited.2539

However, as substrate concentration is increased, the difference in the rates between these two decreases and almost ends up same.2544

Increasing substrate concentration decreases the effect of the inhibitor.2558

Recall that competitive inhibitors bind to the active site of the enzyme, and they compete with the substrate.2578

Therefore, that type of inhibition can be overcome by increasing substrate concentration, and that is exactly what I see here.2585

Therefore, this is a competitive inhibitor.2592

Now, what would the graph look like for a non-competitive inhibitor?2598

Well, at first, if I added some substrate, it would increase the reaction rate because there has to be substrate around for the reaction to occur.2601

But then, I would see this graph level off way down here because this inhibitor is not being affected, no matter I put more substrate in- not helping.2611

I put more substrate in- not increasing because the inhibitor is not competing with the substrate. It is binding outside the active site.2625

This is uninhibited. This is enzyme with an inhibitor, and it showing the effect of increased substrate concentration at overcoming that inhibitor.2636

And finally, this is a competitive inhibitor and finally, non-competitive inhibitor.2645

What this blue line represents is the graph with an enzyme with a competitive inhibitor present.2650

Example two: thermophiles are microorganisms that live in very hot environments.2660

Which figure below depicts the graph of the activity for an enzyme found in a thermophile?2666

Each of these graphs shows reaction rate and temperature- reaction rate versus temperature.2673

Looking at the first one, the reaction rate increases as temperature increases as expected, and then, it peaks. It peaks at about 40°.2680

The optimal temperature for this reaction is 40°, which is probably the optimal temperature for the enzyme that catalyzed the reaction.2695

After that, the reaction rate drops off as the enzyme becomes denatured in warmer temperatures.2707

The second graph shows the reaction rate increasing and then, peaking, and that optimal temperature is a little more than 20, maybe 25 or so.2715

25°C is the optimal temperature for the functioning of the enzyme catalyzing this reaction.2725

And finally, over here, I see the rate of the reaction increasing and peaking at about 70°.2733

Knowing that this is a thermophile and that they live in very hot environments, I would expect the enzyme to function optimally at a warm temperature.2741

Therefore, this graph, the third graph, shows the graph that I would expect depicting the activity of an enzyme found in this type of organism.2751

For humans, the graph would look like this- the first one.2763

Example three: classify each reaction as exergonic or endergonic?2770

Remember that exergonic reactions release energy.2775

Endergonic reactions require an input of energy to take place.2779

This first reaction shows six CO2s plus six waters forming glucose plus oxygen.2784

This is the formation of glucose, and when a molecule is being formed, is being built, a more complex molecule, this is an anabolic reaction. It is therefore, endergonic.2792

The formation of glucose would require energy, so this first one is endergonic.2813

This next reaction shows the hydrolysis of ATP to ADP and inorganic phosphate. so this is showing the breakdown of ATP.2821

That third phosphate group is being removed, and that high energy bond is being broken, so that is going to release energy. Therefore, this is an exergonic reaction.2832

OK, here, again, I see glucose, now, plus six oxygens breaking down to form six CO2s plus six waters.2849

This is the reverse of the first reaction. Instead of the formation of glucose, which is an anabolic reaction, we have the breakdown of glucose.2863

This is a catabolic reaction. Therefore, this is also an exergonic reaction, so endergonic, exergonic, exergonic.2875

Down here, there are two molecules of glucose, and this is forming a molecule of maltose and water.2891

Again, we are building up a molecule. It is the formation of maltose from glucose monomers.2903

Since it is the formation, it is an anabolic reaction, and it is, therefore, endergonic.2915

This is an endergonic reaction. It is going to require the input of energy.2922

In the final example, example four, the graph below depicts energy versus reaction progress for a reaction with no enzyme present.2927

Sketch a graph that would represent the same reaction with the enzyme present.2939

We start out here with the reactants then, get to the transition state.2943

And the difference here in this energy, free energy, is the energy of activation then, the reaction occurs. Products are formed.2954

Notice that the free energy of the products is lower than the free energy of the reactants.2967

Therefore, this is actually going to have a -ΔG, and this is an exergonic reaction- just so some extra information.2978

Now, remember that enzymes lower the activation energy. They do not change overall ΔG.2986

What would I expect the catalyzed reaction to look like is something like this.2995

You have no way of knowing exactly how much the activation energy is - let's move this stuff out of the way - going to be lowered.3001

But you know that it will be lowered, and you would just ask for a sketch, so just the idea.3008

Now, the other thing that you know is that the ΔG is not going to be changed, so this is the reaction when enzyme is present.3013

The activation energy has been lowered. The original activation energy was that high.3026

The activation energy for the catalyzed reaction is just right here.3035

The ΔG, if I measure the G of the products minus the G of the reactants, is unchanged. I have not changed the reaction.3041

This shows the catalyzed reaction.3049

Thank you for visiting, and I will see you again soon.3053