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Lecture Comments (10)

3 answers

Last reply by: Professor Hovasapian
Mon Feb 16, 2015 3:18 PM

Post by Billy Jabbar on April 28, 2014

Great Lecture, but I just want to point out that your expression of both KI and KI' are reversed.  My book and professor presented it the opposite way.

Regardless though, all of the other concepts that include KI were the same so aside from that discrepancy, everything else is okay.

0 answers

Post by tiffany yang on November 14, 2013

along with Cuong's question, can you please explain why reversible inhibitor will not decrease Vmax' whereas irreversible inhibitor will decrease the Vmax? Thank you professor Hovasapian.

If Vmax doesn't change for reversible inhibitor, does that mean V knot when substrate concentration equals to Km doesn't decrease either?

Does reversible inhibitor just means it has a good chance to fall off? so substrate can come back in to bind with enzyme? Thank you so much.

1 answer

Last reply by: Professor Hovasapian
Tue Sep 24, 2013 8:07 PM

Post by Vinit Shanbhag on September 15, 2013

most of the feedback inhibition in metabolic pathways happens thru allostery,, my question is: will those inhibition modes follow the same principles in this lecture?

If your drug is a small molecule which is binds to non enzymatic protein, how shld we express the effect of its inhibition?

2 answers

Last reply by: cuong Le
Mon Apr 1, 2013 1:47 PM

Post by cuong Le on April 1, 2013

For the irreverisble inhibition, what are the graphs look like? Can you please briefly mention?

Enzymes V: Enzyme Inhibition

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
  • Enzymes V: Enzyme Inhibition Overview 0:42
    • Enzyme Inhibitors Overview
    • Classes of Inhibitors
  • Competitive Inhibition 3:08
    • Competitive Inhibition
    • Michaelis & Menten Equation in the Presence of a Competitive Inhibitor
    • Double-Reciprocal Version of the Michaelis & Menten Equation
    • Competitive Inhibition Graph
  • Uncompetitive Inhibition 19:23
    • Uncompetitive Inhibitor
    • Michaelis & Menten Equation for Uncompetitive Inhibition
    • The Lineweaver-Burk Equation for Uncompetitive Inhibition
    • Uncompetitive Inhibition Graph
  • Mixed Inhibition 30:30
    • Mixed Inhibitor
    • Double-Reciprocal Version of the Equation
    • The Lineweaver-Burk Plots for Mixed Inhibition
  • Summary of Reversible Inhibitor Behavior 38:00
    • Summary of Reversible Inhibitor Behavior
    • Note: Non-Competitive Inhibition
  • Irreversible Inhibition 45:15
    • Irreversible Inhibition
    • Penicillin & Transpeptidase Enzyme

Transcription: Enzymes V: Enzyme Inhibition

Hello and welcome back to, and welcome back to Biochemistry.0000

Today, we are going to continue our discussion of enzymes by talking about enzyme inhibition.0004

Enzyme inhibition exactly is exactly what you think it is.0009

We have an enzyme that does something; we want to either completely stop it from doing something, or we want to slow it down a little bit.0014

There is also something called positive inhibition, where you want to actually cause an enzyme to do what it does a little bit better, a little bit faster; but for the most part, we are going to be talking about negative inhibition.0022

Again, inhibition, exactly what you think it means, so let's get started and see what we can do.0036

OK, enzyme inhibitors - excuse me - are molecules that interfere with enzyme action or enzyme activity.0043

Now, it is no surprise that a large percentage of current pharmaceuticals on the market are, in fact, enzyme inhibitors, and that is a huge, huge, huge area or pharmaceutical research.0076

We discover some metabolic pathway; we discover something in that pathway, some enzyme where we know how to control it.0117

We control it; we mess with it- that is what we do.0125

OK, now, there are 2 broad classes of enzyme inhibitors.0128

Excuse me; we have something called reversible and - exactly what you think - irreversible.0140

Now, let's go ahead and deal with the reversible first; let me go ahead and do this in blue.0152

Now, the reversible comes in 3 varieties: something called a competitive inhibitor - it is exactly what you think it is, but we will give a definition in just a minute - and uncompetitive inhibitor and a mixed inhibitor- mixed being combination of competitive and uncompetitive.0161

OK, let's go ahead and deal with these one at a time; the first thing we are going to deal with is competitive inhibition.0187

It is the most intuitive one; the one that is most easily understood.0194

Competitive inhibition is exactly what you think it is, exactly what it sounds like.0199

It is where the inhibitor, which again is a molecule, where the inhibitor molecule, it competes directly against...I will stay with actually, instead of against, does not matter.0212

It is competing with the normal substrate for binding to the active site.0239

It competes directly with the normal substrate for the active site.0244

They both want to be in the active site- substrate competitor.0260

If substrate makes it, the enzyme does what it does; if the competitor makes it to the active site before the substrate does, that is it.0264

It blocks off entrance to the active site for the normal substrate.0271

Excuse me; pictorially, it looks something like this.0279

Let's say you have some enzyme, it looks something like this.0284

OK, excuse me.0290

Let's say our normal substrate looks like that.0295

When the enzyme and the substrate come together - I will put S here, and I will put E for enzyme - we end up with our enzyme substrate complex.0300

Well, the inhibitor, let's say it looks something like that.0312

Now, if the enzyme binds to the inhibitor, you end up with something called the enzyme...this is the enzyme substrate complex.0323

You end up with something called an enzyme inhibitor complex, and now, because this inhibitor is occupying the actual active site of the enzyme, the substrate, itself, cannot come in.0338

It has blocked the enzyme it has inhibited- that is it.0349

Whatever metabolic pathway happens to be involved, it stops at that pathway.0352

It does not do what it is supposed to do; this is competitive inhibition- competition for the active site.0357

In terms of the Es and Ss that we were talking about, before it looks something like this.0364

You have the enzyme plus you have the substrate.0368

They can form something called the enzyme substrate complex that can go on to form enzyme plus product.0372

Now, the enzyme, instead of binding to the substrate, can also bind to the inhibitor forming the enzyme inhibitor complex.0378

This is what it looks like in terms of Es and Ss and things like that.0388

Now, let's go ahead and for this, I am going to introduce something called ki.0393

I just want you to be aware of it; I will write it again.0400

This ki is not a rate constant; this is an equilibrium constant.0405

Here, this ki is equal to the concentration of Ei over the concentration of E times the concentration of I.0411

That is this reaction right here, the equilibrium.0425

It is the dissociation reaction for the enzyme inhibitor complex- OK, products over reactants.0429

These are the reactants; this is the product.0436

This ki, it is the equilibrium constant for the formation of enzyme inhibitor complex, or it is the dissociation constant, if you will, for this complex that way, depending on which direction you want to take it.0438

It is just an equilibrium complex; OK, it will show up a little bit later, but we are not really going to be doing too much with it.0451

It is just something that you should know.0458

OK, now, the Michaelis-Menten equation in the presence of a competitive inhibitor looks like this: v0 = Vmax S, so far so good, alpha Km + S.0462

Now, we have this alpha factor; here, alpha is equal to 1 plus the concentration of inhibitor, oops, ki not Km, right?0505

Yes, they were ki, and ki, as we said, is equal to Ei/E and I.0523

These do not matter all too much; this is what is important, this alpha.0543

Just consider it some number, which is generally going to be bigger than 1.0548

It is the whole idea; it is going to be bigger than 1.0552

OK, it is just some factor, which in this case, is related to the inhibition.0555

OK, now, let's see what we have here.0563

OK, this right here, this alpha Km, this is the observed Km, often called the apparent Km.0569

Remember what Km was; Km was the concentration of substrate that allows you to react at which you are at half maximum velocity.0589

This observed Km is have the normal enzyme, and then, you put this competitive inhibitor into the solution.0600

Now, you are going to experience a new concentration that will take you to half velocity.0610

That is what this is, and we will talk a little bit more about that in just a minute.0617

This is the observed Km, this whole quantity.0620

OK, this is the observed Km, which, again, is the amount of substrate needed to bring you to 1/2 of the maximum velocity.0624

Now, here is what is interesting.0650

Since the substrate and the inhibitor are in direct competition, all we need to do in the presence of a competitive inhibitor...well, again, chemical reactions happen by things running into each other.0657

If you have a whole bunch of substrate molecules, very little inhibitor molecule and some enzyme, well, most of the enzyme is going to be tied up with substrate molecules because they form the majority of the molecules that are bumping into it.0690

If you increase the inhibitor concentration, now, there is a greater probability that that inhibitor will bind to the enzyme, and, of course, it is going to slow the enzyme down.0702

Well, in order to just speed things up again and get things back to where they were before, all you have to do is increase the substrate concentration.0712

No matter how much inhibitor you put, as long as you keep increasing the substrate concentration so that there is so much substrate compared to inhibitor, statistically, more substrate is going to run into the enzyme, and you are going to get back your initial velocity.0719

Km, the amount of substrate that you need in order to achieve half maximum velocity, you throw some inhibitor in there.0735

All of a sudden ,you throw some inhibitor in there, and all of a sudden, the velocity is going to slow down because, now, some of the enzyme is going to be bound up with inhibitor molecule instead of substrate.0745

Well, in order to get it back up, you increase the substrate concentration.0752

Now, you have increased the substrate concentration, and you bring the velocity back up to half Vmax.0757

You add some more inhibitor; well, you add some more substrate to bring velocity back up again to where Vmax was.0763

In competitive inhibition, the maximum velocity does not change.0772

The only thing that changes is the amount of substrate you have to add - normal substrate - in order to bring it up to half velocity.0776

In other words, it is Km that changes, and we will show you that mathematically.0784

Since S and I are in direct competition, all we need to do is increase the substrate concentration in order that more substrate and enzyme come into contact.0790

In this case, in the presence of a competitive inhibitor, the maximum velocity of the enzyme does not change.0815

The competitor does not affect the maximum velocity of the enzyme; it affects how much substrate you have to put in to reach that maximum velocity.0824

In this case, what an inhibitor does, it actually slows down the enzyme action.0831

It does not stop it altogether; it just depends on what the concentration of enzyme and inhibitor is.0838

If you have so much more inhibitor than enzyme, then yes, you are just going to completely eliminate substrate from knocking into the enzyme at all.0842

So, you are going to stop it, but really, what a competitive inhibitor does, it slows the enzyme action down.0850

In this case, in the presence of a competitive inhibitor, maximum velocity does not change.0857

Only Km changes, and it changes by a factor alpha; and you remember that alpha depends on how much inhibitor there is.0868

OK, now, let's look at the double reciprocal version of this Michaelis-Menten equation for competitive inhibition.0878

Let me do this one back to black.0887

Now, the double reciprocal version of the Michaelis-Menten equation is the following.0891

It is 1/v0 = alpha Km/Vmax x 1/S + 1/Vmax.0910

You notice, the intercept 1/Vmax, the Y intercept, what Vmax is, it does not change.0926

Here, Y intercept stays the same.0936

Vmax stays the same.0943

What does change is the slope; that was the slope - right - of the Lineweaver-Burk plot?0951

The slope changes by a factor of that much; in other words, the slope goes up.0957

From your perspective, the slope goes up; I will have a picture in just a minute.0961

Slope rises as inhibitor concentration rises - right - which is alpha.0967

That is what alpha measure; it measures how much inhibitor is there.0975

As this number goes up, it is greater than 1, the Km/Vmax goes up.0978

From your perspective looking at the graph this way, your line is going to go like that.0982

Slope rises so Km decreases, and I will show you why in just a minute.0987

OK, now, let's go ahead and look at an image here.0998

There we go; we might say that this one is no inhibitor.1003

As we move up, as we increase our inhibitor concentration, what happens...we said Vmax does not change.1013

The Lineweaver-Burk plot literally just goes from here; it just switches.1020

The slope rises, rises, rises; well, the Vmax is not changing, but now, the absolute value of 1/Km is actually the absolute value of 1/Km.1025

1/Km is getting closer to 0.1040

Numerically, it is rising because the -1 is bigger than -6, but the absolute value of the 1/Km is getting smaller.1045

It is getting closer to 0; well, if the absolute value of 1/Km is getting smaller, that means Km is getting larger.1054

That is what happens here; in competitive inhibition, the maximum velocity does not change.1061

What changes is the amount of substrate you have to have in order to get back to your half maximum velocity.1067

Competitive inhibition slows down the reaction.1074

It decreases the...I am sorry, increase the Km; it decreases the 1/Km.1082

That is what is happening here; the graph looks like this.1087

As you add inhibitor, you are going to get...the more inhibitor you add as you increase inhibitor concentration, the slope of your graph is going to change, but your Vmax is not.1092

It is going to rotate around that point; that is what is happening.1102

Again, notice that Vmax does not change, but absolute value of 1/Km decreases; so Km increases.1108

And again, it makes perfect sense; the Km increases.1137

You just need to add more substrate in order to allow more substrate molecules to run into the enzyme, in order to get back up to half velocity.1141

Inhibitor slows you down; bringing the substrate concentration back up brings you up.1149

That is what this says; this confirms that Vmax stays the same.1153

Km increases; OK, that is competitive inhibition.1157

Now, let's talk about uncompetitive inhibition; let me go back to blue here, so uncompetitive inhibition.1164

You are probably wondering why did not we call it non-competitive inhibition.1178

Non-competitive inhibition is a special case that happens virtually never experimentally, but it is a special case that we will talk about a little bit later after we talk about mixed inhibition.1182

In order to differentiate it, instead of calling it non, we call it uncompetitive inhibition.1195

Now, uncompetitive inhibition is when your inhibitor molecule binds to a site other than the active site, but it binds only to the enzyme substrate complex.1200

In other words, it binds only after the normal substrate has bound to the enzyme active site, only after the normal substrate is attached.1242

I will just write "is attached" instead of "has bound"; again, uncompetitive inhibition, it looks like this.1271

You have enzyme plus substrate to form something called the enzyme substrate complex.1278

This enzyme substrate complex can break down into enzyme + product, or at this point, the inhibitor can bind to form this enzyme substrate inhibitor complex.1286

Here is an enzyme that has 2 substrates; it has 2 places where things can bind: the normal active site for the normal substrate and some other place on the enzyme or inside the inside the enzyme where an inhibitor can bind.1302

It does not compete with the active site; the substrate binds directly, but at that point, after it is bound, now, the inhibitor gets in the way.1314

It competes indirectly in an uncompetitive fashion.1325

OK, now, for uncompetitive inhibition, the Michaelis-Menten equation looks like this.1330

What you have, now, is v0 = Vmax x the substrate concentration.1346

Again, the numerator stays the same; now, Km is there, plus we are going to have something called alpha prime times substrate concentration.1353

Now, alpha, instead of affecting the Km, it is going to affect the substrate concentration.1362

This is what the form is, and again, alpha prime is equal to 1 + inhibitor concentration/ki prime and ki prime is equal to the equilibrium constant for the formation of the enzyme substrate inhibitor complex from the enzyme substrate complex and inhibitor.1368

That is what that is; I just want to think of it as some number bigger than 1 that is going to is a measure of the uncompetitive effect.1399

OK, let's go ahead and see what is happening here.1408

Now, for high substrate concentrations, this equation, the above equation, becomes v0 = Vmax/alpha.1413

In this case, uncompetitive inhibition Vmax does change, and since alpha prime is a number that is going to be greater than 1, the maximum velocity actually decreases.1444

It really slows down the enzyme; it slows it down from the top end.1464

It makes it so you cannot achieve a maximum velocity, or the maximum velocity that you did achieve without the inhibition is, now, difference, is, now, cut-down.1469

Now, you have a maximum speed that you can get to no just completely slows it down from the top.1480

It does not just affect it, so that if you add more substrate nothing happens.1487

It just literally shuts down the maximum velocity.1492

OK, Vmax does change via alpha.1496

Now, recall that for the competitive inhibition, we had v0 = Vmax x S/alpha Km + S.1503

For high concentrations of substrate in competitive inhibition, for high substrate concentrations, this term goes away.1525

The SS cancels.1536

For high S, v0 still equals Vmax.1541

Again, it confirms what we already know that Vmax does not change for competitive inhibition.1548

Vmax changes for uncompetitive inhibition.1553

Let me see; now, let's do the Lineweaver-Burk plot for uncompetitive inhibition.1559

The Lineweaver-Burk equation for uncompetitive is the following: 1/v0 = Km/Vmax - let me actually put that one in parentheses and not the 1/S - x 1/S + alpha prime/Vmax.1569

In this case, notice, the slope stays the same.1606

The slope does not change.1614

The Y intercept changes; since the Y intercept changes, alpha prime is bigger than 1.1625

1/Vmax, that is the Y intercept; alpha prime/Vmax or alpha prime x 1/Vmax, it is going to raise the Y intercept.1638

As 1/Vmax, as the Y intercept goes up, the Vmax goes down.1650

And again, you will see that in just a second; the Y intercept changes.1658

A graph of this looks as follows; again, we have something like that.1665

That has no inhibition at all.1676

OK, your Vmax, your Km or actually this is your 1/Vmax.1680

This is your -1/Km; the Lineweaver-Burk plot that we just wrote down, the slope does not change.1686

The Y intercept changes; it goes up.1696

Now, every time you add uncompetitive inhibition, you get a new line.1699

You add more inhibitor; you get a new line.1704

Inhibitor, you get a new line; here is increasing inhibitor concentration.1706

As you increase the concentration of uncompetitive inhibitor, the Lineweaver-Burk plot starts to climb by a factor of alpha prime.1712

As the Y intercept goes up, Vmax goes down.1725

Here, notice, well, since the slope does not change, now, the absolute value of the Km is getting larger.1728

Its absolute value of 1/Km is increasing.1740

It is going farther and farther and farther away from 0, so Km decreases.1747

In this particular case, in uncompetitive inhibition, not only does Vmax decrease.1757

Here, Y intercept increases, so Vmax decrease.1764

In the case of uncompetitive inhibition, not only do you change the maximum velocity, you also change the Km.1776

You change the observed amount of substrate that you have to add in order to reach half maximum velocity, and it changes proportionally.1784

The slope does not change, so your maximum velocity goes down.1797

Well, sure enough, if your maximum velocity goes down, your Km is going to go down, and that is what this confirms.1801

Maximum velocity goes down as the intercept goes up; as you add uncompetitive inhibitor, it also changes the Km.1809

In the case of uncompetitive inhibition, both Km and Vmax change.1816

They decrease; we will have a summary of all this, of course, at the end.1822

OK, now, let's see what happens.1827

Now, let's go to our third...let's go back to blue here.1832

Let me draw a little bit of a line; this is going to be called mixed inhibition.1836

This is the third of our reversible inhibition processes.1843

Mixed inhibitor, OK, this is where the inhibitor molecule, again, binds to a site other than the active site for the normal substrate.1850

It also binds, I will just say somewhere other than the active site, but it can bind before the substrate attaches or after the substrate attaches.1868

It can bind either to enzyme directly before the substrate, or it can bind to the enzyme substrate complex, in other words, after the substrate has bound.1902

Here is what it looks like in terms of Es and Ss.1918

You have your enzyme; you have your substrate, and you have your enzyme substrate complex.1922

Well, the enzyme, instead of binding to the substrate first, it can certainly bind to the inhibitor, and it forms something called an enzyme inhibitor complex; and this enzyme inhibitor complex, now, it can bind with substrate to form the enzyme substrate inhibitor complex, or what can happen is enzyme can bind to the substrate like normal, and then, this enzyme substrate can bind to inhibitor to form this thing- that is it.1928

That is what is going on here; here is ki.1960

Here is ki prime; again, it is a little bit of a combination mixed.1965

It is exactly what you think it is; it is a bit of a combination of competitive inhibition that is this one right here and uncompetitive inhibition, which is this one right there.1970

The Michaelis-Menten equation for mixed inhibition is exactly what you think.1986

It is a combination of the 2; the alpha and the alpha prime show up.1996

We have Vmax times the substrate concentration over alpha-Km from the competitive inhibition + alpha prime x S from the uncompetitive inhibition.2000

if we do a double reciprocal of this, it is going to look like this.2015

The double reciprocal version, in other words, the Lineweaver-Burk version of the equation, it is going to be 1/v0 is equal to alpha-Km/Vmax.2023

Do that, and I will do 1/S here plus alpha prime/Vmax.2045

In this particular case - let's go to black - the slope changes, and the Y intercept changes.2052

In this particular case, again, Y intercept changes, so it is going to change the Vmax.2069

The slope is going to change.2075

Under ideal conditions, you will not change the Km, but experimentally, that never shows up.2080

That is what would be called non-competitive inhibition.2084

It is where the Km does not change but the Vmax does.2088

Experimentally, you will not run into that; mixed inhibition is actually a lot like uncompetitive inhibition in a sense that both things change.2092

The graph looks like this.2100

A Lineweaver-Burk plot ends up looking something like this.2104

We have normal; we will just call that one normal no inhibitor.2109

What ends up happening is the following; when you end up having inhibitor - let me go ahead and use red - you are going to get...actually, let's make this a little bit better here.2114

That one is not quite as clear as I want it to be; let me redraw this line.2127

This is normal; let me mark that as...there is no inhibitor there, and then, for red inhibitor, what we end up with this particular case, you are still going to get to this point where the line is going to rotate, but now, what is happening is the uncompetitive inhibition is changing the Vmax, and the competitive inhibition is changing the slope.2132

So, you have vertical movement as well as slope changing.2169

The point where they meet is not going to be on the axis; it is going to be a little bit to the left of the axis.2173

What ends up happening is yes, you are going to have Vmax is going to change.2179

Inhibitor concentration is going this way; as you add inhibitor - in this case it is going to be a mixed inhibitor - you are going to have the uncompetitive component, which is going to change the Vmax.2187

So, as the Y intercept increases, the maximum velocity decreases, and you are also going to be changing the Km.2204

What is going to end up happening is the absolute value of the Km is actually going to be getting smaller.2214

The Km is actually going to rise; this is a combination of competitive inhibition and uncompetitive inhibition.2219

However, just because it is a combination, mixed inhibition is still an inhibitor that binds somewhere else on the enzyme.2227

That is the important thing to remember; as far as physically is concerned, it is still not directly competing with the active site for the normal substrate, but it is showing behavior of a competitive behavior.2240

It is showing some competitive behavior, but it is also showing uncompetitive behavior; so it is just a combination.2256

That is why you have both the alpha prime and the alpha; it looks something like this.2263

In this case, Km does change, and Vmax changes.2268

Vmax goes down; the Km goes up.2272

OK, now, let's go ahead and do a summary of what it is that we have here, so a summary of reversible inhibitor behavior.2278

OK, before I write this down, I am not sure what it is that your teacher is going to ask of you in terms of recognizing inhibition behavior.2300

My guess is that you are not going to be asked to know the form of the Michaelis-Menten equation for the different types of inhibition, and you are certainly not going to be required to know the double reciprocal version of the equation for the various inhibition behavior.2313

My guess is that the only thing that you are going to be asked to recognize is what the Lineweaver-Burk plot looks like for the various types of inhibition behavior.2327

Competitive: slope changes; Vmax does not change.2336

The Km increases.2342

Uncompetitive behavior: slope stays the same, but maximum velocity decreases.2345

Km decreases.2350

And mixed inhibition: slope changes, and Vmax changes.2353

Vmax ends up decreasing, but because the slope is changing, the Km actually increases; and that is what we are going to summarize right here.2357

You are going to be asked more than likely just to identify the behavior based on the Lineweaver-Burk plot.2366

We have our inhibitor.2375

We have our observed; and again, all of this is observed maximum velocity and observed Km.2380

This is what we actually measure when we say observed.2389

OK, now, when there is no inhibitor, your Vmax and your Km are exactly where they should be.2394

Now, for competitive inhibition, Vmax stays the same.2405

It does not change.2412

Your Km, that is, now, the observed Km.2417

It goes up; alpha is a number bigger than 1.2423

It goes up; uncompetitive behavior, uncompetitive competition, I am sorry.2427

Uncompetitive competition, yes, that makes sense- uncompetitive inhibition.2436

There, now, your Vmax is going to go down by a factor alpha prime, and your Km is also going to go down by a factor of alpha prime.2441

Alpha prime is bigger than 1, so both changes.2453

Mixed inhibition, your Vmax is still going to experience a decrease according to alpha prime.2458

Here is where it gets interesting; your Km, it depends on alpha and alpha prime.2473

This and this come together to form this, where alpha and alpha prime both show up.2483

If the inhibitor is behaving more as a competitive inhibitor or is displaying competitive inhibition, the alpha is going to be bigger than alpha prime.2500

You are going to end up with a Km increase; if alpha prime, if it is displaying more uncompetitive behavior than competitive behavior, you are going to end up with a slightly smaller Km.2507

This one is variable; this one depends on that.2520

Mixed inhibition, the alpha and the alpha prime both show up to affect the Km, to affect the amount of substrate concentration that will allow you to be at half maximum velocity- that is it.2524

Now, let's talk a little bit about...we will close it off with this discussion here.2540

Let me do that; OK, let me have another page- I do.2546

Notice, I am just going to mention non-competitive behavior.2551

I am not going to really talk too much about it, but I just want you to notice.2556

When alpha is equal to alpha prime, when the enzyme is displaying both competitive and uncompetitive behavior equally, in other words, when the equilibrium constants for the formation of enzyme inhibitor complex and enzyme substrate inhibitor complex, when that equilibrium is the same, alpha and alpha prime are the same.2560

When alpha equals alpha prime, that is when...remember what the definitions 1 plus the inhibitor com...over the equilibrium constant for formation of the enzyme inhibitor complex.2588

When it equals the alpha prime, which is that over the formation of the enzyme substrate inhibitor complex, when those are equal, this goes away.2609

This goes away; the inhibitor, you end up with this equal to that.2628

What happens is the following, which is ki = ki.2634

In other words, the equilibrium constants are forming at the same rate, then, the alpha x Km/alpha prime, since they are the same, it just equals Km.2643

When alpha = alpha prime, when it displays both competitive and uncompetitive behavior at the same rate to the same degree, you would have something called non-competitive behavior.2658

OK, now, we call it non-competitive.2668

In this particular case, the Km does not change; the Vmax changes- that is it, non-competitive inhibition.2679

You do not really have to know anything about non-competitive inhibition; you just need to recognize that it is when alpha = alpha prime or when competitive and uncompetitive behavior are operating to the same degree- that is it.2686

You do not really see it in practice; you probably never see it.2700

OK, now, let's go ahead and close it off with a quick discussion of irreversible inhibition.2705

I am just going to tell you what it is, give you an example, and then, we will leave it at that, so irreversible inhibition.2711

We have talked about reversible inhibition; irreversible inhibition is exactly what you think it is.2718

It is when you permanently disable the enzyme; there is no way for it to, sort of, recover.2726

OK, an irreversible inhibitor is one that binds covalently to the enzyme active site or elsewhere.2732

I will just put "elsewhere" just in case because you can have some covalent binding at some other site on the enzyme, which permanently deforms the enzyme, walks it into a shape where it can no longer do what it does.2757

So, that is still irreversible inhibition, and that is it; it is locked in that way.2768

It is never going to break free to become some functioning enzyme again.2772

The body has to create the enzyme again if it wants to do what it wants to do; so, one that binds covalently to the enzyme active site or elsewhere and permanently cripples the enzyme- that is it.2777

Now, the penicillins that you take as antibiotics, those are irreversible inhibitors of the bacterial enzyme transpeptidase.2799

Let's go ahead and take a look here; I am not going to really do any diagrams or anything.2811

I am just going to tell you a little bit about them; the penicillins are irreversible inhibitors that bind - the actual penicillin molecule - to bacterial transpeptidase enzyme, and what they do is actually prevent the final cross-linking of amino acid chains in the formation of the bacterial wall/cell wall.2816

The bacterial wall is something called a peptidoglycan; it is a combination of amino acids and sugars.2891

That is all it is, and we will discuss peptidoglycans in a little bit later.2899

OK, let me go to last step here, last page.2917

Now, give you just a little bit more information because I think it is really, really, actually an exciting field.2922

Penicillin binds permanently to a serine hydroxyl in the active site, and it prevents D-alanine from binding at this point- that is it.2929

It binds to this serine residue in the active site.2969

What is supposed to happen is that this D-alanine is supposed to come in, bind to the enzyme, and then, some other amino acid, a glycine, is supposed to come and bind to the D-alanine and kick off the enzyme to release the enzyme again so that it can go and do what it does; and now, you have this chain linked to this chain, this glycine linked to the alanine.2973

Well, the penicillin gets in the way; it binds to the enzyme, and now, it just shuts the enzyme down.2994

Now, it is stuck in the active site; nothing can happen- that is it.3000

That is irreversible inhibition; in order for the bacteria to do what it does, it needs to remake more enzyme.3005

By that time, the immune system has already taken care of the bacterial wall because the bacterial wall, it is not strong the way it should be.3012

It becomes very vulnerable, very, very susceptible, and it is actually quite easy to kill.3021

It is when the bacterial wall is there that it becomes very, very difficult to do any damage to it.3026

At this point, cross-linking cannot take place leaving a vulnerable bacterial cell wall.3032

In the case of penicillin, a penicillin does not actually kill the bacteria.3066

What it does is it stops it in its tracks from becoming what it is supposed to become so that the immune system can come and take care of the rest.3070

It is called bacteriostatic instead of bactericidal.3077

A bactericidal antibiotic actually kills the bacteria; this is bacteriostatic.3081

It just stops it in its tracks and lets other things do what they are supposed to do.3085

OK, that is enzyme inhibition.3091

Thank you for joining us here at; we will see you next time, bye-bye.3095