The first law of thermodynamics says that energy can never be created or destroyed. It can just change forms. According to the second law, this increases the entropy (disorder) of the universe. The Gibbs Free Energy Equation defines the amount of energy in a system that can do work rather than being lost as heat. The change in G (ΔG) over a reaction shows the energy difference between reactants and products. ΔH is the change in enthalpy and ΔS is the change in entropy. Systems want to minimize their free energy. Exergonic reactions are spontaneous and release energy while endergonic reactions require energy to move forward. This energy is exchanged in the form of high-energy bonds in ATP molecules. Reaction-specific enzymes speed up reactions by lowering the activation energy.
with a negative ΔG release energy; these are exergonic reactions.
with a positive ΔG absorb energy; these are endergonic reactions.
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
enzyme binds only to specific substrates and catalyzes a particular
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.
inhibitors are similar in shape to the substrate. They bind to the
enzymes active site and prevent the binding of the substrate.
regulators affect an enzymes activity by binding to sites outside
of the active site. Binding to these sites causes a conformational
change in the active site.
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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
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
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