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I am currently studying biology and have a question.

I know that as pH increases, enzyme activity increase until the enzyme's optimum pH is reached. However, I would like to know why as pH increases towards the optimum pH, does an enzyme's activity increase.

I do not think this has much to do with energy (unlike with temperature change), but does it have something to do with the active site? Basically, in conclusion I would like to ask:

Why does enzyme activity increase going towards an enzyme's optimum pH?

Thank you.

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What do you mean by increasing? Coming from pH 1 and increasing it until the optimum is reached? –  Chris Apr 7 at 12:07
    
@Chris Yes that is what i mean –  C_Intermediate_Learner Apr 7 at 12:12
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3 Answers 3

up vote 4 down vote accepted

Enzymes have a more or less narrow optimal pH at which they work, depending on the conditions of their environment. Pepsin for example is active in the stomach which is pretty acidic and has an optimal pH of 2.0, while Trypsin, which is active in the small intestine has an optimal pH around 8.5.

Changes in the pH first affect the form of the protein, hydrogen bonds between the amino acids of the molecule and so on and also the form of the active center of the enzyme. Small changes in pH do little or nothing, then reversible changes occur and finally the enzyme gets irreversibly denaturated.

The relationship between enzyme activity and pH always looks like this (the curve can be steeper, if the optimal range is small and broader if the range is wider):

enter image description here

It is taken from this website, which is a nice introduction to enzymes. Another one is here.

At the optimal pH for the enzyme the conformation of the protein is as it should be (in the ideal state) while this changes when the pH is not optimal. This can lead to improper substrate binding, changes in the active center and so on.

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Thanks very much :) –  C_Intermediate_Learner Apr 11 at 9:16
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Proteins are polymers of amino acids. Each amino acid has a side chain. Many of these side chains contain ionisable groups. The ionization state of these groups is dependent on the pH. A group that is protonated at pH=2 and neutral, for example, may become deprotonated at pH=8 and become negative. See the Henderson-Hasselbalch equation.

Protein structure is held together by many intermolecular interactions: dipole-dipole, hydrophobic, electrostatic, etc. The correct protein structure is necessary for function. At different pH levels, the protonation and deprotonation of ionisable amino acids affects the intermolecular interactions within the protein and its conformation will change. At a certain pH, the conformation of the protein will be optimal for its function and this is where it will have maximum activity. As pH deviates to either side of the optimum, the conformation changes and the structure will no longer be correct for proper function.

The ionization state of amino acids is also important in the active site but not only because of conformational changes. Catalytic residues must have certain properties to catalyze their specific reactions. As an example, two Asp residues (which have carboxylates on their side chains) in the active site of DNA polymerase are responsible for coordinating two magnesium ions that promote polymerization. If the pH were to decrease to a point where the Asp residues were protonated, they could no longer bind the Mg2+ ions and the enzyme would not function.

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Enzyme function is largely determined by it's 3-D shape. Enzyme shape is affected by pH among a couple other things. The enzyme only retains its optimal shape at the optimal pH, as you creep outside of it the enzymes shape changes and hence so does its function.

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