Question

An enzyme has a broad pH optimum having high rates of activity between pH 4 and 12. X-ray diffraction studies show that the enzyme has an active site with glutamate and arginine residues in close proximity to the substrate. Explain how these two residues might be able to promote concerted acid-base catalysis (which residue is acting as the acid, which is acting as the base). Finally, explain why enzyme activity drops off sharply below pH 4 and above pH 12.

Answer 1

I am explaining here how this two amino acids promote acide-base catalysis:Glutamate residues acting as a the acid and arginine residues acting as the base at their respective pH 4 and 12.Glutamate has a acidic side chain and negative charge. Glutamate at pH 4 equal to pKa, acid chain is uncharged. when glutamate at pH 12 > pKa, carboxylic group loss H+ ion are -ve in charge.and acidic in nature.

Arginine has a basic side chain and positive charge. Arginine at pH 12 equal to pKa, basic chain unchaeged. when arginine at pH 4< Pka, arginine basic side chain accept H+ iona and get Positive charge and basic in nature.

Why enzyme activity drops off sharply below pH 4 and above pH 12: At below pH 4 glutamate are positive in charge and it changed the enzyme active side confirmation and structure and inactivate the enzyme. At above pH 12 arginine are negative in charge and decrease the enzyme binding affinity to the substrate.Due to change in ionisation of two amino acids, sharply drops the enzyme substrate binding below the pH4 and above the pH 12.