Question

A zinc ion in the active site of an enzyme can speed up the reaction by:

A. Making a bound water molecule a more powerful nucleophile by drawing electron density toward itself and facilitating deprotonation of the water

B. Orienting substrates for reaction

C. Stabilizing an anionic intermediate

D. All of these

E. None of these

The role of serine at the active site of serine proteases is to act as a ___ catalyst, while the histidine residue serves as an ___ catalyst.

A. acid-base, covalent

B. anionic; ionic

C. covalent; acid-base

D. strong; weak

E. weak; strong

The active site of an enzyme is being investigated by mutagenesis to determine the stability of the enzyme conformation following substrate binding. A leucine is replaced by glycine. This change will:

A. Destabilize the enzyme

B. Stabilize the enzyme

C. Have no effect on the enzyme

Answer 1

Ans(1): Option D is correct. (All of the above).

Zinc ion functions as a Lewis acid in all catalytic reactions. All of the actions given in options A, B and C can be carried out by the zinc in enzyme catalytic reactions.

Ans(2): Option A is correct. The role of serine at the active site of serine proteases is to act as a acid-base catalyst, while the histidine residue serves as an covalent catalyst.

This is beacause serine has an -OH group which acts as a nucleophile (lewis base) and attacks the carbonyl carbon of the scissile peptide bond of the substrate. Histidine nitrogen has a pair of electronns that has the ability to accept the hydrogen from the serine -OH group and hence, coordinates the attack of the peptide bond.

Ans(3): Option C is correct. (No effect of the enzyme)

This is because leucine and glycine have similar nature, both are hydrophobic. Both donot have any hydrogen donar or acceptor in their side chains.