Question

The imidazole side chain of histidine can function as either a general acid catalyst or a general base catalyst because

I. the imidazole group has a pKa in the physiological pH range.
II. in the physiological pH range, the SULFUR in the ring can be easily protonated/deprotonated. (verified with instructor that sulfur was intended, NOT nitrogen)
III. one guanidino group is protonated, and the other is deprotonated at physiological pH.
IV. The imidazole group is a strong reducing agent at physiological pH
A. I, II
B. II, IV
C. IV
D. I
E. II, III

Answer 1

The imidazole side chain of histidine acts as a general acid or base catalyst because it has a pKa within the physiological pH range, allowing protonation and deprotonation.

Step-by-step explanation:

The imidazole side chain of histidine can function as either a general acid catalyst or a general base catalyst primarily because the imidazole group has a pKa in the physiological pH range. The correct answer is 'D. I,' which corresponds to the imidazole group's ability to be protonated and deprotonated at physiological pH levels. This flexibility allows it to act as both a proton donor and acceptor during enzymatic reactions. The mention of sulfur being easily protonated/deprotonated is incorrect as the imidazole ring of histidine does not contain sulfur; it contains nitrogen atoms that are involved in this process. The guanidino group being referred to is part of the amino acid arginine, not histidine. Lastly, the imidazole group is not known for being a strong reducing agent at physiological pH.

Answer 2

The reason why the imidazole side chain of histidine can function as either acid or base catalyst is: I and II: Option A.

Step-by-step explanation:

The imidazole group of the histidine has a pKa of 6.0, which is closer to the pKa of the physiological environment. Thus, at every reaction, either acidic or basic, the side chain act as a buffering catalyst, which can be useful irrespective of the pH of the reactants. Also, since the reaction of a catalyst is influenced by the pH of the environment, at physiological pH, the sulfur in the ring can be protonated by acidic reaction or deprotonated by the primary reaction within the physiological environment.