In serine protease catalysis (e.g., trypsin), what specific catalytic role do Asp 102 and His 57 play within the catalytic triad (Ser 195–His 57–Asp 102)?

Introduction:
Serine proteases such as trypsin, chymotrypsin, and elastase share a conserved catalytic triad composed of Ser 195, His 57, and Asp 102. This question probes the precise role of Asp 102 and His 57 in activating Ser 195 for nucleophilic attack on the peptide bond carbonyl. Understanding this charge-relay (proton-shuttle) system is central to appreciating how proteases accelerate peptide bond hydrolysis by many orders of magnitude.

Given Data / Assumptions:

• The active site contains Ser 195, His 57, and Asp 102 in a specific hydrogen-bonded geometry.
• Hydrolysis requires nucleophilic attack by Ser 195 Oγ on the substrate carbonyl carbon.
• Proton transfers must occur efficiently during acylation and deacylation steps.

Concept / Approach:
Asp 102 orients and stabilizes the protonated form of His 57. His 57 acts as a general base to abstract a proton from Ser 195, generating a powerful alkoxide nucleophile. During catalysis, His 57 also serves as a general acid to donate a proton to the leaving group and later to water, thereby shuttling protons between participants. This cooperative behavior is the classical charge-relay or proton-shuttle mechanism.

Step-by-Step Solution:

Verification / Alternative check:
Mutational and kinetic studies show large loss of catalytic efficiency when Asp 102 or His 57 is altered, confirming their roles in proton transfer and Ser 195 activation rather than mere structural support.

Why Other Options Are Wrong:

• Neutralize each other’s charges: does not explain nucleophile formation or observed kinetics.
• Keep specificity pocket open: substrate selectivity is determined by pocket size/charge, not by Asp 102–His 57.
• Clamp substrate: binding subsites do this; the triad’s function is chemical catalysis.
• Recruit a metal ion: serine proteases are not metalloenzymes.

Common Pitfalls:
Confusing the oxyanion hole (stabilizes intermediates) with the catalytic triad (performs proton transfers), or thinking Asp 102 directly attacks the substrate.

Final Answer:
Function together as a proton shuttle (charge-relay) that activates Ser 195