Past exam of the biochemistry and Molecular Biology course of the University of Cambridge / 2025 / Paper 2 / Section B / 11 / g / Solution

The displayed separation between the two entrance landmarks increases from approximately $6.2$ to $7.7\text{A˚}$ in TEM-52, indicating a wider opening. The Gly238-to-Ser substitution can alter backbone packing and loop geometry; its effect is not simply the steric size of the new side chain. Increased accessibility or altered productive positioning could increase the effective association rate for penicillin G, producing a lower apparent Michaelis constant without changing the limiting catalytic step. In a simple Michaelis-Menten kinetics scheme, $K_m=(k_{-1}+k_{\mathrm{cat}})/k_1$, so increasing $k_1$ can lower $K_m$.

The structural change is consistent with improved access and the lower measured $K_m$, while similar $V_{\max }$ suggests similar limiting turnover under these conditions. It does not uniquely establish the mechanism or prove tighter equilibrium binding. TEM-52 contains additional substitutions, so a matched single-substitution series would be needed to isolate the contribution of residue 238. Structural work on TEM-52 independently supports the importance of remodeling access to its active site.^[ref]