= Solution
The displayed separation between the two entrance landmarks increases from approximately $6.2$ to $7.7\,\text{\AA}$ 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 $K_m$.
\b[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>.<Predicting the emergence of antibiotic resistance by directed evolution and structural analysis by M. C. Orencia and others>[Abstract and TEM-52 structural analysis: access remodeling and the E104K/M182T/G238S combination]{ref}
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