There were three catalytic triad residues shown (Ser-His-Asp). Which additional residue was shown? The protein backbone at that location is able to make valuable H-bond contacts with the transition state that forms.

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The question describes a catalytic triad consisting of Ser-His-Asp and asks which additional residue was shown, noting that the protein backbone at that location can form valuable H-bond contacts with the transition state. 
Option 1: Gly. Glycine is the smallest amino acid and often appears in flexible regions or at active-site interfaces where minimal steric hindrance is beneficial; its backbone can participate in stabilizing hydrogen bonds with transition states, which aligns with the description of forming valuable H-bond contacts. This makes Gly a plausible candidate for the additional residue showcased.
Option 2: Thr. Threonine contains a hydroxyl group that can participate in hydrogen bonding, and its side-chain could, in principle, engage in interactions near the active site. However, the prompt emphasizes a residue whose presence and backbone location enable valuable H-bond contacts with the transition state specifically described in this context, and Thr is less commonly highlighted in this precise role alongside a catalytic triad in standard textbook representations.
Option 3: Lys. Lysine has a long, basic side chain capable of electrostatic and hydrogen-bond interactions, but the description emphasizes a residue tied to backbone H-bond contacts with the transition state at that location, which is more characteristic of small residues capable of backbone interactions rather than a bulky, charged side chain like that of Lys. This makes Lys a less likely choice in this context.
Option 4: Pro. Proline is unique in that it imposes conformational rigidity due to its cyclic structure, often affecting backbone phi angles. While Pro can influence local geometry and sometimes participate in hydrogen-bond networks, the description highlighting valuable backbone H-bond contacts with the transition state at the catalytic region is not typically attributed to Pro in classic catalytic triad contexts.
Option 5: Ala. Alanine is another small residue, but unlike Gly, Ala has a methyl side chain that adds steric bulk relative to Gly and can alter backbone conformations. The question’s emphasis on a backbone-mediated hydrogen-bond interaction with the transition state lends more support to a residue that offers maximal backbone accessibility, which Gly more readily provides.
In summary, among the options, Gly best fits the role of an additional residue shown that allows the backbone at that location to form valuable hydrogen-bond contacts with the transition state, consistent with common depictions of catalytic sites and their neighboring residues. The other choices may contribute to active-site chemistry in various contexts, but they do not align as closely with the described backbone-mediated transition-state stabilization.

BIOL3612.MERGED.202610 Activity Questions 3B: Chymotrypsin mechanism

There were three catalytic triad residues shown (Ser-His-Asp). Which additional residue was shown? The protein backbone at that location is able to make valuable H-bond contacts with the transition state that forms.

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