One of the catalytic triad members is His. Which one of the following best describes its role?

Question

BlackTom AI · Accepted Answer

The question asks about the role of one member of the catalytic triad, specifically histidine, in enzyme catalysis.
Option 1: 'It is the nucleophilic attacker' — In classic catalytic triads (e.g., serine proteases), the serine residue is the nucleophile that attacks the substrate, not histidine. Histidine acts as a general base/acid to shuttle protons, not as the main nucleophile. Therefore this statement misidentifies the nucleophile in the triad.
Option 2: 'It stabilizes the transition state' — While stabilizing transition states is a general goal of catalytic active sites, histidine’s main, explicit role in the triad is proton transfer via its imidazole side chain. Transition-state stabilization is typically attributed to other features of the active site, such as the oxyanion hole or the general environment, rather than histidine’s primary function. Thus this answer is imprecise or incomplete with respect to histidine’s described role.
Option 3: 'It is able to donate and accept a proton as needed' — This captures the essence of histidine’s function in the catalytic triad: it can both donate and accept protons due to the pKa of its imidazole side chain around physiological pH. This proton shuttle enables activation of serine and facilitates subsequent steps in catalysis, aligning with the established mechanism of serine proteases. This statement fits histidine’s role precisely.
Option 4: 'It is attracted to positive charge, so can stabilize the position of cationic components of the reaction mechanism' — While histidine can engage in electrostatic interactions, the specific, central description of its role in the triad is not primarily about generic stabilization of cationic components. The key functional description is the proton relay capability, not a general attraction to positive charges for stabilization. This makes the statement less accurate as a description of histidine’s designated catalytic role.

Taken together, the options present progressively more accurate or inaccurate attributions of histidine’s role. The most precise characterization is that histidine can donate and accept protons as needed during catalysis, enabling the proton transfers that drive the reaction sequence.

One of the catalytic triad members is His. Which one of the following best describes its role?

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