In proton transport through bacteriorhodopsin, what is the effect of the conformational change of retinal?

The correct choice, which states that the conformational change of retinal in bacteriorhodopsin decreases the pKa of functional groups, is based on the dynamics of how retinal functions as a light-driven proton pump. When retinal absorbs light, it undergoes a cis-to-trans isomerization, triggering a series of conformational changes in the bacteriorhodopsin protein.

This isomerization alters the local environment of key amino acid residues involved in proton transport. Specifically, it can result in the stabilization of protonated forms of certain side chains that participate in the transport mechanism. As the conformational change progresses, it effectively reduces the pKa of nearby functional groups, making them more likely to be protonated or deprotonated at physiological pH. This is fundamental for the process of proton transfer across the membrane, as it allows for the protons to be released or taken up more readily, ultimately leading to an electrochemical gradient across the membrane.

The other options do not align with the primary function of retinal in this context. While ATP synthesis is related to the electrochemical gradient established by proton transport, the conformational change itself does not directly increase the ATP synthesis rate. Rather, it contributes to the creation of the gradient that can