Question

Mutation of Asp85 to serine in bacteriorhodopsin blocks the transport of protons across the membrane by bacteriorhodopsin.

Why would this mutation block the transport of protons by bacteriorhodopsin? Choose one:
A. The serine is unable to bind a proton and transfer it to water.
B. The serine is positively charged and repels the proton.
C. The Asp85 is needed for isomerization of retinal in bacteriorhodopsin, which doesn't happen in the presence of serine.
D. The serine binds tightly to the proton and does not transfer it to water.

Answer 1

The Asp85 to serine mutation blocks proton transport in bacteriorhodopsin because serine cannot effectively transfer the bound proton to water, impeding the formation of an electrochemical gradient.

Step-by-step explanation:

A mutation of Asp85 to serine in bacteriorhodopsin would block the transport of protons across the membrane because the Asp85 residue is necessary for the proton transfer mechanism. Aspartic acid (Asp), which is replaced by serine in this mutation, is able to bind and release protons, thus facilitating proton transport. Serine, however, cannot perform this function effectively. Specifically, among the options provided, the correct reason is that serine binds to the proton but does not transfer it efficiently to water, essentially locking the proton and impeding the proton transport process required by bacteriorhodopsin to generate an electrochemical gradient.