[Judy]

Which step is NOT part of the proton-translocation process in ATP synthase?
A) A negatively charged amino acid in each c subunit becomes neutral upon proton binding.
B) A positively charged amino acid in the a subunit forms an ion pair with a charged amino acid in a subunit.
C) deprotonated c subunits are able to interact only with the a  subunit.
D) An arginine in the a subunit is reversibly protonated and deprotonated in each proton binding event.
E) Proton direction is determined by the relative concentrations of protons on either side of the membrane.


I am not quite sure why D is correct. My guess is that arginine being one of the amino acid within ATP synthase can regulate on/off of the enzyme through protonation/deprotonation. Let me know if my thought is reasonable?

[Babcock_Hall]

Can arginine form a salt bridge to the side chain of a different amino acid, and if so, what kind of amino acid? 

[Judy]

Can arginine form a salt bridge to the side chain of a different amino acid, and if so, what kind of amino acid?

Arginine can form a salt bridge with negatively charged amino acids.
Thanks for your help.

[Babcock_Hall]

Can those negatively charged amino acid side chains bind protons?

[Judy]

Can those negatively charged amino acid side chains bind protons?

Those negatively charge amino acid side chains should be able to bind protons, as opposite charges attract one another. In addition, their binding could potentially help stabilize the entire enzyme. Correct me if I'm wrong, thanks.

[Babcock_Hall]

I would not focus on the whole enzyme.  The negatively charged amino acid side chains can form salt bridges to positively charged side chains such as arginine.  What happens to the salt bridge if the negatively charged group accepts a proton?

[Judy]

I would not focus on the whole enzyme.  The negatively charged amino acid side chains can form salt bridges to positively charged side chains such as arginine.  What happens to the salt bridge if the negatively charged group accepts a proton?

The salt bridge would then accept the proton and delivers it whenever is needed. In the case of ATP synthase, the proton binding on the salt bridge can release during the ATP synthesizing process.

[Babcock_Hall]

If an aspartate in a salt bridge to an arginine becomes protonated, the interaction will weaken.  Try to connect this to the rotary motion of the enzyme.

[Judy]

If an aspartate in a salt bridge to an arginine becomes protonated, the interaction will weaken.  Try to connect this to the rotary motion of the enzyme.

Are you inferring that the rotary motion of enzyme would be slowing down because of the weakened interaction?

[Babcock_Hall]

No, the change in the strength of interaction is what is causing the rotary motion.  There are 10 c-type subunits in one form of ATP synthase.