Examine the structure of the transporter shown here. It is a 2H+ /Hg2+ antiport.

Question

Examine the structure of the transporter shown here. It is a 2H+ /Hg2+ antiport. (That is, it uses the energy in the PMF to expel toxic mercury-II from the bacterial cell.) The red circles represent two glutamic acids right next to each other. Their R-groups can accept H+ , which they tend to do at acidic pH, but less so at neutral or alkaline pH. (Technically, we could say that its pKa ~5, although you don’t have to know what that means to figure this out.) The yellow circle is a lysine, whose R-group is almost always protonated under normal circumstances. (Its pKa ~10.) There are other important amino acids (such as those at the Hg2+ binding site) that are not shown. I just want you to analyze the role of the protons.

C. What would happen if proline was added to the primary sequence of the protein right next to the lysine?

D. Interestingly, mercury is not toxic for cells that contain this transporter, provided the cells are growing at slightly acidic pH (external pH around 6). But if the cells are placed at slightly alkaline pH (external pH about 7.5), now mercury is toxic. Explain this result.

Explanation / Answer

D. Mercury is not toxic at acidic pH because in acidic condition glutamic acid accept H+ ion, flow of H+ inside the cell create proton gradient which result in the release of mercury out of cell. At slight alkaline pH , glutamic acid R group is not accept H+ ion so no entry of H+ ion leads to no proton gradient . Therefore, no release of mercury out of the cell result in toxicity.

C) proline add next to lysine in primary structure of protein not create much effect because proline has no side chain and its carboxyl and amino group involve in peptide bond formation.

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