Catalytic center of cytochrome c oxidase: Effects of protein environment on pKa values of cub histidine ligands

Abstract

The molecular mechanism by which electron transfer (ET) is coupled to proton pumping in cytochrome oxidase is one of the main unsolved problems in biochemistry. Particularly, the nature and position of the proton-loading site is under dispute. The CuB complex has three ligated histidines, whereas only His290 and His291 are ionizable sites with the same pKa values in aqueous solution, but apparently quite different ones within the enzyme. Earlier, a model of proton pumping with the central role of His290 was proposed. Recent calculations indicate that the His291 ligand of the CuB center might play the role of the pumping element, since its protonation state depends on the oxidation state of the binuclear complex (BNC). The present electrostatic study was applied to assess the role of the protein environment on the acidity of the two histidines. Their pKa values and effects of different energy terms were evaluated to discover the nature of their diverse behavior in the enzyme. Here, a new set of pKa values for the non-standard model compounds within the BNC was applied. The enhanced results are compared with results of previous studies in the light of the plausible proton pumping mechanism. The obtained microscopic and apparent pKa values in the oxidized state of BNC are virtually the same, indicating that deprotonated form of His291 accounts for the large pKa increase of His290, since then both titratable sites on then CuB center cannot simultaneously be in the charged state. The present results support the underlined His291 pumping model.