Proton Uptake and Quinone Connection in Bacterial Reaction Centre

Abstract

Movement of the iron sulfur protein extrinsic domain (ISP) during catalysis has focused attention on the role of this group as a substrate, and its participation in formation of the enzyme-substrate (ES-) complex at the quinol oxidizing (Qo-) site. Formation of the ES-complex is reflected in the differential binding of quinol when the ISP is oxidized, and in the displacement of the pK of ISPox, both of which show a binding constant of ~14, as determined from kinetic experiments. In the Qo-site reaction, the first electron transfer from quinol to ISPox is rate limiting (~1.5 103 mol/mol bc1/sec), and shows a high activation barrier (~65 kJ/mol). It is likely that the ES-complex has a configuration similar to that of stigmatellin bound at the site, but with quinol forming H-bonds with His-161 of ISPox and Glu-272 of cytochrome (cyt) b. If this is so, then the slow rate is associated with a relatively short electron transfer path (~7 A). This paradox can be resolved if the reaction is treated as a proton-coupled electron transfer, in which the low probability of transfer of the proton limits that of the electron:...Current speculation on the mechanism for transfer of the second electron (from semiquinone to heme bL) centers on a controversy between single- and double-occupancy models. The role of Glu-272 in processing the second proton is critical; this residue rotates through 120o between conformations in stigmatellin and myxothiazol crystals, providing a pathway through which a proton from the H-bond with quinol in the ES-complex could be transferred to a buried water chain, which contacts the carboxylate moiety in the myxothiazol configuration, and could thus deliver a H+ to the external aqueous phase. Since Glu-272 occupies a substantial fraction of the proximal domain of the Qo-site in the stigmatellin configuration (and hence would in the ES-complex), such a role in normal mechanism would likely preclude a double-occupancy in which a second quinone occupied this domain.