Proton Reactions in Photosynthesis

Managing Proton Reactions in Photosynthesis A. Design Issues for Proton Transfer 

Whether the process is adiabatic or nonadiabatic (tunneling), the range of proton transfer is restricted to distances no more than 1 A and mechanisms rely exclusively on reactive complex formation. Thus, in contrast to electron transfer, the short range, bond-length nature of proton transfer necessitates considerations of structure. 

Although the secrets of maximal rates of proton conduction are well illustrated in gA, multifunctional proteins that couple H+ conduction to other events do not exhibit well-formed, proton-conducting hydrogen bond networks. Indeed, in the bacterial reaction center the putative active path is poorly connected by hydrogen bonds detectable in the best current X-ray structures (2.2 A resolution Stowell et al., 1997). Paddock et al. (1999) have shown that chemical blockage or a simple mutational lesion of this active path diminishes proton transfer rates by at least 1000-fold. Thus, the several well-connected (but not quite continuous) files of water that are seen in the X-ray structures, reaching toward the Qg site from the cytoplasmic side, do not conduct protons at significant rates. 

The implication is that the proton delivery paths are transient and highly dynamic entities. This is explicitly demonstrated in the elegant X-ray studies of bacteriorhodopsin (bR), where the path for reprotonation of Asp-96 is not in existence in the early part of the photocycle, 

In biological electron transfer, the role of H+ depends on the strength of coupling between the electron and proton  

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