Conformationally controlled pK-switching in membrane proteins: One more mechanism specific to the enzyme catalysis?

Internal proton displacements in several membrane photosynthetic enzymes ar e analyzed in relation to general mechanisms of enzymatic catalysis, In the bacterial photosynthetic reaction center (RC) and in bacteriorhodopsin (BR ), carboxy residues (Glu-212 in the RC L-subunit and Asp-96 in BR) serve as indispensable intrinsic proton donors. Both carboxyls are protonated prior to the proton-donation step, because their pK values are shifted to greate r than or equal to 12.0 by the interaction with the protein and/or substrat e. In both cases, the proton transfer reactions are preceded by conformatio nal changes that, supposedly, let water interact with the carboxyls. These changes snitch over the pK values of the carboxyls to less than or equal to 6.0 and 7.1 in the RC and BR, respectively. The sharp increase in the prot on-donating ability of the carboxyls drives the reaction cycles. This kind of catalytic mechanism, where a strong general acid or base emerges, when n eeded, as a result of a conformational change can be denoted as a conformat ionally controlled pK-switching. Generally, the ability of enzymes to go be tween isoenergetic conformations that differ widely in the reactivity of th e catalytic group(s) may be of crucial importance to the understanding of e nzymatic catalysis, Particularly; the pK-switching concept could help to re concile the contradictory views on the functional protonation state of the redox-active tyrosine Y-Z in the oxygen-evolving photosystem II. It is conc eivable that Y-Z switches its pK from similar to 4.5 to greater than or equ al to 10.0 upon the last, rate-limiting step of mater oxidation, By turning into a strong base, tyrosine assists then in abstracting a proton from the bound substrate water and helps to drive the dioxygen formation. (C) 1999 Federation of European Biochemical Societies.