Acid-base catalysis single proton transfer

The results of the above analysis of reactions involving two proton transfers may be compared with those for reactions in which only a single transfer is kinetically relevant (Sec. III.3.a). For a single transfer the existence of an equilibrium between catalyst and substrate always produces the appearance of specific catalysis by hydrogen or hydroxyl ions, and the detection of general acid-base catalysis therefore excludes... 

Serine proteases are widely distributed and have many different functions. They are products of at least two evolutionary pathways, which originate in prokaryotes. Many of them resemble trypsin, chymotrypsin, elastase, or sub-tilisin in specificity, but serine proteases with quite different specificities have been isolated recently. A recent NMR study of a bacterial protease labelled with at carbon 2 of its single imidazole groups implicates a buried side chain of aspartic acid as the ultimate base for proton transfers in catalysis and eliminates a charge separation from reaction schemes for catalysis. Much of the catalytic effectiveness of serine proteases can be attributed to substrate binding, but the interactions which yield a Michaelis complex are supplemented by others which stabilize intermediates on the reaction pathway. 

The same conclusion applies for L-alanine/pyruvate (2S7), pyridoxamine/pyru-vate (238), and dialkylamino acid transaminase (239). Second, small but significant amounts of direct hydrogen transfer (as either or H) between the a-carbon and C-4 have been detected during catalysis by the latter two enzymes (238, 239). Thus, the clear implication of these results is that a single active site base operating at the si face of the aldimine facilitates proton transfer between the a-carbon and C-4 [Eq. (47)]. 

We have seen in the last chapter that many catalysed reactions are determined kinetically by a single acid-base reaction between the catalyst and the substrate, and when this is so /c or k in Equation (105) is just the second-order velocity constant for this reaction. The interpretation is not quite so simple when the reaction involves two consecutive proton transfers, of which the first is effectively at equilibrium. Thus for acid catalysis we may have... 

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