MECHANISMS OF ENZYMATIC REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 69

proton extraction from kinetic” acidity of the

may be increased by a concerted formation of either hydrogen-bonded or metal-
coordinated enolates by the general acid catalysis (Gerlt, 1999; Gerlt and Gassman, 1992).

bond is 12-34 kcal/mole. A '

An essential contribution to lowering the activation barrier is proposed by the
formation of a low barrier hydrogen bond (LBHB) with short distance between the proton
donors and acceptors as demonstrated in the isomerase (KSI) reaction
(Ha et al., 2000). This enzyme catalyzes cleavage and formation of the substrate C-H bond
at a diffusion-controlled limit. The crystallographic and NMR investigations of KSI with
its competitative inhibitors (equilenin and androsteron) showed a large perturbation of pKa
values of both inhibitors and a key catalytic residue at the active site. NMR spectroscopic
investigation evidenced “unusual” ionization of a hydroxyl group

of an

4 at The protonation of the
catalytic residue Asp38 in the interaction with a carboxylate group of an
inhibitor also takes place. The pKa differences between catalytic groups and substrates
can be significantly reduced in the active site environment and thereby eliminate a large
fraction of thermodynamic and activation barriers in general acid/base reactions. Similar
effects have been found in glutamate, triose-P, mandelate racemiases, aconitase and citraite
synthase (Gerlt and Gassman, 1992; Cleland and Northrop, 1999; and references therein)
In serine proteases the hydrogen bonds between Asp and the His of the catalytic triad is
normally weak. At the substrate presence the histidine becomes “unusually” protonated
and a LBHB forms between Asp and His. The LBHB formation is proven by the low field

inhibitor with a catalytic residue Tyr 1