MECHANISMS OF ENZYMATIC REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 68

The basic idea underlying modern approaches to enzyme transition states is that finding
a substrate molecule in the transition state in an enzyme active is connected with protein
by multiple bonds. At a relatively modest energy of an individual bond, the energy from
multiple bonds of only 2-3 kcal/mole can generate energy of 20 -30 kcal/mole in the
transition state (Cleland and Northrope, 1999 and references therein). Such control of a
strong interaction by the cooperation of many weak bonds has been called the “Lilliput
principle” (Lumry, 2002). Because energy of different interactions such as van der Waals
contacts, electrostatic ion-ion, ion-dipole and dipole-dipole, and hydrogen bonds are
strongly dependent on media polarity, the transition of an interacting pair from water to a
less polar medium is accompanied by drastic increases in the interaction energy and,
therefore, by a change in the group's reactivity.
Values of local dielectric constants in proteins and enzyme active centers are estimated
within the range of (Marcus and Sutin, 1985; Fogel et al. 1994; Likhtenshtein,
1996; Honig and Nicolls, 1995; Cleland and Northrop, 1999) and, therefore, electrostatic
interactions in these media are essentially more favorable as compared to a aqueous
solution. Hydrogen bonds in aqueous solution are relatively weak, with energy formation
and in length. In nonprotic solvents of lower dielectric

constant, hydrogen bonds become stronger and shorter
between two oxygen atoms). As the bonds shorten, the barriers between two positions
decrease and the possibility of nuclear tunneling increases. Such a bond is called a “low
barrier hydrogen bonds” (LBHB).
In low dielectric organic solvents and enzyme active sites a number of hydrogen bonds
between groups with similar pKa exhibit highly deshielded 1H NMR peaks (>16 ppm),
low isotopic fraction factors and relatively short H-bonds (data on neutron and x-ray
diffraction analysis (Gerlt and Gassman, 1992; Zundel , 2000; Cleland and Northrop,
1999).
Fig. 2.13 illustrates the electrostatic effects in transition state in enolase reaction
(Larson et al., 1996). During this reaction a proton is removed by Lys-345 from C-2 of 2-
phosphoglycerate to give an enolyzed, charged intermediate. This intermediate is stabilized
by electrostatic interaction with five positive charges supplied by two ions and a
protonated lysine. The 10-11 electrostatic interactions were found in the transition state of
formate dehydrogenase and carbamoyl synthetase (Bruice and Benkovic, 2000) Another
example of multifunctional interactions during enzymatic reactions in intermediate is the
X-ray structure of tetrahedral intermediate in the chymotrypsin active site (Fig. 1.1).
One of the most important factors providing acceleration of enzymatic reactions as
compared to chemical reactions is drastic changes of chemical reactivity catalytic groups
inside and outside the enzyme protein globule. Drawing the charges of metal ions,
carboxylate and protonated residues into the protein interior is accompanied by essential
alternation of its acid-base and redox properties. This effect can be illustrated by the
reaction of cleavage and formation of an bond in enzymatic reactions of
racemization, transamination, and isomerization (Ha et al., 2000 and references therein).
In these reactions a proton is abstracted from a carbon adjacent to carbonyl, carboxylic
acid, or the carboxylate anion group by active cite residues. In water the of
of most aldehydes, ketons, thioesters, and carboxylate anions lies between16-32, whereas
of most carboxylate bases is usually < 7. Thus, the thermodynamic barrier for the

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