MECHANISMS OF ENZYMATIC REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 75

In 1977 the topic author (Likhtenshtein, 1977b), analyzing possible mechanisms of
hydrolysis of polyaminosaccharides catalyzed by lysozyme, had concluded that the
concerted attack of carboxylate and carboxyl and water molecule on the substrate is
characterized by very low theoretical value synchronization factor Such a
low probability of synchronous motion of nuclei along the reaction coordinate can not be
compensated by low energy activation of concerted process. Recently, strong experimental
evidence has been provided that the reaction occurs through the formation of covalent
glycosyl-enzyme intermediate during catalytical cycle of hen egg-white lysozyme (Fig
2.17) (Vocado et al., 2001). The formation of the intermediate was proved using
electrospray ionization mass spectrometry and X-ray diffraction. The proposed reaction
mechanism includes substrate distortion, formation of a covalent intermediate.
Nevertheless, the concerted elementary process required simultaneous motion of nine
nuclei and estimated synchronization factor and also is not possible.
Though the aforementioned estimates that illustrate the principle of “optimal motion”
are based on simplified models and approximation formulas (2.44 - 2.46), they give
independently indirect evidences in favor of, a similarity between transition and
pretransition states of the enzymatic reactions.

2.10. Radical mechanisms of enzyme catalysis

In 1949 Chance in his classical work has constructed the following radical mechanism of
substrate oxidation catalyzed by peroxidase (E)

According to the Chance mechanism, the interaction of with the enzyme gives
“compound I “ The oxidation of the donor molecules leads to “compound II”
which oxidizes the second donor molecule. The radical intermediates were detected
experimentally for such substrates as amines and phenols with relatively high reduction
potential (Dunford and Stillman, 1976). The one-electron steps with the formation of free
radicals at oxidation of amines and phenols have been proved in the ceruloplasmin, laccase
and ascorbic oxidase reactions (Malsmstrom et al., 1975).
Recently enzymatic mechanisms that proceed by free radical chemistry initiated by the
5'deoxyadenosyl radical were discovered. (Frey, 2001). Three radicals were

specroscopically characterized in reaction of the interconversion of L-lysin and
by lysine 2,3-aminomutase. The enzyme center undergoes the chemical cleavage
of S-adenosylmethionine (SAM) with the reversible formation of 5'-deoxyadenosyl
radical. In other reactions with SAM, iron-sulfur proteins generate this radical which
activate an enzyme to abstraction a hydrogen atom from an enzymatic glycyl residue to
form a glycyl radical. 5'deoxyadenosyl radical also arises in adenosylcobalamin reaction
as the result of hemolytic cleavage of the cobalt-carbon bonds. In the following reaction
this radical initiates abstraction hydrogen atoms from substrates.