SOME PROTEIN PROPERTIES IMPORTANT FOR ENZYME CATALYSIS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 163

resolved fluorescence techniques led to a more flexible enzyme structure, which is
probably responsible for enhanced enzymic activity (Nucci et al., 1993; D Auria et al.,

1999; Shames et al., 2000; Likhtenshtein et al., 2000). Nevertheless, these effects are not
accompanied by

'

marked change in integral protein properties such as heat capacity,
degree of helicity and number and properties of intramolecular hydrogen bonds. Such an
apparent discrepancy between the integral and local properties was found to be a general
feature of “regular” proteins and enzymes, and appears to be a consequence of the hinge-
like mechanism of protein dynamics. The fast reversible motion of relatively rigid blocks
(polypeptide chains, hydrophobic and polar clusters and domains) can contribute
significantly to mobility as well as to some local properties of biophysical labels,
although they don't markedly affect the above-mentioned integral physical properties of
proteins.
Two mechanisms of the effect of conformational flexibility on
activation caused by a temperature increase and the addition of activators can be
inferred. The first mechanism is related to the steric hindrances to the substrate approach
to the tunnel of the active site. Flexibility can make it easier for the substrate to access
the catalytic group of the active site. The second mechanism concerns the chemical
activity of the E387 group. According to the X-ray structural model (Aguilar et al .,
1997), this group in the enzyme resting state is connected by a hydrogen bond to
histidine R79. Such a connection can dampen the nucleophylic activity of the E387
group. The conformational transition can break the hydrogen bond and therefore activate
the carboxyl nucleophile. The fact is that the Michaelis-Menten constant for
substrate hydrolysis shows only a slight, if any, dependence on temperature (Nucci etal.
1993) evidences in favour of the latter mechanism.
We may also speculate concerning a reason for the increase in the protein rigidity
and correspondent decrease in sensitivity of the spin labels' rotational diffusion to
temperature increase above Efficiency of the chemical processes requires
optimum flexibility of the enzyme active site. If the low-temperature tendency toward an
increase in the enzyme conformational flexibility in the active area would continue at
high temperatures, such an optimization would be destroyed. Thus, the conformational
transition may be necessary for maintaining a balance between activity and stability of
the enzyme at high temperatures.

a