  Dynamic cooperativity in monomeric enzymes is characterized in terms of a
non-Michaelis-Menten kinetic behaviour. The latter is believed to be associated
with mechanisms that include multiple reaction pathways due to enzymatic
conformational fluctuations. Recent advances in single-molecule fluorescence
spectroscopy have provided new fundamental insights on the possible mechanisms
underlying reactions catalyzed by fluctuating enzymes. Here, we present a
bottom-up approach to understand enzyme turnover kinetics at physiologically
relevant mesoscopic concentrations informed by mechanisms extracted from
single-molecule stochastic trajectories. The stochastic approach, presented
here, shows the emergence of dynamic cooperativity in terms of a slowing down
of the Michaelis-Menten (MM) kinetics resulting in negative cooperativity. For
fewer enzymes, dynamic cooperativity emerges due to the combined effects of
enzymatic conformational fluctuations and molecular discreteness. The increase
in the number of enzymes, however, suppresses the effect of enzymatic
conformational fluctuations such that dynamic cooperativity emerges solely due
to the discrete changes in the number of reacting species. These results
confirm that the turnover kinetics of fluctuating enzyme based on the
parallel-pathway MM mechanism switches over to the single-pathway MM mechanism
with the increase in the number of enzymes. For large enzyme numbers,
convergence to the exact MM equation occurs in the limit of very high substrate
concentration as the stochastic kinetics approaches the deterministic
behaviour.
