  The celebrated Michaelis-Menten (MM) expression provides a fundamental
relation between the rate of enzyme catalysis and substrate concentration. The
validity of this classical expression is, however, restricted to macroscopic
amounts of enzymes and substrates and, thus, to processes with negligible
fluctuations. Recent experiments have measured fluctuations in the catalytic
rate to reveal that the MM equation, though valid for bulk amounts, is not
obeyed at the molecular level. In this mini-review, we show how new statistical
measures of fluctuations in the catalytic rate identify a regime in which the
MM equation is always violated. This regime, characterized by temporal
correlations between enzymatic turnovers, is absent for a single enzyme and
unobservably short in the classical limit.
