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A covalent enzyme intermediate bearing an acyl group during catalysis. Its formation and breakdown can occur at different rates, producing burst kinetics if the first released reaction product reports a fast step.

A detectable burst kinetics phase requires rapid formation and detection of an initial reaction product, followed by a slower step that regenerates competent enzyme. A common hydrolysis mechanism iswhere $E^{\ast }$ can be a covalent acyl-enzyme intermediate. With excess substrate, rapid binding and $k_2\gg k_3$, the first turnover rapidly releases $P_1$ and traps much of the enzyme as $E^{\ast }$. Subsequent turnovers wait for the slow regeneration step. The measured product must report the fast step: monitoring only $P_2$ need not reveal a burst. Fast mixing and sufficient time resolution are needed, while spontaneous substrate hydrolysis, depletion and enzyme inactivation must be excluded.

For an illustrative saturated two-step cycle, initially unmodified enzyme concentration $E_T$ givesThe second term approaches a burst amplitude near $E_T$ when $k_2\gg k_3$, followed by the slower steady-state slope. A covalent intermediate is a useful example, not a logical requirement for every possible burst mechanism.