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When enzyme complexes relax much faster than substrate concentrations, the quasi-steady-state approximation sets the time derivatives of the fast complexes to zero and solves their algebraic balance equations.

It approximates the fast intermediate by a steady state balance without requiring the entire chemical system to be at equilibrium.

A steady state has time-independent concentrations because formation and removal rates balance for the species considered. Nonzero reaction fluxes can continue, so a steady state need not be chemical equilibrium. A quasi-steady-state approximation imposes this balance approximately for a fast intermediate while slower species evolve.