Qssa
Quasi-Steady-State Approximation (QSSA) is a method in chemical kinetics and systems biology used to simplify reaction networks by assuming that certain reactive intermediates rapidly reach a steady state.
Origin: The concept emerged in the early 20th century in work by Briggs and Haldane on enzyme
In a simple enzyme-catalyzed reaction E + S ⇌ ES → E + P, the rate equations yield d[ES]/dt = k1[E][S]
Variants include the standard (sQSSA) and the total (tQSSA) formulations; the latter uses total enzyme concentration
Limitations: not valid if substrate concentration is comparable to enzyme concentration, in cases with slow ES
Applications: enzyme kinetics analysis, metabolic pathway modeling, and systems biology; used to derive Michaelis-Menten type rate