Enzyme kinetics terminology: in the Briggs–Haldane (Michaelis–Menten) framework, which differential condition defines the pseudo steady state (quasi-steady-state) assumption for the enzyme–substrate complex?

Introduction / Context:
The Michaelis–Menten model commonly employs the pseudo (quasi) steady-state assumption (QSSA) to derive a tractable rate law. This approximation asserts that the concentration of the enzyme–substrate complex ES rapidly reaches a near-constant value relative to slower changes in substrate and product, enabling algebraic elimination of ES from the differential equations.

Given Data / Assumptions:

• We track concentrations of free enzyme (E), substrate (S), complex (ES), and product (P).
• QSSA focuses on ES dynamics after an initial transient.
• Total enzyme CE = E + ES is conserved (if no synthesis/degradation).

Concept / Approach:
Under QSSA, the time derivative of ES is approximately zero: d(ES)/dt ≈ 0. This means ES forms and breaks down at similar rates, allowing steady concentration despite ongoing conversion of S to P. The condition is valid when substrate is much greater than enzyme and after the short pre-steady-state phase.

Step-by-Step Solution:

Verification / Alternative check:
Pre-steady-state kinetic experiments show an initial burst phase after which ES is nearly constant, supporting QSSA under typical conditions (S » E).

Why Other Options Are Wrong:

• d(CE)/dt = 0: enzyme conservation, not QSSA.
• d(Cp)/dt = 0: implies no product formation; not the assumption.
• d(Cs)/dt = d(CES)/dt: not a standard kinetic condition.
• d(Cs)/dt = 0: corresponds to zero-order in S (near saturation), not the general QSSA definition.

Common Pitfalls:
Confusing total enzyme conservation with the ES steady-state requirement; ignoring the initial transient when QSSA may not hold.

Final Answer:
d(CES)/dt = 0 (enzyme–substrate complex concentration is approximately constant)