Rapid Equilibrium and Steady-State Hypothesis

The effect of substrate concentration on enzyme kinetics was first proposed by Henri at the beginning of the XX century. Making an analogy with reversible chemical reactions between two substrates, Henri proposed that conversion of substrate into product involved a reversible reaction between enzyme and substrate to form an active intermediate that brakes down delivering the product. These ideas were taken a few years later by Michaelis and Menten (1913) who proposed the first formal hypothesis for enzyme catalysis based on two sequential steps, as suggested by Henri in the first step the substrate is captured in the active site of the enzyme, while in the second step the amino acid residues at that site chemically process the substrate to 

According to the (pseudo) equilibrium proposal of Michaelis and Menten, the substrate binding step is so fast as compared to the rate of enzyme-substrate complex (ES) breakdown into product that the reaction of formation of ES can be considered at equilibrium. This is so in most cases, which validates this hypothesis, even though it is obviously an oversimplification, since no equilibrium is possible as long as ES is being converted into product. According to this  

6 represents the parametric expression for the enzymatic reaction rate as a function of substrate concentration. 

A few years later, Briggs and Haldane (1925) argued against the validity of the rapid equilibrium hypothesis and proposed a steady-state hypothesis according to which, after a very short transient phase, the ES complex remains constant throughout the whole reaction period, as shown in Fig. 3.2. 

Strictly speaking, according to the steady-sate hypothesis  

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For simple kinetic mechanisms, like irreversible one-substrate reactions, both rapid equilibrium and steady-state hypothesis lead to rate equations that are formally equal in parametric terms, so when those parameters are experimentally determined, results are the same no matter what hypothesis is considered. Kinetic parameters are to be experimentally determined to obtain validated rate expressions to be used for the design or performance evaluation of enzyme reactors. 

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