Structural Biochemistry/Enzyme/Maximum velocity

Introduction

When an enzyme is applied to a reaction, it allows for the reactants to quickly produce the products. However, over time the use of an enzyme on substrates does not continue to accelerate the reaction. This occurs when the enzyme is fully saturated with the substrate. At this point, all of the active sites of the enzymes are occupied. Thus, the reaction reaches a maximal velocity asymptotically and eventually levels off to a constant rate. The maximum velocity is evidence that the enzyme-substrate complex exists. The maximum velocity is also known as the turnover rate, or the number of substrate molecules it can convert to product per unit time. When the enzyme is saturated, all enzymes in the solution are found in ES complexes and thus the formation of the intermediate no longer affects the rate. This means that Vmax is determined only by the rate constant for the formation of product from the ES complex.

File:Maximum velocity.GIF

There is an older method to determine K_M and V_max. It is used before the invention of computer because it was hard to draw the curve of Michaelis-Menten equation. Therefore, it is impossible to get the exact values of K_M and V_max from the curve. This method transforms the Michaelis-Menten equation to a linear line by taking the reciprocal of both sides of the MichaeliS-Menton equation. The y-axis is 1 / V_max and the x-axis is 1 / [S]. The slope is K_M / V_max.

1 / V₀ = (K_M / V_max) • (1 / S) + (1 / V_max)

Effects of Reversible Inhibitors

Since the maximum velocity is essentially derived from the rate of product formation from the enzyme-substrate complex (k₂) and the initial enzyme concentration ([E_o]), the maximum velocity is affected by certain types of reversible inhibitors. (Note: V_max = k₂[E_o].) Out of the different types of reversible inhibitors—competitive, uncompetitive, and noncompetitive—V_max is affected only by uncompetitive inhibitors and noncompetitive inhibitors. For uncompetitive inhibitors, the enzyme-substrate-inhibitor complex decreases the full potential of product formation from initial enzyme concentration, which lowers the maximum velocity. As for noncompetitive inhibitors, the maximum velocity is also lowered because the inhibitor lowers the concentration of the enzyme. A new maximum velocity is formed in the presence of a pure noncompetitive inhibitor, which will be referred to as V_max,i here. This new value can be related to the maximum velocity by: V_max,i = V_max*1/(1+[I]/K_i), with [I] representing the concentration of the inhibitor and K_i is the rate constant of the formation of the enzyme-substrate-inhibitor complex.