Effect of Substrate Concentration on Reaction Velocity

Different enzymes show different responses to changes in substrate concentration, temperature, and pH.  Enzymic responses to these factors give us valuable clues as to how enzymes function in living cells (that is, in vivo).
 Substrate concentration
1. Maximal velocity: The rate or velocity of a reaction (v) is the number of substrate molecules converted to product per unit time. Velocity is usually expressed as μmol of product formed per minute. The rate of an enzyme-catalyzed reaction increases with substrate concentration until a maximal velocity (V_max) is reached (Fig. 1). The leveling off of the reaction rate at high substrate concentrations reflects the saturation with substrate of all available binding sites on the enzyme molecules present.

Figure 1: Effect of substrate concentration on reaction velocity.
2. Shape of the enzyme kinetics curve: Most enzymes show Michaelis-Menten kinetics , in which the plot of initial reaction velocity (v_o) against substrate concentration is hyperbolic (similar in shape to that of the oxygen-dissociation curve of myoglobin. In contrast, allosteric enzymes do not follow Michaelis-Menten kinetics and show a sigmoidal curve (see Fig. 1) that is similar in shape to the oxygendissociation curve of hemoglobin .

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