Environmental Effects on Enzyme Action

Environmental factors such as pH, ionic strength, and temperature affect enzyme activity. They must be controlled when making in vitro measurements of enzyme activity and heeded in vivo when abnormal conditions such as acidosis, alkalosis, and fevers may exist. 

Because active sites of enzymes frequently have ionizable groups that must be in a specific ionic form to maintain the conformation of the active site, bind the substrate, or catalyze the reaction, it follows that pH will influence the velocity of enzyme-catalyzed reactions. In addition, the substrate may contain ionizable groups, and only a specific ionic form can bind to the enzyme or undergo catalysis. 

The pH optimum is not necessarily identical with intracellular pH. Many other factors determine the pH stability of an enzyme. These include ionic strength and temperature, enzyme concentration, and substrate and cofactor concentrations among others. Despite these constraints, the pH vs V0 profiles are 

Neutral salts have a significant effect on the solubility of proteins. The effects of salts can be expressed in terms of their ionic strength (u) when they are in solution. Ionic strength can be calculated as 

Most enzymes show a 50-300% increase in reaction rate when the temperature is increased by 10°, and the ratio of rate constants at two temperatures 10° apart is usually between 1.5 and 4.0 for most enzymes. This value is termed Q10 and is derived from the Arrhenius equation [Equation (5.9)], which can be integrated to give 

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