Control of Enzyme Activity 89

I. The activity of an enzyme is affected by a variety of factors. A. The rate of enzyme-catalyzed reactions increases with increasing temperature, up to a maximum.

1. This is because increasing the temperature increases the energy in the total population of reactant molecules, thus increasing the proportion of reactants that have the activation energy.

2. At a few degrees above body temperature, however, most enzymes start to denature, which decreases the rate of the reactions that they catalyze.

B. Each enzyme has optimal activity at a characteristic pH—called the pH optimum for that enzyme.

1. Deviations from the pH optimum will decrease the reaction rate because the pH affects the shape of the enzyme and charges within the active site.

2. The pH optima of different enzymes can vary widely— pepsin has a pH optimum of 2, for example, while trypsin is most active at a pH of 9.

C. Many enzymes require metal ions in order to be active. These ions are therefore said to be cofactors for the enzymes.

D. Many enzymes require smaller organic molecules for activity. These smaller organic molecules are called coenzymes.

1. Coenzymes are derived from water-soluble vitamins.

2. Coenzymes transport hydrogen atoms and small substrate molecules from one enzyme to another.

E. Some enzymes are produced as inactive forms that are later activated within the cell.

1. Activation may be achieved by phosphorylation of the enzyme, in which case the enzyme can later be inactivated by dephosphorylation.

2. Phosphorylation of enzymes is catalyzed by an enzyme called protein kinase.

3. Protein kinase itself may be inactive and require the binding of a second messenger called cyclic AMP in order to become activated.

F. The rate of enzymatic reactions increases when either the substrate concentration or the enzyme concentration is increased.

1. If the enzyme concentration remains constant, the rate of the reaction increases as the substrate concentration is raised, up to a maximum rate.

2. When the rate of the reaction does not increase upon further addition of substrate, the enzyme is said to be saturated.

II. Metabolic pathways involve a number of enzyme-catalyzed reactions.

A. A number of enzymes usually cooperate to convert an initial substrate to a final product by way of several intermediates.

B. Metabolic pathways are produced by multienzyme systems in which the product of one enzyme becomes the substrate of the next.

C. If an enzyme is defective due to an abnormal gene, the intermediates that are formed following the step catalyzed by the defective enzyme will decrease, and the intermediates that are formed prior to the defective step will accumulate.

1. Diseases that result from defective enzymes are called inborn errors of metabolism.

2. Accumulation of intermediates often results in damage to the organ in which the defective enzyme is found.

D. Many metabolic pathways are branched, so that one intermediate can serve as the substrate for two different enzymes.

E. The activity of a particular pathway can be regulated by end-product inhibition.

1. In end-product inhibition, one of the products of the pathway inhibits the activity of a key enzyme.

2. This is an example of allosteric inhibition, in which the product combines with its specific site on the enzyme, changing the conformation of the active site.

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Essentials of Human Physiology

Essentials of Human Physiology

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