Enzyme Activity

In addition to changing the rate of enzyme-mediated reactions by changing the concentration of either substrate or enzyme, the rate can be altered by changing enzyme activity. A change in enzyme activity occurs when the properties of the enzyme's active site are altered by either allosteric or covalent modulation. Such modulation alters the rate at which the binding site converts substrate to product, the affinity of the binding site for substrate, or both.

Figure 4-12 illustrates the effect of increasing the affinity of an enzyme's active site without changing the substrate or enzyme concentration. Provided the substrate concentration is less than the saturating concentration, the increased affinity of the enzyme's binding site results in an increased number of active sites bound to substrate, and thus an increase in the reaction rate.

The regulation of metabolism through the control of enzyme activity is an extremely complex process since, in many cases, the activity of an enzyme can be altered by more than one agent (Figure 4-13). The modulator molecules that allosterically alter enzyme activities are product molecules of other cellular reactions. The result is that the overall rates of metabolism can be adjusted to meet various metabolic demands, as will be illustrated in the next section. In contrast, covalent modulation of enzyme activity is mediated by protein kinase enzymes that are themselves activated by various chemical signals received by the cell, for example, from a hormone. (The regulatory mechanisms controlling the activity of protein kinase enzymes will be described in Chapter 7.)

Figure 4-14 summarizes the factors that regulate the rate of an enzyme-mediated reaction.

Active site

Enzyme

Enzyme

Site of Sites of Sites of Site of covalent allosteric allosteric covalent activation activation inhibition inhibition

FIGURE 4-13

On a single enzyme, multiple sites can modulate enzyme activity and hence the reaction rate by allosteric and covalent activation or inhibition.

Site of Sites of Sites of Site of covalent allosteric allosteric covalent activation activation inhibition inhibition

FIGURE 4-13

On a single enzyme, multiple sites can modulate enzyme activity and hence the reaction rate by allosteric and covalent activation or inhibition.

Vander et al.: Human I I. Basic Cell Functions I 4. Protein Activity and I I © The McGraw-Hill

Physiology: The Cellular Metabolism Companies, 2001 Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

Cell Body Function

FIGURE 4-14

Factors that affect the rate of enzyme-mediated reactions.

FIGURE 4-14

Factors that affect the rate of enzyme-mediated reactions.

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