Enzymes and the Chemical Work of Cells
■ The function of nearly all proteins depends on their ability to bind other molecules (ligands). Ligand-binding sites
▲ FIGURE 3-21 Structure and function of pyruvate dehydrogenase, a large multimeric enzyme complex that converts pyruvate into acetyl CoA. (a) The complex consists of 24 copies of pyruvate decarboxylase (E1), 24 copies of lipoamide transacetylase (E2), and 12 copies of dihydrolipoyl dehydrogenase (E3). The E1 and E3 subunits are bound to the outside of the core formed by the E2 subunits. (b) The reactions catalyzed by the complex include several enzyme-bound intermediates (not shown). The tight structural integration of the three enzymes increases the rate of the overall reaction and minimizes possible side reactions.
on proteins and the corresponding ligands are chemically and topologically complementary.
■ The affinity of a protein for a particular ligand refers to the strength of binding; its specificity refers to the preferential binding of one or a few closely related ligands.
■ Enzymes are catalytic proteins that accelerate the rate of cellular reactions by lowering the activation energy and stabilizing transition-state intermediates (see Figure 3-16).
■ An enzyme active site comprises two functional parts: a substrate-binding region and a catalytic region. The amino acids composing the active site are not necessarily adjacent in the amino acid sequence but are brought into proximity in the native conformation.
■ From plots of reaction rate versus substrate concentration, two characteristic parameters of an enzyme can be determined: the Michaelis constant Km, a measure of the enzyme's affinity for substrate, and the maximal velocity Vmax, a measure of its catalytic power (see Figure 3-19).
■ Enzymes in a common pathway are located within specific cell compartments and may be further associated as domains of a monomeric protein, subunits of a multimeric protein, or components of a protein complex assembled on a common scaffold (see Figure 3-20).
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