| Almost all biological functions are supported by chemical reactions catalyzed by biological catalysts, enzymes. Efficient metabolism is controlled by orderly, sequential, and branching metabolic pathways. Enzymes accelerate chemical reactions under physiologic conditions, 37°C and neutral pH. However, an enzyme cannot alter the equilibrium of a reaction, but can only accelerate the reaction rate, by decreasing the activation energy of the reaction (Fig. 5.1). Regulation of enzymatic activities allows metabolism to adapt to rapidly changing conditions. Nearly all enzymes are proteins, although some ribonucleic acid molecules, termed ribozymes, also have catalytic activity (Chapter 31). Based on analysis of the human genome, it is estimated that about a quarter of human genes encode for enzymes that catalyze metabolic reactions.
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