O

Peptide bond

HO R

Dipeptide molecule +

Water

Figure

When two amino acid molecules unite by dehydration synthesis, a peptide bond forms between a carbon atom and a nitrogen atom. In the reverse reaction, a dipeptide is hydrolyzed to two amino acids.

Hydrolysis breaks down carbohydrates into monosaccharides; fats into glycerol and fatty acids (see fig. 4.2; read from right to left); proteins into amino acids (see fig. 4.3; read from right to left); and nucleic acids into nucleotides.

Hydrolysis does not occur automatically, even though in the body, water molecules are readily available to provide the necessary —H and —OH. For example, water-soluble substances such as the disaccharide sucrose (table sugar) dissolve in a glass of water but do not undergo hydrolysis. Like dehydration synthesis, hydroly sis requires the help of specific enzymes, which are discussed in the next section.

The reactions of metabolism are often reversible. However, the enzyme that speeds, or catalyzes, an anabolic reaction is often different from that which catalyzes the corresponding catabolic reaction.

Both catabolism and anabolism must be carefully controlled so that the breakdown or energy-releasing reactions occur at rates that are adjusted to the requirements of the building up or energy-utilizing reactions.

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