Proteins constitute more than 50% of the dry weight of cells. Of all the macromolecules, they are the most versatile in what they do in cells. A typical bacterial cell contains 600 to 800 different kinds of proteins at any one time.
In the microbial world, proteins are responsible for:
■ Catalyzing all reactions of the cell required for life.
■ The structure and shape of certain structures such as ribosomes, the protein-building machinery in all cells.
■ Taking nutrients into the cell. ■ binding proteins, p. 56
■ Turning genes on and off. ■ gene regulation, p. 183
■ Certain properties of various membranes in the cell.
■ inner membrane, p. 59, ■ outer membrane, p. 59
Proteins are composed of numerous combinations of 20 major amino acids. The properties of a protein depend mainly on its shape, which in turn depends on the arrangement of the amino acids that make up the protein.
All amino acids have at one end a carbon atom to which a carboxyl group and an amino group are bonded (figure 2.13). This carbon atom also is bonded to a side chain or backbone (labeled R), which gives each amino acid its characteristic properties. In solution at pH 7, both the amino and carboxyl groups are ionized such that the —NH2 group is —NH3+ and the —COOH group is —COO:, with the overall charge being zero. The amino acids are subdivided into several different groups based on similarities in their side chains (figure 2.14). One important property of the side chains is whether they are polar or non-polar; this feature determines the solubility
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