Amino Acids Share Common Structural Features
All 20 of the common amino acids are -amino acids.
They have a carboxyl group and an amino group bonded
to the same carbon atom (the carbon) (Fig. 3–2). They
differ from each other in their side chains, or R groups,
which vary in structure, size, and electric charge, and
which influence the solubility of the amino acids in water. In addition to these 20 amino acids there are many
less common ones. Some are residues modified after a
protein has been synthesized; others are amino acids
present in living organisms but not as constituents of
proteins. The common amino acids of proteins have
been assigned three-letter abbreviations and one-letter
symbols (Table 3–1), which are used as shorthand to indicate the composition and sequence of amino acids
polymerized in proteins.
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