Amino Acids and Proteins

• A protein's unique characteristics, including its three-dimensional folded structure, are dictated by its linear sequence of amino acids, termed its primary structure.
• The primary structures of all of the diverse human proteins are synthesized from 20 amino acids arranged in a linear sequence determined by the genetic code.
• Each three-base (nucleotide) sequence within the coding region of a gene (the genetic code) specifies which amino acid should be present in a protein. The genetic code is discussed further in Chapter 12.
• All amino acids contain a central α-carbon, joined to a carboxylic acid group, an amino group, a hydrogen, and a side chain, which varies between the 20 different amino acids.
• At physiological pH the amino acids are zwitterions; the amino group is positively charged, and the carboxylate is negatively charged.
• In proteins, amino acids are joined into linear polymers called polypeptide chains via peptide bonds, which are formed between the carboxylic acid of one amino acid and the amino group of the next amino acid.
• Amino acid side chains can be classified either by polarity (charged, nonpolar hydrophobic, or uncharged polar) or structural features (aliphatic, cyclic, or aromatic).
• Depending on their side chain characteristics, certain amino acids cluster together to exclude water (hydrophobic effect), whereas others participate in hydrogen bonding. Cysteine can form disulfide bonds, whereas charged amino acids can form ionic bonds.
• Amino acids in proteins can be modified by phosphorylation, carboxylation, or other reactions after the protein is synthesized (posttranslational modifications).
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• Alterations in the genetic code may lead to mutations in the protein's primary structure, which can affect the protein's function.
• Proteins with the same function but different primary structure (isoforms and isozymes) can exist in different tissues or during different phases of development.