Structural modification (amino acids)

Amino acids can be modified in several ways:

• phosphorylation at serine and threonine residues; one means of secondary messenger and hormonal activation
• addition of carbohydrate groups to give proteoglycans:
  • usually bound to nitrogen or oxygen atom
  • may protect against proteolysis
  • role in membrane recognition
• addition of lipid to give lipoprotein
• formation of covalent bonds e.g.:
  • disulphide bridges between cysteine residues
  • peptide bonds between -C00H and -NH2 groups; the basis of polypeptide formation
• formation of hydrogen bonds between hydrogen and oxygen atoms in adjacent amino acid residues
• formation of van der Waals bonds between adjacent hydrophobic portions of 2 amino acids

These sorts of interaction determine the arrangement of higher levels of protein structure; hence, they have a vital role to play in protein function.