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.