<Li> biotinylation: covalent attachment of a biotin moiety using a biotinylation reagent, typically for the purpose of labeling a protein . </Li> <Li> carbamylation: the addition of Isocyanic acid to a protein's N - terminus or the side - chain of Lys or Cys residues, typically resulting from exposure to urea solutions . </Li> <Li> oxidation: addition of one or more Oxygen atoms to a susceptible side - chain, principally of Met, Trp, His or Cys residues . Formation of disulfide bonds between Cys residues . </Li> <Li> pegylation: covalent attachment of polyethylene glycol (PEG) using a pegylation reagent, typically to the N - terminus or the side - chains of Lys residues . Pegylation is used to improve the efficacy of protein pharmaceuticals . </Li>

Describe at least three types of post translational modifications