The protein ubiquitination pathway. In eukaryotes, three enzymes (denoted by E1, E2, and E3) carry out the polyubiquitination of proteins in a process that involves ATP and two enzyme-ubiquitin intermediates. The free carboxyl group of the ubiquitin C-terminal Gly residue is linked through an amide bond to the ɛ-amino group of a Lys residue of the target protein. Additional cycles produce polyubiquitin, a covalent polymer that targets the protein for destruction.