biopolymer modification protein modification Proteins are subjected to three classes of protein modifications, i.e. pre-, co- and post-translational modifications. A majority of modifications are made when the protein is already folded; these are genuine post-translational modifications (PTMs). Some modifications are made while the polypeptide is still being synthesized on the ribosome; these are co-translational modifications. A few 'non-standard' amino-acids are incorporated into proteins by modification of some 'standard' amino-acids while they are charged on special tRNAs; these are pre-translational modifications. GO:0006464 pathway UPa:UPA00460 protein metabolism