(A) The hydroxyl groups of Ser, Thr and Tyr can be phosphorylated and glycosylated in some proteins.

(B) Disulfide bonds between cysteine residues can be formed in proteins to be secreted.

(C) Methionine side chains may be oxidized in air without enzymatic catalysis.

(D) Long-chain fatty acyl groups may be attached to proteins as membrane anchors.

(E) Carbohydrate side chains of glycoproteins are attached during or after synthesis of the polypeptide.