Final answer:
The most common cross linkages in proteins are disulfide linkages, which are formed by the oxidation of adjacent cysteine residues, resulting in a stabilizing effect on the protein's tertiary structure, with insulin being a notable example.
Step-by-step explanation:
The most common cross linkages in protein forming before secretion are disulfide linkages, created by the oxidation of two cysteine side chains in the endoplasmic reticulum (ER). When the highly reactive sulfhydryl (-SH) groups of two cysteine residues, which are adjacent in the folded protein, are brought together, they can form a covalent disulfide bond (-S-S-). This process is catalyzed by an enzyme in the ER. The newly formed bond is the result of an oxidation reaction, converting two cysteine (Cys) residues into a cystine (Cys-Cys) dimer. Such disulfide bonds are critical for the stabilization of the protein's tertiary structure and are found in numerous proteins, including insulin.