Final answer:
The covalent modification that occurs mainly in the ER is the formation of disulfide bonds.
Step-by-step explanation:
The covalent modification that occurs mainly in the ER is the formation of disulfide bonds.
The amino acid cysteine has a sulfhydryl (SH) group as a side chain. Two sulfhydryl groups can react in the presence of oxygen to form a disulfide (S-S) bond. In the ER, proteins are folded and undergo oxidation, allowing the formation of disulfide bonds. These bonds contribute to the stability and proper folding of proteins within the ER.