Modyfikacje białek w RER

N-linked glycosylation and disulfide bond formation are two important protein modifications that occur in the ER.

During glycosylation, the oligosaccharyltransferase complex adds branched oligosaccharide molecules to some select asparagine residues of an incoming polypeptide chain.

In contrast, disulfide bond formation occurs between two closely spaced cysteine residues on the same or different polypeptide chains.

It involves two key players — protein disulfide isomerase, or PDI, and ER oxidoreductase I, or Ero1.

The PDI molecule resembles a horseshoe with disulfide bonds in its active site.

Oxidized PDI has an open conformation and binds unfolded polypeptides. First, the reduced cysteine residue of the polypeptide forms a disulfide link with the enzyme's active site.

This intermediate then interacts with another closely spaced cysteine on the polypeptide chain, resulting in a disulfide bond between the two residues.

Subsequently, the PDI changes to a closed conformation, releasing the polypeptide.

Ero1 converts the reduced PDI back to its oxidized state, preparing it for another round of reaction.