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Protein Modifications in the RER

Protein Modifications in the RER

Modification of secretory and transmembrane proteins entering the rough ER begins in the ER lumen. These modifications aid in protein folding and stabilize the acquired tertiary structure. Protein modifications in the rough ER co-occur at different stages of protein folding.
Broadly, these modifications can be categorized into four main categories — glycosylation, formation of disulfide bonds, assembly of protein subunits, and specific proteolytic cleavages like removal of signal sequences.

Sulfur Assimilation

Sulfur Assimilation

Sulfur is an essential element in biological systems, contributing to synthesizing key biomolecules, including amino acids such as cysteine and methionine, and cofactors such as coenzyme A and biotin. Microorganisms primarily assimilate sulfur as sulfate (SO₄²⁻) from the environment, which must undergo a series of biochemical transformations before it can be incorporated into cellular components. As sulfate is highly oxidized, it must undergo assimilatory sulfate reduction to become...

Preparation and Reactions of Sulfides

Preparation and Reactions of Sulfides

Sulfides are the sulfur analog of ethers, just as thiols are the sulfur analog of alcohol. Like ethers, sulfides also consist of two hydrocarbon groups bonded to the central sulfur atom. Depending upon the type of groups present, sulfides can be symmetrical or asymmetrical. Symmetrical sulfides can be prepared via an SN2 reaction between 2 equivalents of an alkyl halide and one equivalent of sodium sulfide.

Export of Misfolded Proteins out of the ER

Export of Misfolded Proteins out of the ER

After folding, the ER assesses the quality of secretory and membrane proteins. The correctly folded proteins are cleared by the calnexin cycle for transport to their final destination, while misfolded proteins are held back in the ER lumen. The ER chaperones attempt to unfold and refold the misfolded proteins but sometimes fail to achieve the correct native conformation. Such terminally misfolded proteins are then exported to the cytosol by ER-associated degradation or ERAD pathway for...

SN2 Reaction: Stereochemistry

S_N2 Reaction: Stereochemistry

In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.

Preparation and Reactions of Thiols

Preparation and Reactions of Thiols

Thiols are prepared using the hydrosulfide anion as a nucleophile in a nucleophilic substitution reaction with alkyl halides. For instance, bromobutane reacts with sodium hydrosulfide to give butanethiol.

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Related Experiment Video

Updated: Jul 11, 2026

Combining Non-reducing SDS-PAGE Analysis and Chemical Crosslinking to Detect Multimeric Complexes Stabilized by Disulfide Linkages in Mammalian Cells in Culture

Combining Non-reducing SDS-PAGE Analysis and Chemical Crosslinking to Detect Multimeric Complexes Stabilized by Disulfide Linkages in Mammalian Cells in Culture

Published on: May 2, 2019

Substrate recognition by the protein disulfide isomerases.

Feras Hatahet¹, Lloyd W Ruddock

¹Biocenter Oulu and Department of Biochemistry, University of Oulu, Finland.

The FEBS Journal

|September 26, 2007

Summary

Protein disulfide isomerase (PDI) enzymes assist protein folding in the endoplasmic reticulum by catalyzing disulfide bond formation. This review explores PDI interactions with substrates and their roles in protein folding.

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Combining Non-reducing SDS-PAGE Analysis and Chemical Crosslinking to Detect Multimeric Complexes Stabilized by Disulfide Linkages in Mammalian Cells in Culture

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Area of Science:

Biochemistry
Molecular Biology
Cell Biology

Background:

Protein folding in the endoplasmic reticulum relies on disulfide bonds for stability and regulation.
Disulfide bond formation is a critical, often rate-limiting, step in protein folding.
Molecular chaperones and catalysts, like the protein disulfide isomerase (PDI) family, facilitate correct protein conformation.

Purpose of the Study:

To review the interactions between human PDI family members and their substrates.
To highlight recent research on identifying PDI substrate-binding sites.
To discuss the determination of natural PDI substrates in vivo.

Main Methods:

Literature review focusing on PDI family interactions.
Analysis of recent studies on substrate-binding site characterization.
Examination of research identifying in vivo substrates.

Main Results:

The human endoplasmic reticulum contains 17 reported PDI family members.
Functional differentiation among PDI family members is not fully understood.
Mechanisms of action for PDI enzymes are still under investigation.

Conclusions:

Understanding PDI-substrate interactions is crucial for elucidating protein folding pathways.
Further research is needed to fully characterize the roles of individual PDI family members.
Identifying natural substrates will advance our knowledge of PDI function in vivo.