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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.

Phase II Reactions: Sulfation and Conjugation with α-Amino Acids

Phase II Reactions: Sulfation and Conjugation with α-Amino Acids

Sulfation and α-amino acid conjugation are two critical biotransformation reactions in drug metabolism. Sulfation, a phase II biotransformation reaction, involves adding a polar sulfate group to a drug, enhancing its water solubility and promoting excretion. This process can either co-occur with or occur independently of glucuronidation. Nonmicrosomal sulfotransferase enzymes catalyze the process. The reaction involves 3'-phosphoadenosine-5'-phosphosulfate or PAPS coenzyme activation, sulfur...

Covalently Linked Protein Regulators

Covalently Linked Protein Regulators

Proteins can undergo many types of post-translational modifications, often in response to changes in their environment. These modifications play an important role in the function and stability of these proteins. Covalently linked molecules include functional groups, such as methyl, acetyl, and phosphate groups, and also small proteins, such as ubiquitin. There are around 200 different types of covalent regulators that have been identified.
These groups modify specific amino acids in a protein.

Carboxylic Acids to Acid Chlorides

Carboxylic Acids to Acid Chlorides

Carboxylic acids react with SOCl2 or PCl5 to form acid chlorides. Amongst the carboxylic acid derivatives, acid chlorides are the most reactive and synthetically important derivatives. They are useful reagents for Friedel–Crafts acylation of some aromatic compounds.

Phase II Reactions: Glutathione Conjugation and Mercapturic Acid Formation

Phase II Reactions: Glutathione Conjugation and Mercapturic Acid Formation

Glutathione, a tripeptide made up of glutamate, cysteine, and glycine, is a critical player in the detoxification of drugs and xenobiotics via a process known as glutathione conjugation or mercapturic acid formation. This phase II biotransformation reaction involves the covalent binding of glutathione to a drug or its metabolite, enhancing the compound's water solubility and enabling its excretion.
Several distinctive characteristics distinguish glutathione conjugation from other phase II...

Phase II Reactions: Acetylation Reactions

Phase II Reactions: Acetylation Reactions

Acetylation, a phase II biotransformation reaction, introduces an acetyl group to drugs or their metabolites. Acetyltransferase enzymes facilitate this reaction, which resembles α-amino acid conjugation due to the addition of a functional group to the drug molecule.
The substrates for acetylation are typically drugs or their metabolites with an amino, sulfonamide, or hydrazine functional group. Acetylation can occur at several points in the drug molecule, including primary, secondary, and...

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Related Experiment Video

Updated: Jun 26, 2026

Synthesis and Structure Determination of µ-Conotoxin PIIIA Isomers with Different Disulfide Connectivities

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Published on: October 2, 2018

Protein C-terminal modification through thioacid/azide amidation.

Xiaohong Zhang¹, Fupeng Li, Xiao-Wei Lu

¹Division of Chemical Biology and Biotechnology, School of Biological Sciences, Nanyang Technological University, 60 Nanyang Drive, Singapore 637551.

Bioconjugate Chemistry

|January 15, 2009

Summary

Researchers developed a novel protein modification method using thioacid/azide amidation for C-terminus-directed bioconjugation. This technique allows for selective attachment of functional tags to recombinant proteins, like PEGylation or biotinylation of ubiquitin.

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Last Updated: Jun 26, 2026

Synthesis and Structure Determination of µ-Conotoxin PIIIA Isomers with Different Disulfide Connectivities

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Published on: October 2, 2018

Constructing Thioether/Vinyl Sulfide-tethered Helical Peptides Via Photo-induced Thiol-ene/yne Hydrothiolation

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Targeting Cysteine Thiols for in Vitro Site-specific Glycosylation of Recombinant Proteins

Targeting Cysteine Thiols for in Vitro Site-specific Glycosylation of Recombinant Proteins

Published on: October 4, 2017

Area of Science:

Biochemistry
Chemical Biology
Protein Engineering

Background:

Protein bioconjugation relies on selective chemistries orthogonal to protein functionalities.
Existing methods face challenges with diverse functional groups present in proteins.

Purpose of the Study:

To introduce a new method for C-terminus-directed modification of recombinant proteins.
To demonstrate the utility of thioacid/azide amidation for site-selective protein modification.

Main Methods:

Utilized hydrothiolytic cleavage of a thioester intermediate in protein splicing to generate a C-terminal thioacid group.
Employed chemoselective amidation of the C-terminal thioacid with an electron-poor organic azide carrying a biofunctional tag.
Used small ubiquitin as a model system to validate the bioconjugation approach.

Main Results:

Successfully achieved C-terminal PEGylation and biotinylation of ubiquitin.
Demonstrated the selective modification of recombinant proteins via thioacid/azide amidation.
Confirmed the practical utility and mechanistic novelty of the developed bioconjugation method.

Conclusions:

Thioacid/azide amidation is a novel and practical method for site-selective protein modification.
This approach enables efficient C-terminus-directed bioconjugation of recombinant proteins.
The method offers a valuable tool for protein engineering and bioconjugate preparation.