Citrullination Inactivates Nicotinamide- N-methyltransferase.

Area of Science:

• Biochemistry
• Enzymology
• Post-translational modifications

Background:

• Nicotinamide-N-methyltransferase (NNMT) is crucial for nicotinamide metabolism.
• NNMT is linked to chronic diseases like cancer and Parkinson's.
• Post-translational modifications of NNMT, such as phosphorylation, are known but functionally uncharacterized.

Purpose of the Study:

• To elucidate the mechanism by which citrullination inactivates NNMT.
• To identify specific sites of citrullination on NNMT.
• To understand the structural consequences of NNMT citrullination.

Main Methods:

• Tandem mass spectrometry to identify citrination sites.
• Site-directed mutagenesis to alter specific arginine residues.
• Enzyme kinetics assays to measure methyltransferase activity.
• Circular dichroism spectroscopy to assess protein structure.

Main Results:

• Three sites of NNMT citrullination were identified.
• Citrullination of Arginine 132 (R132) was found to be critical for inactivation.
• Mutagenesis and structural analyses confirmed that R132 citrullination perturbs NNMT structure, leading to loss of activity.

Conclusions:

• Citrullination is a novel mechanism for NNMT inactivation.
• R132 is a key residue targeted by Protein Arginine Deiminases (PADs).
• Structural changes induced by citrullination disrupt NNMT function, offering potential therapeutic targets.