Structural requirements for catalysis and dimerization of human methionine adenosyltransferase I/III

Area of Science:

• Biochemistry
• Enzymology
• Structural Biology

Background:

• Methionine adenosyltransferase (MAT) is crucial for cellular methylation.
• Human hepatic MAT (hMAT) exists in different isoforms, with hMAT III being a key player.
• Understanding hMAT's structure-function relationship is vital for its biological roles.

Purpose of the Study:

• To elucidate the structural determinants governing catalysis and dimerization of human hepatic methionine adenosyltransferase (hMAT).
• To identify specific amino acid residues critical for hMAT's enzymatic function and quaternary structure.

Main Methods:

• Site-directed mutagenesis was employed to alter specific amino acid residues in hMAT.
• Homology modeling was used, inferring hMAT III structure from the Escherichia coli MAT crystal structure.
• Catalytic activity and dimerization capabilities of mutated hMAT variants were assessed.

Main Results:

• Mutations in the active site or ATP binding region, except for one, significantly impaired hMAT catalytic activity.
• Dimerization was disrupted by single mutations in only three specific residues located on one monomer.
• The homology model indicated these residues stabilize monomers and form inter-subunit bridges via metal-phosphate ions.
• Dimerization was confirmed to be dependent on the presence of phosphate ions.

Conclusions:

• Specific amino acid residues are critical for both hMAT catalysis and dimerization.
• Phosphate ions play an indispensable role in the dimerization process of hMAT.
• The active site of hMAT is located at the interface between subunits, involving conserved residues.