Structure, function and physiological role of glycine N-methyltransferase

Area of Science:

• Biochemistry
• Enzymology
• Structural Biology

Background:

• Glycine N-methyltransferase (GNMT) is an abundant liver enzyme catalyzing methyl group transfer from S-adenosylmethionine (AdoMet) to glycine.
• Unlike typical methyltransferases, GNMT forms a unique tetrameric structure with a central pore.
• GNMT plays a role in methionine metabolism and is found in various mammalian livers.

Purpose of the Study:

• To investigate the structural and functional characteristics of glycine N-methyltransferase.
• To elucidate the role of N-terminal acetylation in GNMT's cooperative kinetics.
• To understand the enzyme's interaction with 5-methyltetrahydropteroylpentaglutamate and its implications.

Main Methods:

• X-ray crystallography of recombinant rat GNMT.
• Amino acid sequence analysis of GNMT from different species.
• Enzyme kinetics studies at various pH values.
• Inhibition studies using 5-methyltetrahydropteroylpentaglutamate.

Main Results:

• Crystallography revealed a flat, square tetrameric structure of GNMT with a central hole.
• GNMT from different mammalian livers shows similar amino acid sequences; N-terminal acetylation was confirmed in rat and rabbit enzymes.
• Enzyme kinetics demonstrated sigmoidal behavior with AdoMet and hyperbolic with glycine.
• Recombinant GNMT lacking N-terminal acetylation showed no AdoMet cooperativity at neutral pH.
• 5-methyltetrahydropteroylpentaglutamate acts as a tight-binding inhibitor.

Conclusions:

• N-terminal acetylation is crucial for the cooperative kinetics of GNMT with AdoMet.
• The unique tetrameric structure influences GNMT's functional properties.
• GNMT's interaction with pteroylglutamate derivatives suggests a role in regulating folate and methionine metabolism.