Structural Studies on a Glucosamine/Glucosaminide N-Acetyltransferase.

Area of Science:

• Biochemistry
• Enzymology
• Structural Biology

Background:

• Glucosamine/glucosaminide N-acetyltransferase (GlmA) catalyzes acetylation of glucosamine using acetyl CoA.
• The enzyme is implicated in bacterial cell wall rescue and acts on glucosamine and its oligosaccharides.

Purpose of the Study:

• To elucidate the structure and kinetics of GlmA from Clostridium acetobutylicum.
• To investigate the roles of specific active site residues (Asp 287 and Tyr 297) in catalysis.

Main Methods:

• X-ray crystallography to determine eight structures of GlmA at high resolution (≤2.0 Å).
• Site-directed mutagenesis to create D287N and Y297F variants.
• Enzyme kinetics assays to analyze reaction mechanisms and substrate interactions.

Main Results:

• GlmA adopts a tandem GNAT superfamily fold with two similar domains; the C-terminal domain is catalytically active.
• Asp 287 and Tyr 297 are crucial for substrate binding but not as acid/base catalysts.
• Kinetic analysis revealed an ordered reaction mechanism: acetyl CoA binds first, followed by glucosamine, with N-acetylglucosamine released before CoA.

Conclusions:

• The study provides novel structural and kinetic insights into GlmA and the tandem GNAT superfamily.
• Specific residues play distinct roles in substrate binding versus catalytic activity.
• The ordered kinetic mechanism of GlmA is now clarified.