4″-Methyl-S-Adenosyl-l-Methionines Are Substrates for 4-Methylazetidinecarboxylic Acid Synthases

Area of Science:

• Biochemistry
• Molecular Biology
• Natural Product Biosynthesis

Background:

• 4-Methylazetidinecarboxylic acids (MeAZEs) are uncommon nonproteinogenic amino acids found in peptides like bonnevillamide D and vioprolide A.
• The biosynthesis of MeAZEs involves C-methylation and azetidine ring formation from S-adenosyl-l-methionine (SAM), but the sequence of these steps was unclear.

Purpose of the Study:

• To elucidate the order of C-methylation and azetidine ring formation in MeAZE biosynthesis.
• To identify the specific enzymes responsible for these transformations.

Main Methods:

• Enzyme assays using recombinant proteins Orf5 and VioH.
• Structural modeling to assess the role of the C-methyl group in cyclization.
• Analysis of the bonnevillamide biosynthetic gene cluster (BGC).

Main Results:

• Orf5 converts (4″R)-4″-methyl-SAM to cis-MeAZE.
• VioH transforms (4″S)-4″-methyl-SAM to trans-MeAZE.
• Structural modeling confirmed the C-methyl group is crucial for efficient cyclization.

Conclusions:

• C-methylation of SAM is a prerequisite for azetidine ring formation in MeAZE biosynthesis.
• This study clarifies the biosynthetic logic of MeAZEs, identifying key enzymes and reaction steps.