A Methyltransferase Catalyzing Reactions More Than Methylation

Area of Science:

• Biochemistry
• Enzymology
• Drug Discovery

Background:

• S-Adenosyl-l-methionine (SAM)-dependent methyltransferases (MTs) are crucial enzymes in biological methylation.
• Proteins with an MT-fold can exhibit catalytic functions beyond simple methylation, posing a challenge for bioinformatics.
• Mitomycins are clinically important antitumor antibiotics whose biosynthesis involves complex enzymatic pathways.

Purpose of the Study:

• To investigate the catalytic repertoire of the methyltransferase MitM involved in mitomycin biosynthesis.
• To elucidate the mechanisms by which MitM's MT-fold enables diverse enzymatic reactions.
• To identify novel mitomycin derivatives with potential antitumor activity.

Main Methods:

• Gene inactivation and biochemical characterization of the MitM enzyme.
• Substrate/product cocrystallization studies to determine enzyme-bound structures.
• Site-specific mutagenesis to probe catalytic mechanisms.

Main Results:

• MitM functions as a C9a-O-methyltransferase for the aziridinomitosane (AMS) skeleton, influencing stereoselectivity and N-methylation.
• MitM also catalyzes C9a-O-methoxy elimination, aziridine hydrolysis/opening, and subsequent C1-O- and C2-N-methylations.
• The study identified new antitumor mitomycins previously unrecognized in the producing strain.

Conclusions:

• The MT-fold of MitM is repurposed for extraordinary catalytic versatility, extending beyond methylation.
• This finding highlights the potential for uncharacterized MT-fold proteins to possess diverse catalytic functions.
• The discovery of new mitomycins opens avenues for novel antitumor drug development.