THE X-RAY STRUCTURE OF A COBALAMIN BIOSYNTHETIC ENZYME, COBALT-PRECORRIN-4 METHYLTRANSFERASE

Biosynthesis of the corrin ring of vitamin B-12 requires the action of six S-adenosyl-L-methionine (AdoMet) dependent transmethylases, close ly related in sequence. The first X-ray structure of one of these, cob alt-precorrin-4 transmethylase, CbiF, from Bacillus megaterium has bee n determined to a resolution of 2.4 Angstrom. CbiF contains two alpha/ beta domains forming a trough in which S-adenosyl-L-homocysteine (AdoH cy) binds. The location of AdoHcy and a number of conserved residues, helps define the precorrin binding site. A second crystal form determi ned at 3.1 Angstrom resolution highlights the flexibility of two loops around this site. CbiF employs a unique mode of AdoHcy binding and re presents a new class of transmethylase.