The biosynthesis of ansamycin antibiotics, including rifamycin B, involves the synthesis ofan aromatic precursor, 3-amino-5-hydroxybenzoic acid (AHBA), which serves as starter for the assemblyof the antibiotics' polyketide backbone. The terminal enzyme of AHBA formation, AHBA synthase, is adimeric, pyridoxal 5'-phosphate (PLP) dependent enzyme with pronounced sequence homology to a numberof PLP enzymes involved in the biosynthesis of antibiotic sugar moieties. The structure of AHBA synthasefrom
Amycolatopsis mediterranei has been determined to 2.0 Å resolution, with bound cofactor, PLP,and in a complex with PLP and an inhibitor (gabaculine). The overall fold of AHBA synthase is similarto that of the aspartate aminotransferase family of PLP-dependent enzymes, with a large domain containinga seven-stranded
-sheet surrounded by
-helices and a smaller domain consisting of a four-strandedantiparallel
-sheet and four
-helices. The uninhibited form of the enzyme shows the cofactor covalentlylinked to Lys188 in an internal aldimine linkage. On binding the inhibitor, gabaculine, the internal aldiminelinkage is broken, and a covalent bond is observed between the cofactor and inhibitor. The active site iscomposed of residues from two subunits of AHBA synthase, indicating that AHBA synthase is active asa dimer.