Eads J C, Beeby M, Scapin G, Yu T W, Floss H G
School of Biochemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K.
Biochemistry. 1999 Aug 3;38(31):9840-9. doi: 10.1021/bi990018q.
The biosynthesis of ansamycin antibiotics, including rifamycin B, involves the synthesis of an aromatic precursor, 3-amino-5-hydroxybenzoic acid (AHBA), which serves as starter for the assembly of the antibiotics' polyketide backbone. The terminal enzyme of AHBA formation, AHBA synthase, is a dimeric, pyridoxal 5'-phosphate (PLP) dependent enzyme with pronounced sequence homology to a number of PLP enzymes involved in the biosynthesis of antibiotic sugar moieties. The structure of AHBA synthase from Amycolatopsis mediterranei has been determined to 2.0 A resolution, with bound cofactor, PLP, and in a complex with PLP and an inhibitor (gabaculine). The overall fold of AHBA synthase is similar to that of the aspartate aminotransferase family of PLP-dependent enzymes, with a large domain containing a seven-stranded beta-sheet surrounded by alpha-helices and a smaller domain consisting of a four-stranded antiparallel beta-sheet and four alpha-helices. The uninhibited form of the enzyme shows the cofactor covalently linked to Lys188 in an internal aldimine linkage. On binding the inhibitor, gabaculine, the internal aldimine linkage is broken, and a covalent bond is observed between the cofactor and inhibitor. The active site is composed of residues from two subunits of AHBA synthase, indicating that AHBA synthase is active as a dimer.
安莎霉素类抗生素（包括利福霉素B）的生物合成涉及一种芳香族前体3-氨基-5-羟基苯甲酸（AHBA）的合成，该前体作为抗生素聚酮骨架组装的起始物。AHBA合成的末端酶，即AHBA合酶，是一种二聚体、依赖于磷酸吡哆醛（PLP）的酶，与参与抗生素糖部分生物合成的许多PLP酶具有明显的序列同源性。已确定来自地中海拟无枝酸菌的AHBA合酶的结构分辨率为2.0埃，带有结合的辅因子PLP，并处于与PLP和一种抑制剂（γ-氨基丁酸）的复合物中。AHBA合酶的整体折叠与依赖于PLP的天冬氨酸转氨酶家族相似，有一个大结构域，包含一个由α螺旋包围的七股β折叠，还有一个较小的结构域，由一个四股反平行β折叠和四个α螺旋组成。该酶的未受抑制形式显示辅因子通过内部醛亚胺键与Lys188共价连接。在结合抑制剂γ-氨基丁酸后，内部醛亚胺键断裂，观察到辅因子与抑制剂之间形成共价键。活性位点由AHBA合酶两个亚基的残基组成，表明AHBA合酶作为二聚体具有活性。
