The crystal structure of benzophenone synthase from Garcinia mangostana L. pericarps reveals the basis for substrate specificity and catalysis

Benzophenone synthase (BPS) catalyzes the production of 2,4,6-trihydroxy­benzophenone via the condensation of benzoyl-CoA and three units of malonyl-CoA. The biosynthetic pathway proceeds with the formation of the prenylated xanthone α-mangostin from 2,4,6-trihydroxybenzophenone. Structural elucidation was performed to gain a better understanding of the structural basis of the function of Garcinia mangostana L. (mangosteen) BPS (GmBPS). The structure reveals the common core consisting of a five-layer αβαβα fold as found in other type III polyketide synthase enzymes. The three residues Met264, Tyr266 and Gly339 are proposed to have a significant impact on the substrate-binding specificity of the active site. Crystallographic and docking studies indicate why benzoyl-CoA is preferred over 4-coumaroyl-CoA as the substrate for GmBPS. Met264 and Tyr266 in GmBPS are properly oriented for accommodation of the 2,4,6-trihydroxybenzophenone product but not of naringenin. Gly339 offers a minimal steric hindrance to accommodate the extended substrate. Moreover, the structural arrangement of Thr133 provides the elongation activity and consequently facilitates extension of the polyketide chain. In addition to its impact on the substrate selectivity, Ala257 expands the horizontal cavity and might serve to facilitate the initiation/cyclization reaction. The detailed structure of GmBPS explains its catalytic function, facilitating further structure-based engineering to alter its substrate specificity and obtain the desired products.