Crystal structure of L-arginine : inosamine-phosphate amidinotransferase StrB1 from Streptomyces griseus: An enzyme involved in streptomycin biosynthesis

Inosamine-phosphate amidinotransferases catalyze two nonconsecutive transam idination reactions in the biosynthesis of the streptomycin family of antib iotics. L-Arginine: inosamine-phosphate amidinotransferase StrB 1 from Stre ptomyces griseus (StrB1) was cloned as an N-terminal hexa-histidine fusion protein, purified by affinity chromatography, and crystallized, and its cry stal structure was solved by Patterson search methods at 3.1 Angstrom resol ution. The structure is composed of five beta beta alpha beta-modules which are arranged circularly into a pseudo-5-fold symmetric particle. The three -dimensional structure is closely related to the structure of human L-argin ine:glycine amidinotransferase (AT), but five loops (the 40-, 170-, 220-, 2 50-, and 270-loop) are organized very differently, The major changes are fo und in loops around the active site which open the narrow active site chann el of AT to form an open and solvent-exposed cavity, In particular, module II of StrB1 is AT-like but lacks a 10-residue alpha-helix in the 170-loop. The concomitant reorganization of neighboring surface loops that surround t he active site, i.e., the 40-loop and the 270-loop, results in an arrangeme nt of loops which allows an unrestricted access of substrates to the cavity . However, the residues which are involved in substrate binding and catalys is are conserved in AT and StrB1 and are at equivalent topological position s, suggesting a similar reaction mechanism among amidinotransferases. The b inding site for L-arginine had been deduced from its complex with AT. Molec ular modeling revealed a possible binding mode for the second substrate scy llo-inosamine 4-phosphate.