Crystal structure at 2.4 A resolution of Borrelia burgdorferi inosine 5 '-monophosphate dehydrogenase: Evidence of a substrate-induced hinged-lid motion by loop 6

The conversion of inosine 5'-monophosphate (IMP) to xanthosine 5'-monophosp hate (XMP) is the committed and rate-limiting reaction in de novo guanine n ucleotide biosynthesis. Inosine 5' monophosphate dehydrogenase (IMPDH) is t he enzyme that catalyzes the oxidation of IMP to XMP with the concomitant r eduction of nicotinamide adenine dinucleotide (from NAD(+) to NADH). Becaus e of its critical role in purine biosynthesis, IMPDH is a drug design targe t for anticancer, antiinfective, and immunosuppressive chemotherapy. We hav e determined the crystal structure of IMPDH from Borrelia burgdorferi, the bacterial spirochete that causes Lyme disease, with a sulfate ion bound in the IMP phosphate binding site. This is the first structure of IMPDH in the absence of substrate or cofactor where the active-site loop (loop 6), whic h contains the essential catalytic residue Cys 229, is clearly defined in t he electron density. We report that a seven residue region of loop 6, inclu ding Cys229, is tilted more than 6 Angstrom away from its position in subst rate- or substrate analogue-bound structures of IMPDH, suggestive of a conf ormational change. The location of this loop between beta 6 and alpha 6 lin ks IMPDH to a family of Pier barrel enzymes known to utilize this loop as a functional lid during catalysis, Least-squares minimization. root-mean-squ are deviation analysis, and inspection of the molecular surface of the loop 6 region in the substrate-free B. burgdorferi IMPDH and XMP*-bound Chinese hamster IMPDH show that loop 6 follows a similar pattern of hinged rigid-b ody motion and indicates that IMPDH may be using loop 6 to bind and sequest er substrate and to recruit an essential catalytic residue.