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'-monophosphate (XMP) is the committed and rate-limiting reaction in de novo guanine nucleotide biosynthesis. Inosine 5' monophosphate dehydrogenase (IMPDH) is the enzyme that catalyzes the oxidation of IMP to XMP with the concomitant reduction of nicotinamide adenine dinucleotide (from NAD(+) to NADH). Because of its critical role in purine biosynthesis, IMPDH is a drug design target for anticancer, antiinfective, and immunosuppressive chemotherapy. We have 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), which contains the essential catalytic residue Cys 229, is clearly defined in the electron density. We report that a seven residue region of loop 6, including Cys229, is tilted more than 6 Angstrom away from its position in substrate- or substrate analogue-bound structures of IMPDH, suggestive of a conformational change. The location of this loop between beta 6 and alpha 6 links IMPDH to a family of Pier barrel enzymes known to utilize this loop as a functional lid during catalysis, Least-squares minimization. root-mean-square 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-body motion and indicates that IMPDH may be using loop 6 to bind and sequester substrate and to recruit an essential catalytic residue.