Mechanistic Enzymology

   How does an enzyme catalyze two reactions?


  Inosine monophosphate dehydrogenase (IMPDH) catalyzes the oxidation of inosine monophosphate (IMP) to xanthosine monophosphate (XMP) with the concomitant reduction of NAD+.  This reaction controls the entry of purines into the guanine nucleotide pool, making IMPDH a billion-dollar target for immunosuppressive, antiviral and anticancer drugs.  The IMPDH reaction involves two different chemical transformations: hydride transfer and hydrolysis of a covalent intermediate, and thus provides unique opportunity to investigate how proteins efficiently change conformations to accommodate multiple substrate-binding events, transition states and product release steps.  This enzyme displays several remarkable mechanistic features, including a large conformational change in mid-catalytic stream, an Arg residue that acts as a general base catalyst and a K+ that acts as a ball and socket joint.  The related enzyme GMP reductase also catalyzes two reactions, deamination and hydride transfer.  Whereas in IMPDH the protein changes conformation to accommodate both transformations, in GMPR the cofactor changes conformations.  We are collaborating with Wei Yang (FSU) to combine detailed mechanistic investigations with QM/MM simulations to understand these intriguing reactions.  This project is funded by NIH/NIGMS GM054403.

Sun, Xin E.; Hansen, Bjarne G. and Hedstrom, Lizbeth. Kinetically Controlled Drug Resistance: How Penicillium brevicompactum Survives Mycophenolic Acid. J. Biol. Chem., 286, 40595-600 (2011). [abstract]

Hansen, Bjarne G., Sun, Xin E., Genee, Hans J.; Kaas, Christian S.; Nielsen, Jakob B.; Mortensen, Uffe H.; Frisvad, Jens C. and Hedstrom, Lizbeth. Adaptive evolution of drug targets in producer and non-producer organisms. Biochemical Journal 441, 218-226 (2011). [abstract]

Patton, Gregory C.; Stenmark, Pål; Gollapalli, Deviprasad R.; Sevastik, Robin; Kursula, Petri; Flodin, Susanne; Schuler, Herwig; Swales, Colin T.*; Eklund, Hans; Himo, Fahmi;

Nordlund, Pär and Hedstrom, Lizbeth. Cofactor mobility determines reaction outcome in the IMPDH/GMPR (β/α)8 barrel enzymes. Nat. Chem. Biol. 7, 950-8 (2011). [abstract]

Riera, Thomas V.; Zheng, Lianqing; Josephine, Helen R.; Min, Donghong; Yang, Wei and Hedstrom, Lizbeth.  Allosteric activation via kinetic control: Potassium accelerates a conformational change in IMP dehydrogenase. Biochemistry 50, 8508-18 (2011). [abstract]

Min, Donghong; Josephine, Helen R.; Li, Hongzhi; Lakner, Clemens; MacPherson, Iain S.; Naylor, Gavin. J.P.; Swofford, David; Hedstrom,, Lizbeth and Yang, Wei.  An Enzymatic Atavist Revealed in Dual Pathways for Water Activation.  PLoS Biology 6, e206 (2008). [abstract]  featured article in PLoS Biology, C&E News and

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