My laboratory uses approaches derived from both chemistry and biology. Projects include problems in inhibitor design, enzyme catalysis, protein degradation and the mechanism of visual disease.  Techniques vary with the particular project, and can entail molecular biology, organic synthesis, protein crystallography and NMR spectroscopy as well as protein purification, enzyme kinetics and mutagenesis. Our ongoing projects are outlined below. For more information, please see the lab web site.

Dynamic structural determinants of drug selectivity and reaction specificity.  Understanding of how structure determines function is a central challenge in biochemistry.  The IMPDH/GMPR family of (b/a)8 proteins provide a striking example of how subtle differences in protein sequence, and hence in structure, can profoundly change reaction outcomes.  These enzymes share a common set of catalytic residues and bind the same ligands with similar affinities.  The reactions utilize the same covalent intermediate, yet with markedly different outcomes.  Both IMPDH and GMPR catalyze two chemical transformations at a single active site.  IMPDH performs a hydride transfer reaction followed by a hydrolysis reaction, and a protein conformational changes rearranges the active site to accommodate both reactions.  This conformational change determines drug sensitivity.  GMPR performs a deamination reaction followed by a hydride transfer reaction.  In this case, the cofactor has a different position in each reaction.  We are now trying to understand what structural features determine this very different dynamic behavior.  Our long-range goal is to develop computational models that accurately recapitulate and quantitatively predict the catalytic properties of enzymes in the IMPDH/GMPR family.  This work will advance the field of computational chemistry and provide important insights into how enzymes work.  This work is a collaboration with Wei Yang of Florida State University.

Targeting a prokaryotic enzyme in a eukaryotic pathogen. The protozoan parasite Cryptosporidium parvum is an emerging opportunistic pathogen and potential bio-warfare agent. The C. parvum oocyte is resistant to the usual methods of water treatment, which has caused spectacular outbreaks such as the infection of 40% of the inhabitants of Milwaukee in 1993. C. parvum is resistant to the usual antiparasitic drugs and currently used chemotherapy is ineffective.  In collaboration with Boris Striepen at UGA, we have been engaged in a medicinal chemistry program targeting C. parvum IMPDH.  Curiously, the parasite obtained its IMPDH gene via horizontal transfer from a bacteria, so the parasite enzyme is very different from its host.  We have a collection of more than fifty low nanomolar inhibitors of the parasite enzyme, some of which show promising antiparasitic activity.  These compounds also inhibit IMPDHs from pathogenic bacteria such as Streptococcus pyogenes, Helicobacter pylori and Francisella tularensis.  We are now investigating the potential of these compounds as broad spectrum antibiotics.  This work is a collaboration with Joanna Goldberg and colleagues at UVA.

IMPeD: inhibitor mediated protein degradation.  A formidable toolkit exists for manipulating protein expression at the transcriptional level, but the methods for post-translational modulation of proteins are few.  A small molecule that induces degradation of endogenous proteins would clearly be a tremendously useful tool for probing protein function and an exciting new approach for chemotherapy.  We serendipitously discovered a small molecule tag that induces the degradation of target proteins.  We are currently investigating the mechanism of degradation and applying this method to clinically important targets.

Pathophysiological mechanisms of retinal disease. Many inherited retinal diseases are caused by mutations in proteins of the visual cycle, which can easily explain why disease is photoreceptor-specific. However, retinal disease can also result from mutations in widely expressed proteins. The photoreceptor-specific effects of these mutations are perplexing and pathophysiological mechanisms are undefined. One such protein is inosine monophosphate dehydrogenase type 1 (IMPDH1), which catalyzes a key step in guanine nucleotide biosynthesis. The effects of the IMPDH1 mutations cannot be explained by the loss of enzyme activity. We have recently discovered that IMPDH binds nucleic acids and demonstrated that the disease-causing mutations perturb nucleic acid binding. We are now investigating how this defect causes the specific apoptosis of photoreceptor cells with the aim of developing strategies for therapy.

Mechanistic enzymology.  Filamentous fungi produce many important natural products such as penicillin and mycophenolic acid.  New projects are available investigating the enzymes involved in these biosynthetic pathways.

Selected Publications:

Cofactor mobility determines reaction outcome in the IMPDH/GMPR (β/α)8 barrel enzymes. 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. Nat. Chem. Biol., in press.

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

Structural Determinants of Inhibitor Selectivity in Prokaryotic IMP Dehydrogenases.Gollapalli, Deviprasad R.; MacPherson, Iain S.; Liechti, George; Goldberg, Joanna B. and Hedstrom, Lizbeth. Chemistry and Biology 17, 1084-1091 (2010).  [abstract]

The structural basis of Cryptosporidium -specific IMP dehydrogenase inhibitor selectivity. MacPherson IS, Kirubakaran S, Gorla SK, Riera TV, D'Aquino JA, Zhang M, Cuny GD, Hedstrom L. J Am Chem Soc. 2010 Feb 3;132(4):1230-1. [abstract]

Triazole inhibitors of Cryptosporidium parvum inosine 5'-monophosphate dehydrogenase.  Maurya SK, Gollapalli DR, Kirubakaran S, Zhang M, Johnson CR, Benjamin NN, Hedstrom L, Cuny GD. J Med Chem. 2009 Aug 13;52(15):4623-30. [abstract]

An enzymatic atavist revealed in dual pathways for water activation. Min D, Josephine HR, Li H, Lakner C, MacPherson IS, Naylor GJ, et al. PLoS Biol. 2008;6(8):e206. [full text in PubMed Central] [abstract]

Targeting a prokaryotic protein in a eukaryotic pathogen: identification of lead compounds against cryptosporidiosis. Umejiego NN, Gollapalli D, Sharling L, Volftsun A, Lu J, Benjamin NN, et al. Chem Biol. 2008;15(1):70-7. [abstract]

IMP dehydrogenase type 1 associates with polyribosomes translating rhodopsin mRNA. Mortimer SE, Xu D, McGrew D, Hamaguchi N, Lim HC, Bowne SJ, et al. J Biol Chem. 2008;283(52):36354-60. [full text in PubMed Central] [abstract]

View Complete Publication List on PubMed: Liz Hedstrom

Last update: September 8, 2011