A Kinetic Alignment of Orthologous Inosine-5′-monophosphate Dehydrogenases

IMP dehydrogenase (IMPDH) catalyzes two very different chemical transformations, a dehydrogenase reaction and a hydrolysis reaction. The enzyme toggles between the open conformation required for the dehydrogenase reaction and the closed conformation of the hydrolase reaction by moving a mobile flap...

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Bibliographic Details
Published in:Biochemistry (Easton) 2008-08, Vol.47 (33), p.8689-8696
Main Authors: Riera, Thomas V, Wang, Wen, Josephine, Helen R, Hedstrom, Lizbeth
Format: Article
Language:English
Subjects:
Amino Acid Sequence
Animals
Binding Sites
Cryptosporidium parvum - enzymology
Guanosine Monophosphate - chemistry
Guanosine Monophosphate - metabolism
IMP Dehydrogenase - chemistry
IMP Dehydrogenase - metabolism
Inosine Monophosphate - chemistry
Inosine Monophosphate - metabolism
Kinetics
Models, Molecular
Molecular Conformation
Molecular Sequence Data
NAD - analogs & derivatives
NAD - chemistry
NAD - metabolism
Protein Binding
Ribonucleotides - chemistry
Ribonucleotides - metabolism
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Summary:IMP dehydrogenase (IMPDH) catalyzes two very different chemical transformations, a dehydrogenase reaction and a hydrolysis reaction. The enzyme toggles between the open conformation required for the dehydrogenase reaction and the closed conformation of the hydrolase reaction by moving a mobile flap into the NAD site. Despite these multiple functional constraints, the residues of the flap and NAD site are highly diverged, and the equilibrium between open and closed conformations (K c ) varies widely. In order to understand how differences in the dynamic properties of the flap influence the catalytic cycle, we have delineated the kinetic mechanism of IMPDH from the pathogenic protozoan parasite Cryptosporidium parvum (CpIMPDH), which was obtained from a bacterial source through horizontal gene transfer, and its host counterpart, human IMPDH type 2 (hIMPDH2). Interestingly, the intrinsic binding energy of NAD+ differentially distributes across the dinucleotide binding sites of these two enzymes as well as in the previously characterized IMPDH from Tritrichomonas foetus (TfIMPDH). Both the dehydrogenase and hydrolase reactions display significant differences in the host and parasite enzymes, in keeping with the phylogenetic and structural divergence of their active sites. Despite large differences in K c , the catalytic power of both the dehydrogenase and hydrolase conformations are similar in CpIMPDH and TfIMPDH. This observation suggests that the closure of the flap simply sets the stage for catalysis rather than plays a more active role in the chemical transformation. This work provides the essential mechanistic framework for drug discovery.
ISSN:0006-2960
1520-4995
DOI:10.1021/bi800674a