BIOL 241 |
| IMP dehydrogenase (IMPDH) is the penultimate enzyme in guanine nucleotide biosynthesis. IMPDH first catalyzes the transfer of a hydride from IMP to NAD+ producing NADH and the covalent intermediate, E-XMP*. NADH release is followed by hydrolysis of E-XMP* yielding the product XMP. All IMPDHs are activated by potassium. This work investigates the mechanism of activation by comparing the potassium versus basal IMPDH reactions. Potassium does not affect KM IMP or the Kds for either substrate for free enzyme. Potassium increases kcat 45-fold and decreases KM NAD 10-fold from its basal value. Pre-steady-state bursts of NADH production and release are also potassium dependent. Solvent isotope effects, steady-state accumulation of E-14C-XMP* and product release rates suggest that the rate-limiting step is a conformational change after hydrolysis in the basal reaction. Potassium lowers this energy barrier such that hydrolysis and NADH release become rate-limiting. Supported by NIH GM54403, AI55268. |
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Enzymes
4:30 PM-6:30 PM, Wednesday, 13 September 2006 Moscone Center -- Hall D, Poster
Sci-Mix
Division of Biological Chemistry |