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Science 20 October 2006:
Vol. 314. no. 5798, p. 381
DOI: 10.1126/science.314.5798.381m
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The enzyme isopropylmalate dehydrogenase (IMDH) is required for the synthesis of the essential amino acid leucine. All IMDHs use NAD (the oxidized form of nicotinamide adenine dinucleotide) as a coenzyme. A mutant version of Escherichia coli IMDH that was engineered to use NAD phosphate (NADP) is less fit than the wild-type enzyme because of intense co-product inhibition by the reduced form of NADP (NADPH), which is present at high intracellular concentrations. Miller et al. (p. 458) screened for versions of the NADP-utilizing IMDH that break this constraint and show increased fitness. Of the fitter mutants, none increased the rate constant kcat for NADP, uncoupled the affinities for NADP and NADPH, or broke the trade-off in NAD-NADP performance. Instead, fitness was increased by increasing expression of the mutant IMDH and reducing its affinity for NADP and NADPH.




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