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Understanding and Improving NADPH‐Dependent Reactions by Nongrowing Escherichia coli Cells
Author(s) -
Walton Adam Z.,
Stewart Jon D.
Publication year - 2004
Publication title -
biotechnology progress
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.572
H-Index - 129
eISSN - 1520-6033
pISSN - 8756-7938
DOI - 10.1021/bp030044m
Subject(s) - yield (engineering) , intracellular , escherichia coli , biochemistry , substrate (aquarium) , cofactor , chemistry , monooxygenase , cyclohexanone , yeast , enzyme , catalysis , biology , cytochrome p450 , ecology , materials science , metallurgy , gene
We have shown that whole Escherichia coli cells overexpressing NADPH‐dependent cyclohexanone monooxygenase carry out a model Baeyer‐Villiger oxidation with high volumetric productivity (0.79 g ϵ‐caprolactone/L·h ) under nongrowing conditions (Walton, A. Z.; Stewart, J. D. Biotechnol. Prog. 2002 , 18, 262–268). This is approximately 20‐fold higher than the space‐time yield for reactions that used growing cells of the same strain. Here, we show that the intracellular stability of cyclohexanone monooxygenase and the rate of substrate transport across the cell membrane were the key limitations on the overall reaction duration and rate, respectively. Directly measuring the levels of intracellular nicotinamide cofactors under bioprocess conditions suggested that E. coli cells could support even more efficient NADPH‐dependent bioconversions if a more suitable enzyme‐substrate pair were identified. This was demonstrated by reducing ethyl acetoacetate with whole cells of an E. coli strain that overexpressed an NADPH‐dependent, short‐chain dehydrogenase from bakerapos;s yeast ( Saccharomyces cerevisiae ). Under glucose‐fed, nongrowing conditions, this reduction proceeded with a space‐time yield of 2.0 g/L·h and a final product titer of 15.8 g/L using a biocatalyst:substrate ratio (g/g) of only 0.37. These values are significantly higher than those obtained previously. Moreover, the stoichiometry linking ketone reduction and glucose consumption (2.3 ± 0.1) suggested that the citric acid cycle supplied the bulk of the intracellular NADPH under our process conditions. This information can be used to improve the efficiency of glucose utilization even further by metabolic engineering strategies that increase carbon flux through the pentose phosphate pathway.