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Intracellular cofactors--such as NADH, NAD, ATP, ADP, and P$\sb{\rm i}$--are important co-substrates that can affect the rate of the cell's catabolic and respiratory reactions. Pure-culture experiments with Pseudomonas putida P$\sb{\rm p}$F1 performed in a special chemostat apparatus, using acetate as electron donor and O$\sb2$ as electron acceptor, revealed that the cofactor concentrations vary highly systematically with changes in availability of the external electron donor and acceptor. In general, the NAD/NADH ratio increased as the DO concentration rose or the acetate concentration fell, while the ATP/ADP$\cdot$P$\sb{\rm i}$ value increased as the electron-donor utilization rate decreased.
A structured model for substrate-utilization kinetics under simultaneous limitation of electron donor and acceptor (i.e., dual-limitation) was developed by incorporating those cofactor ratios as variables. The model, which described the observed electron-donor utilization rate satisfactorily under dual- and single-limited conditions, demonstrated that the systematic responses with the NAD/NADH ratio accelerate the limiting reaction (e.g., catabolic reaction) at the cost of slowing the nonlimiting reaction (e.g., respiration), maximizing the overall growth rate under a given dual-limitation condition. Linkage of the systematic internal responses to external primary electron-donor and acceptor concentrations transformed the structured model into a double-Monod model; thus, the co-limitation by both substrates reduced the overall reaction rate multiplicatively.
The systematic changes in the cellular NAD(H) concentrations could be applied to biodegradation of halogenated or nonhalogenated hydrocarbons whose degradation requires reducing power. A secondary-substrate utilization model predicted a maximum reductive dehalogenation at a saturating concentration of electron donor. The % removal, however, was very sensitive to substrate-concentration changes for the combination of high electron-acceptor and low electron-donor concentrations--a very common situation in biological treatment processes. For nonhalogenated hydrocarbons that are degraded by mono or dioxygenase reactions, consistently high % removals could be achieved when the concentrations of primary electron donor and acceptor are high together. Since the oxygen concentration, in addition to the reducing power, controls the oxygenase reaction, no satisfactory removal could be expected when either concentration was low.
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