CARBON AND ENERGY FLUX CONSTRAINTS IN CONTINUOUS CULTURES OF ALCALIGENES-EUTROPHUS GROWN ON PHENOL

The growth behaviour of Alcaligenes eutrophus on phenol was investigat ed in continuous cultures to identify the phenomena limiting both grow th efficiency and substrate degradation rates. It was shown that the f ixed stoichiometry of the meta pathway of phenol degradation, leading to equimolar quantities of pyruvate and acetate, and the structure of the central pathways, which do not allow gluconeogenesis of acetate du ring growth on phenol, provoke the accumulation of polyhydroxybutyrate (PHB) under certain growth conditions. Acetate is predominantly used as an energy source and PHB accumulates when the cells are carbon-limi ted rather than energy-limited. The maximum rates of phenol degradatio n can be attributed to the expression of the enzymes of the catabolic pathway. This is particularly true of phenol hydroxylase and 2-hydroxy muconate semialdehyde (2-hms) dehydrogenase, whose substrates accumula ted to physiologically significant concentrations at high growth rates . Indeed the concentration of 2-hms that accumulated in the medium ind icated that this enzyme was substrate-saturated at maximum growth rate s. However, the specific activity profiles of other catabolic enzymes associated with phenol degradation were close to the estimated flux th rough the pathway. This suggests a complex control structure in which several enzymes contribute to the control of pathway flux, as would be expected in a catabolic pathway.