FLUXES OF CARBON, PHOSPHORYLATION, AND REDOX INTERMEDIATES DURING GROWTH OF SACCHAROMYCES-CEREVISIAE ON DIFFERENT CARBON-SOURCES

In the present work we develop a method for estimating anabolic fluxes when yeast are growing on various carbon substrates (glucose, glycero l, lactate, pyruvate, acetate, or ethanol) in minimal medium. Fluxes t hrough the central amphibolic pathways were calculated from the produc t of the total required amount of a specified carbon intermediate time s the growth rate. The required amount of each carbon intermediate was estimated from the experimentally determined macromolecular compositi on of cells grown in each carbon source and the monomer composition of macromolecules. Substrates sharing most metabolic pathways such as et hanol and acetate, despite changes in the macromolecular composition, namely carbohydrate content (34% +/- 1 and 21% +/- 3, respectively), d id not show large variations in the overall fluxes through the main am phibolic pathways. For instance, in order to supply anabolic precursor s to sustain growth rates in the range of 0.16/h to 0.205/h, similar l arge fluxes through Acetyl CoA synthase were required by acetate (4.2 mmol/hr g dw) or ethanol (5.2 mmol/h g dw). The V-max activities of ke y enzymes of the main amphibolic pathways measured in permeabilized ye ast cells allowed to confirm, qualitatively, the operation of those pa thways for all substrates and were consistent on most substrates with the estimated fluxes required to sustain growth. When ATP produced fro m oxidation of the NADH synthesized along with the key intermediary me tabolites was taken into account, higher Y-ATP(max) values (36 with re spect to 24 g dw/mol ATP) were obtained for glucose. The same result w as obtained for glycerol, ethanol, and acetate. A yield index (YI) was defined as the ratio of the theoretically estimated substrate flux re quired to sustain a given growth rate over the experimentally measured flux of substrate consumption. Comparison of YI between growth on var ious carbon sources led us to conclude that ethanol (YI = 0.84), aceta te (YI = 0.77), and lactate (YI = 0.77) displayed the most efficient u se of substrate for biomass production. For the other substrates, the YI decayed in the following order: pyruvate > glycerol > glucose.