A mathematical model for yeast respiro-fermentative physiology

A mechanistic model is presented that describes the respiro-fermentative ph ysiology of yeast. The model assumes the presence of multiple types of gluc ose carriers and multiple assimilation pathways. Respiro-fermentative physi ology is explained by the mechanistic response of the different types of ca rriers and assimilation pathways on the substrate concentration. At low sub strate concentrations, glucose is taken up mainly via a high affinity carri er with a low maximum uptake rate. At high substrate concentrations, this c arrier becomes saturated and the main pathway for glucose uptake is via a l ow affinity carrier with a high maximum uptake rate. The price to pay for t he high uptake rate is a lowered assimilation efficiency, resulting in a lo w biomass yield. Product formation occurs via the pathway with the high upt ake rate. The model explains the link between substrate concentration and p roduct formation generally observed in the literature on yeast and bacteria . Model parameter values are estimated by fitting data from the literature. The model distinguishes itself from other models in that it does not rely on the presence of switches, such as the 'critical dilution rate', or on th e assumption that the respiratory capacity reaches its maximum during respi ro-fermentative metabolism. The present theory is not designed exclusively for the phenomenon of respiro-fermentative physiology: it describes the deg radation of substances by heterotrophic microorganisms in general. Copyrigh t (C) 2000 John Wiley & Sons, Ltd.