Glucose as an auxiliary substrate

Summary A theoretical consideration is presented of the comparative efficiency of carbon conversion of glucose by the Embden-Meyerhof-Parnas (EMP) and the oxidative hexosemonophosphate (HMP) pathways. As a result it is shown that maximum carbon conversion, that is 89%, is possible when glucose is assimilated via the EMP pathway. This value is diminished in proportion to the participation of the HMP pathway in carbon assimilation and is halved when glucose is incorporated entirely via this pathway. If NADPH is included as a source of energy, glucose may behave both as an excess carbon and an excess energy substrate, the latter being the case when greater portions of the HMP pathway operate, and the extent of this is in turn dependent on the P/O quotient. If NADPH cannot be used for ATP synthesis, glucose remains an excess carbon substrate throughout, although when the HMP pathway accounts for more than 26% of glucose assimilation an increasing excess of reduction equivalents is produced. These results are interpreted in terms of mixed-substrate utilization for improving growth yield when glucose is to be used as the excess carbon component.