The mixed substrate concept, applied for microbial syntheses of metabolites

Microbial overproduction of metabolites is a response to suboptimal conditions for growth and multiplication. It is an energy-wasting process in terms of life insofar as a part of energy of the carbon source remains in the metabolite. From an energetic point of view microbial overproduction can be divided into two categories: i) energy-consuming, ii) energy-yielding. The amount of energy required or made available is considered to be responsible for discrepancies between carbon metabolism-determined possible and experimentally obtained yields. Since the expenditure of energy must be provided by oxidation of carbon source more substrate is consumed than required according to the metabolic pathway. In the case of energy-yielding synthesis energy must be discharged. Various possibilities exist. Since metabolic sequences not involved in the synthesis of the proper product are not switched off completely other synthetic processes and even growth can occur. The energy is thus discharged at the expense of substrate. To increase the experimental yield the energy produced or consumed has to be maintained low. This can be achieved by means of substrate mixtures. The synthesis of by-products and growth are difficult to prevent completely. However, growth can be quite desirable since the catalyst is renewed thus making the product synthesis possible.