The auxiliary substrate concept — an approach for overcoming limits of microbial performances

Abstract

and OutlookIn biotechnological processes, fundamental performances of microorganisms are used. The economy of these processes is essentially determined by the efficiency, velocity (productivity) and quality of thc products. Therefore it is a permanent task and challenge for basic and biotechnological research to seek out measures for improving the actually attained parameters. The auxiliary substrate concept supplics an approach. It is based on the fact that chemo-organo-heterotrophic substrates differ in the carbon:energy ratio, thus, growth yield is limited in energy and/or reducing power. I t SAYS that. by simultaneous utilization of physiologically similar substrates (mixed substrates), the growth yield increases. The substrates are to combine in such a way that with their simultaneous utilization a minimum of carbon is dissimilated merely for the purpose of the generation of biologically useful energy and/or reducing power. Since all chemo-organo-heterotrophic substrates are more or less energy-deficient, an increase in growth efficiency can be expected if the individual substrates of the mixture are assimilated more efficiently than the respective substrates alone. This may result, for instance, from an immediate assimilation of a substrate (according to the "manner of finished part construction"). An increased growth rate is rather the rule than the exception in mixed substrate utilization.In product syntheses the substrates are. depending on the concrete product and metabolic pathway. either energy-excess or energy-deficient, or, in other words, the processes are energy-generating or energy-consuming, respectively. If this is responsible for discrepancies between the possible yields determined by the carbon metabolism and the experimentally obtained yields. the discrepancies should be able to be decreased and the yields increased by mixing substrates. The substrates are to choose and combine so that, due to simultaneous utilization, the product formation process becomes energy neutral. As a rule, the enhanced efficiency is accompanied by an increased velocity. This does not only apply to syntheses, but also to degradation (and detoxification) reactions. Even supposedly inert compounds or persistent substances can be activated by simultaneous (co-)metabolization of another (an auxiliary substrate, victim substrate or co-substrate) and converted at a considerable rate. It is of interest for syntheses of products but in particular for degradation and decontamination of harmful and waste products in the environment that the residual concentrations of the substrates are smaller than those achieved if the compounds of a mixture are metabolized separately. The auxiliary substrate concept has proven to be fruitful, both for theoretical and practical questions. It was practically already being used before it was formulated (mixed substrate utilization, cometabolism). However, an abundance of regulatory and energetic aspects are waiting to be investigated in more detail. 212Acta Biotechnologica 13 (1993) 3

A Theoretical Study on the Metabolic Requirements Resulting from α-Ketoglutarate-Dependent Cleavage of Phenoxyalkanoates

“…The calculations were performed on the basis of the Y ATP concept (1,22), taking into account a biomass synthesis efficiency of 10.5 g of bacterial dry mass/mol of ATP (3). Biomass synthesis was assumed to start from a central carbon precursor (29), which is considered to be 3-phosphoglycerate (PGA).…”

Formate as an Auxiliary Substrate for Glucose-Limited Cultivation of Penicillium chrysogenum : Impact on Penicillin G Production and Biomass Yield

“…Since part of the glucose has to be used for the generation of ATP equivalents in dissimilation, it can be called an energy-deficient substrate (4). Due to this intrinsic free energy deficiency, experimentally obtained biomass yields on glucose are generally lower than the maximum biomass yield that could theoretically be reached when assimilatory reactions are fully optimized (5,9).…”

“…An increase of the carbon source-to-biomass conversion efficiency has been demonstrated for many combinations of carbon sources and auxiliary substrates, including acetate-thiosulfate (14), acetate-formate (11), and glucose-formate (9,13,16). Formate is a suitable model auxiliary energy substrate for yeasts and fungi, as many of these organisms contain an NAD ϩ -linked formate dehydrogenase (EC 1.2.1.1; FDH) that catalyzes the oxidation of formate to CO 2 but are unable to assimilate formate (3,4,10,16,28,33,38). The NADH formed in the FDH reaction can be coupled to the mitochondrial respiratory chain and thus lead to ATP production.…”

Formate-stimulated Oxidation of Methanol byPseudomonas putida9816

“…It has been suggested that this deˆciency could be overcome by providing an additional energy source, such as formate. 16) However, the addition of formate did not increase the yield signiˆcantly. Growth remained low: between 2 mg W l ・d (100 mM MeOH as sole substrate) and 7 mg W l ・d (100 mM MeOH＋10 mM Ft).…”