Overflow metabolism

Most of the amino acids are consumed by insect cells, with the exception of alanine which is produced however, it has been reported that alanine overflow metabolism is energetically wasteful as it is with mammalian cells [63]. The alanine production by insect cells has been interpreted as a strategy to avoid the accumulation of toxic ammonia produced from amino acid catabolism [64]. 

Ljunggren J, Haggstrom L (1992), Glutamine limited fed-batch culture reduces the overflow metabolism of amino acids in myeloma cells, Cytotechnology 8 45-56. 

Mineral nutrient limitation of microbial degradation has been put forward as an explanation for accumulation of carbon-rich DOM after a Phae-ocystis bloom (Thingstad and Billen 1994). The increase in carbohydrate/POC due to overflow metabolism will give rise to a substrate with a C/P and C/N ratio that is unfavorable to bacteria. Since the C/P ratio of bacteria may be considerably lower than that of phytoplankton (Vadstein et al. 1988), especially phosphate limitation may hamper microbial degradation (Thingstad et al. 

All of them are product and process dependent but generally an increased yield of product on the substrate [128] and an increased growth rate are common properties that S. cerevisiae is required to have in order to produce an efficient processes. More specific desired features are the high productivity of ethanol in brewing and wine making, of CO2 in baking, and minimization of overflow metabolism (ethanol and glycerol) and increased biomass yield on the substrate in applications where product formation is directly coupled to biomass. 

The secondary metabolites are produced from key intermediates of the primary metabolism pathways. They are often found in limited quantity, can occur transiently in the cell cycle and can be unique for a particular group of organisms or even species. Secondary metabolites typically represent a chemically very diverse group of small molecules (molecular mass <2000 amu) and include (1) products from overflow metabolism as a consequence of nutrient limitation, (2) compounds for defense, (3) regulatory molecules, (4) signaling molecules, or (5) molecules that serve the requirements of evolutionary exploration within the physicochemical space available on this planet. Secondary metabolites can be grouped according to the primary metabolic pathway from which they are derived or in terms of their structural similarity. 

Xu B, Jahic M, Bomsten G, Enfors S O (1999). Glucose overflow metabolism and mixed acid fermentation in aerobic large-scale fed-batch processes with Echerichia coli. Appl. Microbiol. Biotechnol. 51 564-571. 

Holwerda, E.K., Thorne, P.G., Olson, D.G., Amador-Noguez, D., Engle, N.L., Tschaplinski, T.J., van Dijken, J.P., and Lynd, L.R. (2014) The exometabolome of Clostridium thermocellum reveals overflow metabolism at high cellulose loading. Biotechnol Biofuels, 7 (1), 155. 

The peculiar metabolic features of S. cerevisiae explain why this yeast is especially suitable for overproduction of metabolites, while other yeasts with predominantly respiratory phenotype have advantages for the production of biomass-related products such as recombinant proteins. Natural hmitation of sugar uptake hmits overflow metabolism in these yeasts, channeling substrate into growth-related products. Table 18.1 provides an overview of yeasts used in biotechnology. 

Walshaw D.L., Wilkinson A., Mundy M., Smith M. and Poole P.S., 1997, Regulation of the TCA cycle and the general aminoacid permease by overflow metabolism in Rhizobium leguminosarum. Microbiology 143 2209-2221. 

Transport of fumaric acid outside of the fungal cell is poorly understood. It is important to understand how and at what conditions fumaric acid is transported out of the cell. The ambient pH is one of the main factors found to influence the excretion of organic acids in filamentous fungi and there are different hypothesis on the mechanisms involved (overflow metabolism hypothesis, charge balance hypothesis, and aggressive acidification hypothesis all detailed in Vrabl et al., 2012). 

Xu B, Jahic M, Enfors SO (1999) Modeling of overflow metabolism in batch and fed-batch cultures of Escherichia coli. Biotechnol Prog 15 81-90... 

In the process of the cultivation of recombinant E. coli, low yields are observed due to the formation and accumulation of acetic acid resulting from overflow metabolism. Thus, it is necessary to prevent and monitor the formation of acetic acid, which can be avoided by the introduction of glucose to the medium. However, the rate of glucose addition must be monitored in order to maintain the growth below a critical value. In this context, a flow injection system was developed for the determination of acetic acid in E. coli cultivations with electrochemical detection based on immobilized AK, PK, and LDH, with amperometric detection of NADH. A limitation associated with this system was that the enzyme AK lost 90% of its activity after only one fermentation. The authors thus suggested that, due to the low stability of immobilized AK in the fermentation measurements, alternative enzymes needed to be found (Tang et al., 1997). 

---