Pecularities of methylotrophs concerning overflow metabolism, especially the synthesis of polyhydroxyalkanoates

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The accumulation of poly-β-hydroxybutyrate (PHB) is widespread in prokaryotes. The synthesis of PHB is independent of the carbon source and type of nutrition. Methylotrophic bacteria can synthesize PHB, and poly-hydroxyalkanoates (PHA), too, if suitable mixed substrates are used. Apart from the genetic disposition, the capability of accumulating PHB seems to be correlated with the serine pathway, as the synthesis of exopolysaccharides is probably coupled with the hexulosephosphate pathway. This capability prevents the obviously NADH-limited ‘serine pathway (bacteria)’ from wasting NADH, which could be due to the high capacity of the dissimilatory sequence. Provided ‘RMP bacteria’ were able to synthesize PHB, methanol differs from other substrates insofar as the synthesis can be associated with a supply or consumption of reducing equivalents. The synthesis of PHB from acetate is also energy- and reducing power-consuming. In this respect, and concerning the role of the TCA cycle, methanol and acetate resemble each other and jointly differ from glucose. That PHB syntheses from different substrates can be energy-consuming or -generating, opens up chance for improving efficiency. The prescription for this is provided by the mixed or auxiliary substrate concept. By choosing suitable substrates it is possible to enhance the carbon conversion efficiency, and in addition, to synthesize heteropolyesters.