Effects of growth rate and oxygen tension on glucose dehydrogenase activity in Acinetobacter calcoaceticus LMD 79.41

Dynamic decoupling of biomass and wax ester biosynthesis in Acinetobacter baylyi by an autonomously regulated switch

For improving the microbial production of fuels and chemicals, gene knock-outs and overexpression are routinely applied to intensify the carbon flow from substrate to product. However, their possibilities in dynamic control of the flux between the biomass and product synthesis are limited, whereas dynamic metabolic switches can be used for optimizing the distribution of carbon and resources. The production of single cell oils is especially challenging, as the synthesis is strictly regulated, competes directly with biomass, and requires defined conditions, such as nitrogen limitation. Here, we engineered a metabolic switch for redirecting carbon flow from biomass to wax ester production in Acinetobacter baylyi ADP1 using acetate as a carbon source. Isocitrate lyase, an essential enzyme for growth on acetate, was expressed under an arabinose inducible promoter. The autonomous downregulation of the expression is based on the gradual oxidation of the arabinose inducer by a glucose dehydrogenase gcd. The depletion of the inducer, occurring simultaneously to acetate consumption, switches the cells from a biomass mode to a lipid synthesis mode, enabling the efficient channelling of carbon to wax esters in a simple batch culture. In the engineered strain, the yield and titer of wax esters were improved by 3.8 and 3.1 folds, respectively, over the control strain. In addition, the engineered strain accumulated wax esters 19% of cell dry weight, being the highest reported among microbes. The study provides important insights into the dynamic engineering of the biomass-dependent synthesis pathways for the improved production of biocompounds from low-cost and sustainable substrates.

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Efficient conversion of acetate to storage lipids (wax ester) is demonstrated.

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AraC-pBAD promoter coupled with glucose dehydrogenase was used as a dynamic switch.

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The autonomous switch allowed dynamic shift from biomass to lipid synthesis mode.

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Wax ester yield and titer were improved by 3–4 folds over the wild type strain.

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The highest amount of wax esters produced in microbes, 19% of CDW, was achieved.

The biochemistry, physiology and genetics of PQQ and PQQ-containing enzymes

Pyrrolo-quinoline quinone (PQQ) is the non-covalently bound prosthetic group of many quinoproteins catalysing reactions in the periplasm of Gram-negative bacteria. Most of these involve the oxidation of alcohols or aldose sugars. PQQ is formed by fusion of glutamate and tyrosine, but details of the biosynthetic pathway are not known; a polypeptide precursor in the cytoplasm is probably involved, the completed PQQ being transported into the periplasm. In addition to the soluble methanol dehydrogenase of methylotrophs, there are three classes of alcohol dehydrogenases; type I is similar to methanol dehydrogenase; type II is a soluble quinohaemoprotein, having a C-terminal extension containing haem C; type III is similar but it has two additional subunits (one of which is a multihaem cytochrome c), bound in an unusual way to the periplasmic membrane. There are two types of glucose dehydrogenase; one is an atypical soluble quinoprotein which is probably not involved in energy transduction. The more widely distributed glucose dehydrogenases are integral membrane proteins, bound to the membrane by transmembrane helices at the N-terminus. The structures of the catalytic domains of type III alcohol dehydrogenase and membrane glucose dehydrogenase have been modelled successfully on the methanol dehydrogenase structure (determined by X-ray crystallography). Their mechanisms are likely to be similar in many ways and probably always involve a calcium ion (or other divalent cation) at the active site. The electron transport chains involving the soluble alcohol dehydrogenases usually consist only of soluble c-type cytochromes and the appropriate terminal oxidases. The membrane-bound quinohaemoprotein alcohol dehydrogenases pass electrons to membrane ubiquinone which is then oxidized directly by ubiquinol oxidases. The electron acceptor for membrane glucose dehydrogenase is ubiquinone which is subsequently oxidized directly by ubiquinol oxidases or by electron transfer chains involving cytochrome bc1, cytochrome c and cytochrome c oxidases. The function of most of these systems is to produce energy for growth on alcohol or aldose substrates, but there is some debate about the function of glucose dehydrogenases in those bacteria which contain one or more alternative pathways for glucose utilization. Synthesis of the quinoprotein respiratory systems requires production of PQQ, haem and the dehydrogenase subunits, transport of these into the periplasm, and incorporation together with divalent cations, into active quinoproteins and quinohaemoproteins. Six genes required for regulation of synthesis of methanol dehydrogenase have been identified in Methylobacterium, and there is evidence that two, two-component regulatory systems are involved.

Selection of glucose-assimilating variants of Acinetobacter calcoaceticus LMD 79.41 in chemostat culture

Glucose metabolism has been studied in two strains of Acinetobacter calcoaceticus. Strain LMD 82.3, was able to grow on glucose and possessed glucose dehydrogenase (EC 1.1.99.17). Glucose oxidation by whole cells was stimulated by PQQ, the prosthetic group of glucose dehydrogenase. PQQ not only increased the rate of glucose oxidation and gluconic acid production but also shortened the lag phase for growth on glucose. Strain LMD 79.41 also possessed glucose dehydrogenase but was unable to grow on glucose. Batch cultures and carbon-limited chemostat cultures growing on acetate in the presence of glucose oxidized the sugar to gluconic acid, which was not further metabolized. However, after prolonged cultivation on mixtures of acetate and glucose, carbon-limited chemostat cultures suddenly acquired the capacity to utilize gluconate. This phenomenon was accompanied by the appearance of gluconate kinase and a repression of isocitrate lyase synthesis. In contrast to the starter culture, cells from chemostats which had been fully adapted to gluconate utilization, were able to utilize glucose as a sole carbon and energy source in liquid and solid media.