A stoichiometric model for the metabolism of Gluconacetobacter xylinus
Integration of life sciences & engineering
Biochemical Engineering (T5-1)
Keywords: Acetan, metabolic model, Gluconacetobacter xylinus
Velasco-Bedrán, H.*, Aréchiga-Viramontes, U., López-Isunza, F.
Departamento de Ingeniería de Procesos e Hidráulica. Universidad Autónoma Metropolitana-Iztapalapa. Avenida San Rafael Atlixco # 186. Colonia Vicentina, Iztapalapa, 09340, México, D. F. *Departamento de Ingeniería Bioquímica. Escuela Nacional de Ciencias Biológicas. Instituto Politécnico Nacional. Prolongación de Carpio esquina Plan de Ayala, Casco de Santo Tomás, 11340, México, D. F.
felipe@xanum.uam.mx, uriel@xanum.uam.mx, hvelasco@encb.ipn.mx
Gluconacetobacter xylinus is an acetic acid bacteria-related species with a complex metabolism which has received recent attention, though its biochemical features have been pointed out as early as 1948. The past years have seen studies on its ability to synthesize cellulose and a species specific water soluble polymer, acetan, with potential commercial use. This microorganism is able to grow on ethanol producing acetic acid; it can also oxidize glucose to gluconic acid, or glucose 6-P (G6P) via the pentose phosphate pathway (PP). In the absence of ethanol it can also metabolize acetic acid. Unable to phosphorylate G6P, the fermentation pathway is precluded so G. xylinus is an obligate aerobe. Besides the PP pathway, G. xylinus runs the glyoxilate shunt of the Tricarboxylic Acid cycle, Gluconeogenesis from pyruvate, and a Phospho-keto and Phospho-aldolase cycle. Recent research has claimed that the ethanol and sucrose catabolic pathways are independent and has named them “upper” (glucose or fructose) and “lower” (ethanol) glycolitic pathways.
This work concerns with an inner metabolite steady-state stoichiometric model of the biochemistry of this bacterium, comprising 10 metabolic reaction lumps. The mass conservation equations for reduced Nicotine Amide Dinucleotide (NADH), reduced Pyrrolo quinolinquinone (PQQH2), Adenosin Triphosphate (ATP), Acetate, Acyl-phosphate and G6P are established. Together with a total mass balance, the resulting equation system is solved and an ATP conservation equation is resolved, in terms of acetate, ethanol and sucrose uptake rates and of biomass production rate. This is used to analyze the results of continuous and batch culture experiments. The CSTR experiments tested different media compositions (sucrose and acetate, ethanol and acetate, and the three carbon sources), at three different dilution rates. Cellulose and Acetan were synthesized in the medium containing solely ethanol and acetic acid. The cell contents of PQQ, NAD and ATP were assayed and compared with the model predictions. The results of batch experiments from recent research are also discussed. We conclude that the contention of “independent upper and lower glycolitic” routes is not supported by experiment. On the contrary we uphold the view of a unified biochemistry for the physiology of Gluconacetobacter xylinus.
Presented Wednesday 19, 11:40 to 12:00, in session Biochemical Engineering (T5-1).
