Propionate anaerobic metabolism

Similarly, the regulation of PDK activity is modified in adult muscle PDC. For example, PDK activity is inhibited by pyruvate and propionate (metabolites elevated during anaerobic metabolism) and is less sensitive to stimulation by elevated NADH/NAD+ and acetyl CoA/CoA ratios (Fig. 14.2) (Thissen et al, 1986 Chen et al, 1998). The effects of NADH and acetyl CoA on PDK activity are mediated by the degree of E3-catalysed oxidation and E2-catalysed acetylation of the inner lipoyl domain of E2 (Roche and Cate, 1977 Rahmatullah and Roche, 1985, 1987 Ravindran et al, 1996 Yang et al, 1998), so that the regulation of this phenomenon is complex and involves multiple interacting components. 

All parasitic flatworms capable of anaerobic metabolism favour malate as the primary mitochondrial substrate and the oxidative decarboxylations of first malate and then pyruvate generate intramitochondrial reducing power in the form of NADH (Fig. 20.1). In contrast, the pathways used to reoxidize intramitochondrial NADH are quite diverse and depend on the stage or species of parasite under examination, but in all cases, redox balance is maintained and electron-transport associated ATP is generated by the NADH-reduction of fumarate to succinate. In the cestode, hi. diminuta, succinate and acetate are the major end products of anaerobic malate dismutation and are excreted in the predicted 2 1 ratio. In the trematode F. hepatica, succinate is then further decarboxylated to propionate with an additional substrate level phosphorylation coupled to the decarboxylation of methylmalonyl CoA. F. hepatica forms primarily propionate and acetate as end products, again in a ratio of 2 1 to maintain redox balance. 

The anaerobic metabolism of acrylate and 3-mercaptopropionate (3-MPA) was studied in slurries of coastal marine sediments. The rate of these compounds is important because they are derived from the algal osmolyte dimethylsulfoniopropionate (DMSP), which is a major organic sulfur compound in marine environments. Micromolar levels of acrylate were fermented rapidly in the slurries to a mixture of acetate and propionate (1 2 molar ratio). Sulfate-reducing bacteria subsequently removed the acetate and propionate. 3-MPA has only recently been detected in natural environments. In our experiments 3-MPA was formed by chemical addition of sulfide to aciylate and was then consumed by biological processes. 3-MPA is a known inhibitor of fatty acid oxidation in mammalian systems. In accord with this fact, high concentrations of 3-MPA caused acetate to accumulate in sediment slurries. At lower concentrations, however, 3-MPA was metabolized by anaerobic bacteria. We conclude that the degradation of DMSP may ultimately lead to the production of substrates which are readily metabolized by microbes in the sediments. 

Ethanol is formed by the anaerobic metabolism of yeasts like Saccharomyces and many other species. In the presence of sulfite salts or in alkaline solutions, the alcohol formation can be changed to glycerin formation. Clostridium and Bacillus species participate in the production of butanol-acetone-butyric acid. Besides n-butanol, acetone and butyric acid, other organic compounds like propionic and lactic acids, 2-propanol, ethanol, and acetyl methylcarbinol (3-oxo-2-butanol) as well as C02 and H2 are produced as by-products. Some bacteria generate 2-propanol from acetone and others form acetone from ethanol. 

Houwen FP, C Dijkema, AIM Stams, AJB Zehnder (1991) Propionate metabolism in anaerobic bacteria determination of carboxylation reactions with C-NMR spectroscopy. Biochim Biophys Acta 1056 126-132. 

Both the synthesis of propionate and its metabolism may take place under anaerobic conditions. In Desulfobulbuspropionicum, degradation could plausibly take place by reversal of the steps used for its synthesis from acetate (Stams et al. 1984)—carboxylation of propionate to methylmalonate followed by coenzyme Bi2-mediated rearrangement to succinate, which then enters the tricarboxylic acid cycle. The converse decarboxylation of succinate to propionate has been observed in Propionigenium modestum (Schink and Pfennig 1982),... 

Clostridium species are anaerobic, spore-forming microbes. The formic, acetic, propionic, and butyric acids produced as a result of their metabolic activity can enhance the corrosion of steel. 

Soil t,/2 = 10 d in sandy soils and t,/2 30 d in sandy clay soils while under anaerobic conditions, results were similar except that the very rapid cleavage of the ester bond by hydrolysis within one hour to propionic acid derivatives was experienced and within 2 d, up to 86% of the parent compound was metabolized into various free acid metabolites and up to 3.7% of phenol metabolites (Herbicide Handbook 1989) ... 

The large intestine extends from the ileocecal valve to the anus. It is wider than the small intestine except for the descending colon, which when empty may have the same diameter as the small intestine. Major functions of the colon are absorption of water, Na+, and other electrolytes, as well as temporary storage of excreta followed by their elimination. The colon harbors large numbers of mostly anaerobic bacteria that can cause disease if they invade tissues. These bacteria metabolize carbohydrates to lactate, short-chain fatty acids (acetate, propionate, and butyrate), and gases (CO2, CH4, and H2). Ammonia, a toxic waste product, is produced from urea and other nitrogenous compounds. Other toxic substances are also produced in the colon. Ammonia and amines (aromatic or aliphatic) are absorbed and transported to the liver via the portal blood, where the former is converted to urea (Chapter 17) and the latter is detoxified. The liver thus protects the rest of the body from toxic substances produced in the colon. Colonic bacteria can also be a source of certain vitamins (e.g., vitamin K, Chapter 36). 

The strictly anaerobic beer-spoilage bacteria share a few metabolic and physiological features. For instance, they produce acetic and propionic adds from simple sugars. However, clear differences exist in their nutrition and physiology, which is reflected in their beer-spoilage ability and can be exploited for their detection and identification. The key characteristics discriminating the beer-spoilage species from each other are shown in Tables 9.1 and 9.2. 

The formation and studying of the collections of propionic acid bacteria proceeded simultaneously with investigations of their biochemistry, first of all, biochemistry of their unique mode of fermentation. Propionic acid fermentation was discovered by A. Fitz, later, it was studied by H.G. Wood and C.H. Werkman. It was in propionibacteria that the heterotrophic assimilation of CO2 was discovered by H.G. Wood. Owing to the studies by Wood, Werkman and their school, then by H.A. Barker and F. Lipmann as well as E.A. Delwiche in the USA, the chemistry of this unique fermentation was elucidated. Another development at the second stage of biochemical investigations concerns the discovery of aerobic metabolism in propionic acid bacteria, previously considered anaerobic. Important contributions to this field of study were made by the school of A H. Stouthamer in the Netherlands and in our laboratory at the Moscow State University. These investigations demonstrated a surprising lability of the metabolism of propionic acid bacteria, which were found to be well equipped for both the... 

---