Fermentation mixed acid

The metabolism of anaerobic chytrids has not been studied in great detail, but it is known that most anaerobic chytrids studied so far produce formate, acetate, succinate, lactate and ethanol besides hydrogen and carbon dioxide when growing on cellulose, glucose or fructose as a carbon source (Julliand et al. 1998). Such a mixed acid fermentation is very similar to bacterial mixed acid fermentations that are, for example, well known for facultative anaerobic enteric bacteria, such as Escherichia coli. 

The a-oxoacid dehydrogenases yield CoA derivatives which may enter biosynthetic reactions. Alternatively, the acyl-CoA compounds may be cleaved with generation of ATP. The pyruvate formate-lyase system also operates as part of an ATP-generating system for anaerobic organisms, for example, in the "mixed acid fermentation" of enterobacteria such as E. coli (Chapter 17). These two reactions, which are compared in Fig. 15-16, constitute an important pair of processes both of which accomplish substrate-level phosphorylation. They should be compared with the previously considered examples of substrate level phosphorylation depicted in Eq. 14-23 and Fig. 15-16. 

Some of the glucose is converted to D-lactic and to succinic acids (pathway/, Fig. 17-9) hence the name mixed acid fermentation. Table 17-1 gives typical yields of the mixed acid fermentation of E. coli. Among the four major products are acetate, ethanol, H2, and C02, as shown in Eq. 17-25. However, at high pH formate accumulated instead of C02. 

Products of the Mixed Acid Fermentation by E. coli at Low and High Values of pHa... 

In some mixed acid fermentations (e.g., that of Shigella) formic acid accumulates, but in other cases (e.g., with E. coli at pH 6) it is converted to C02 and H2 (Eq. 17-25). The equilibration of formic acid with C02 and hydrogen is catalyzed by the formic hydrogen-lyase system which consists of two iron-sulfur enzymes. The selenium-containing formate dehydrogenase (Eq. 16-63) catalyzes oxidation of... 

Mixed acid fermentations are not limited to bacteria. For example, trichomonads, parasitic flagellated protozoa, have no mitochondria. They export pyruvate into the bloodstreams of their hosts and also contain particles called hydrogenosomes which can convert pyruvate to acetate, succinate, C02, and H2.144 Hydrogenosomes are bounded by double membranes and have a common evolutionary relationship with both mitochondria and bacteria. The enzyme that catalyzes pyruvate cleavage in hydrogenosomes apparently does not contain lipoate and may be related to the pyruvate-ferredoxin oxidoreductase of clostridia (Eq. 15-35). The hydrogenosomes also contain an active hydrogenase. 

Many invertebrate animals are true facultative anaerobes, able to survive for long periods, sometimes indefinitely, without oxygen.145-147 Among these are Ascaris (Fig. 1-14), oysters, and other molluscs. Succinate and alanine are among the main end products of anaerobic metabolism. The former may arise by a mixed acid fermentation that also produces pyruvate. 

Tanaka, K., Komiyama, A., Sonomoto, K., Ishizaki, A., Hall, S. J., and Stanbury, P. F. 2002. Two different pathways for D-xylose metabolism and the effect of xylose concentration on the yield coefficient of L-lactate in mixed-acid fermentation by... 

Rachman, M.A., Furutani, Y., Nakashimada, Y., Kakizono, T., and Nishio, N. 1997. Enhanced hydrogen production in altered mixed acid fermentation of glucose by Entero-bacter aerogenes. J. Ferm. Bioeng. 83, 358-363. 

One is the formate pathway characteristic of the mixed acid fermentation by bacteria such as Escherichia coli. The pathway was proposed from stoichiometric considerations, such as the molar amount of formate at pH around 7 is equal to the molar amount of C02 and H2 at pH... 

Of course in practice in fermentations such as the mixed acid fermentation carried out by enteric bacteria, acetate and ATP production are less due to the need to balance the fermentation by regenerating NAD through the reduction of acetyl-CoA to ethanol thus providing necessary substrate (NAD) required for further sugar degradation. 

METABOLIC ENGINEERING APPROACHES FOR THE IMPROVEMENT OF BACTERIAL HYDROGEN PRODUCTION BASED ON ESCHERICHIA COLI MIXED ACID FERMENTATION... 

Recent progress in molecular biology on the anaerobic metabolism of Escherichia coli has elucidated the enzymes and genes responsible for fermentative hydrogen production (1,2). Ecoli produces hydrogen by mixed acid fermentation, mainly from glucose. In Ecoli,... 

In this review, we summarize the molecular biology of E.coli hydrogen production based on mixed acid fermentation, and our recent progress and approaches to enhance hydrogen production by metabolic engineering. 

MIXED ACID FERMENTATION - BIOCHEMISTRY AND MOLECULAR BIOLOGY-1. Biochemical Information... 

A brief introduction of the anaerobic mixed acid fermentation pathway of E.coli, remarking hydrogen production from glucose by pyruvate formate lyase (PFL)/FHL system is presented in Figure 1. 

Under anaerobic conditions, in the absence of alternative and much more energetically favorable electron acceptors, such as nitrate or fumarate, Ecoli utilizes the mixed acid fermentation pathway. Since the fermentation products are not only carbon dioxide and... 

The effects were most pronounced during the transformation of bacterial cells to spheroplasts. It was shown that the lower production rate of molecular hydrogen by spheroplastic cells was due not only to a suggested decrease in mixed-acid fermentation, but to a reduction in hydrogen lyase activity as well. 

The details of the process and the oxidation-reduction balance can be pictured as in Eq. 17-25. Pyruvate is cleaved by the pyruvate formate-lyase reaction (Eq. 15-37) to acetyl-CoA and formic acid. Half of the acetyl-GoA is cleaved to acetate via acetyl-P with generation of ATP, while the other half is reduced in two steps to ethanol using the two molecules of NADH produced in the initial oxidation of triose phosphate (Eq. 17-25). The overall energy yield is three molecules of ATP per glucose. The "efficiency" is thus (3 x 34.5) H- 225 = 46%. Some of the glucose is converted to D-lactic and to succinic acids (pathway/. Fig. 17-9) hence the name mixed acid fermentation. Table 17-1 gives typical yields of the mixed acid fermentation of... 

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