Carbohydrate metabolism anaerobic

The main product of anaerobic degradation of sugars by these organisms is lactic acid. Other products of bacterial carbohydrate metabolism include extracellular dextrans (see p. 40)—insoluble polymers of glucose that help bacteria to protect themselves from their environment. Bacteria and dextrans are components of dental plaque, which forms on inadequately cleaned teeth. When Ca salts and other minerals are deposited in plaque as well, tartar is formed. 

Anaerobic Oxidation of ducose. Historically, the first system of carbohydrate metabolism to be studied was the conversion by yeast of glucose to alcohol (fermentation) according to the equation CnH,Of,2CH)CH,OH + 2CO . The biochemical process is complex, involving the successive catalytic actions of 12 enzymes and known as the Emhden-Meyerhof pathway This series of reactions is summarized in the entry on Glycolysis. 

Van Oordt, B.E., Tielens, A.G. and van den Bergh, S.G. (1989) Aerobic to anaerobic transition in the carbohydrate metabolism of Schistosoma mansoni cercariae during transformation in vitro. Parasitology 98, 409-415. 

In cestodes the major pathways of carbohydrate metabolism, described above, are anaerobic, although a number of the key reactions involved occur inside the mitochondria. Nevertheless, all species examined so far appear to utilise oxygen, at least in vitro. This raises two important questions is oxygen uptake accompanied by ATP synthesis, and, if oxidative phosphorylation occurs, what is its overall contribution to the energy balance of the cestode ... 

Lactic acid, present in blood entirely as lactate ion (pK = 3.86), is an intermediate of carbohydrate metabolism and is derived mainly from muscle cells and erythrocytes (see Chapter 25). It represents the end product of anaerobic metabolism and is normally metabolized by the liver. The blood lactate concentration is, therefore, affected by the rate of production and the rate of metabolism, both of which are dependent on adequate tissue perfusion. An increase in the concentration of lactate to >2 mmol/L and the associated increased is considered lactic acidosis. 

Studies of the carbohydrate metabolism of T. cruzi (21) have shown that phos-phoenolpyruvate serves as the acceptor of the primary COj-fixation reaction. This resulted in the formation of oxaloacetate and malate and the excretion of succinate. The central role of PEPCK in energy metabolism in insect-stage trypanosomatids has been illustrated in the case of T. cruzi epimastigotes, using 3-mercaptopicolinic aeid, a powerful inhibitor of this enzyme (22). Inhibition led to a twofold reduction in the anaerobic production of succinate and a similar decrease in glucose consumption, while the production of alanine, via the transamination of pyruvate, increased threefold. 

ANAEROBIC CARBOHYDRATE METABOLISM Yeasts growing in media containing high concentrations of fermentable carbohydrate invariably metabolize it fermentatively to produce ethanol and CO2. If air is present, and when the sugar concentration has been lowered, the ethanol is respired using the metabolic routes described above. Under the anaerobic conditions of a brewery fermentation the hexoses derived from wort fermentable carbohydrates are catabolized by the EMP pathway (Fig. 17.2) to pyruvic acid. The pyruvate produced is decarboxylated by the enzyme pyruvate decarboxylase, with the formation of acetaldehyde and CO2. The enzyme requires the cofactor thiamine pyrophosphate (TPP) for activity and the reaction is shown in Fig. 17.10. The acetaldehyde formed acts (in the absence of the respiratory chain) as an electron acceptor and is used to oxidize NADH with the formation of ethanol ... 

Fig. 17.11 Summary of carbohydrate metabolism in yeast growing anaerobically, i.e. in fermentation. The dotted lines represent the oxidative pathway for the formation of succinate (and possibly fumarate and malate) from pyruvate. The alternative reductive pathway is indicated by solid lines. 

Transketolase (TK) is involved in anaerobic carbohydrate metabolisms such as the nonoxidative phase of the pentose phosphate pathway. In plants and photosynthetic bacteria, TK is involved in the Calvin-Benson cycle. TK catalyses the transfer of a 2-carbon dihydroxyethyl group from a ketose phosphate (donor substrate such as D-xylulose 5-phosphate) to the Cl position of an aldose phosphate (acceptor substrate such as o-ribose 5-phosphate) (Figure 4.3) (Schneider and Lindqvist 1998). The first product is an aldose phosphate released from the donor (such as glyceraldehyde 3-phosphate) and the second is a ketose phosphate (such as sedoheptulose 7-phosphate), in which the 2-carbon fragment is attached to the acceptor. Examples of the substrates and the products mentioned above are for the first reaction of the pentose phosphate pathway. In the second reaction of the same pathway, the acceptor is D-ery-throse 4-phosphate and the second product is o-fructose 6-phosphate. A snapshot X-ray crystallographic study revealed that an ot-carbanion/enamine a,p-dihydroxyethyl ThDP is formed as a key intermediate (Fiedler et al. 2002). Then, a nucleophilic attack of the a-carbanion intermediate on the acceptor substrate occurs. 

Phosphate is again present in an energy-rich form (namely the enol ester). It can be transferred by phosphopyruvate kinase to ADP this transfer affords pyruvic acid, which is the most important metabolite of both anaerobic and aerobic carbohydrate metabolism. 

The preceding paragraphs undoubtedly have revealed the complicated and diverse nature of carbohydrate metabolism, both on the level of interconversions among the carbohydrates and on that of degradative reactions for the production of energy. Part of the energy is derived anaerobically by substrate-linked phosphorylation the major part, however, is liberated in the respiratory chain. The situation is further complicated by the obvious fact that carbohydrate metabolism is not an isolated system of reactions, but is closely tied to other pathways and reaction cycles through common intermediates. A separate chapter (Chapt. XVIII) is devoted to such interrelationships. 

Helminths are characterized by a high rate of carbohydrate metabolism associated with incomplete substrate oxidation. This is the case whether they live anaerobically (as intestinal worms do) or aerobically (like schistosomes). The Meyerhof sequence is the major metabolic pathway in worms for the utilization of carbohydrate. Trehalose (see above) plays an important part in helminth carbohydrate metabolism. For a review of helminth biochemistry, see von Brand (1974). 

Metabolic Functions. The formation of phosphate esters is the essential initial process in carbohydrate metaboHsm (see Carbohydrates). The glycolytic, ie, anaerobic or Embden-Meyerhof pathway comprises a series of nine such esters. The phosphogluconate pathway, starting with glucose, comprises a succession of 12 phosphate esters. 

---