Hexose catabolism

Fig. 3.4 The glycolytic pathway produces NADH which under regular conditions is oxidized to NAD+ while reducing acetaldehyde (ACA) to ethanol (EtOH), thereby in turn reducing NAD+ in order to keep hexose catabolism running. The actual cytosolic NADH concentration is determined by the respective conversion rates of the enzymes involved in the oxidation and regeneration of the compound. If these enzymes convert additional non-natural substrates (xenobiotics, i.e. drugs), the conversion rate changes. As a consequence, the cytosolic NADH concentration differs from the natural condition. Furthermore, if a xenobiotic acts as an enzyme inhibitor, e.g. for ADH, then NAD+ regeneration is substantially affected, which eventually results in altered cytosolic NADH concentration. Therefore the presence of a xenobiotic in the cell is conceivably a perturbation factor. Under the conditions where glycolytic oscillations...

Hexose catabolism has been studied in detail in two thermophilic archaebacterial genera, Sulfolobus and Thermoplasma, organisms which are phenotypically close (they are both thermoacidophiles) but phylogenetically distinct. Interestingly, in both genera a further modification of the Entner-Doudoroff pathway has been found. 

The nature of the pathways of hexose catabolism (section 2) was related to the phenotypic characteristics of the organisms, and it is clear that the form of the citric acid cycle employed can be similarly correlated. The three forms of the cycle (oxidative, reductive and partial) are discussed in turn. 

From studies of eubacterial and eukaryotic metabolism, it has been previously argued that the Embden-Meyerhof glycolytic pathway is the ancient energy-conserving route of hexose catabolism [3]. However, a key enzyme of this catabolic route. 

Previous considerations of the pathways of hexose catabolism in archaebacteria have indicated that it is most sensible to consider them with respect to the phenotypes of the organisms, that is the extreme halophiles, the thermophiles and the methanogens[l,2]. This is the strategy to be adopted here, although it is reiterated that these groupings do not necessarily reflect genotypic relationships, a subject comprehensively covered in other chapters. 

Fig. 11. —Possible Routes of Hexitol and Hexose Catabolism by Yeasts (after Barnett603).

Pyruvate-dependent lyases serve catabolic functions in vivo in the degradation of sialic acids and KDO (2-keto-3-deoxy-manno-octosonate), and in that of 2-keto-3-deoxy aldonic acid intermediates from hexose or pentose catabolism. 

There is known one more catabolic route for carbohydrates commonly referred to as the pentose phosphate cycle (also called hexose mono phosphate shunt, or phosphogluconate pathway). 

The strategy employed by most cells in the catabolism of several 6-carbon sugars is to convert them to glucose 6-phosphate and, in the several steps outlined in Fig. 10-2, to cleave this hexose phosphate to two equivalent molecules of glyceraldehyde 3-phosphate. This triose phosphate can then be metabolized further. Notice the chemical nature of the reactions involved in... 

In vivo, pyruvate lyases perform a catabolic function. The synthetically most interesting types are those involved in the degradation of sialic acids or the structurally related octulosonic acid KDO, which are higher sugars typically found in mammalian or bacterial glycoconjugates [62-64], respectively. Also, hexose or pentose catabolism may proceed via pyruvate cleavage from intermediate 2-keto-3-deoxy derivatives which result from dehydration of the corresponding aldonic acids. Since these aldol additions are freely reversible, the often unfavourable equilibrium constants require that reactions in the direction of synthesis have to be driven by an excess of one of the components, preferably pyruvate for economic reasons, in order to achieve a satisfactory conversion. 

Cytosol Glycolysis, glycogenesis and glycogenolysis, hexose monophosphate pathway, fatty acid synthesis, purine and pyrimidine catabolism, aminoacyl-tRNA synthetases... 

Whilst the majority of investigations into halophilic hexose metabolism has been concerned with the catabolism of glucose, it has been recently reported [104,105] that Haloarcula vallismortis catabolises fructose via a modified Embden-Meyerhof pathway. Fructose is phosphorylated to fructose 1-phosphate via a ketokinase, and is then converted to fructose 1,6-bisphosphate via 1-phosphofructokinase. Aldol cleavage generates dihydroxyacetone-phosphate and glyceraldehyde 3-phosphate, both of which can be further metabolised via the glycolytic sequence described earlier. It remains to be established whether other halophilic archaebacteria can also catabolise fructose in this manner. 

VIII. The Catabolism of Other Hexoses 1. D-Fructose and D-Mannose... 

There are various, interrelated reasons why the abilities of yeasts to utilize pentoses and alditols should be associated. (i) Pentoses and pentitols may be reversibly interconverted by dehydrogenases (it) catabolic routes may be shared and (Hi) many of the alditol dehydrogenases have particularly wide substrate-specificity, so that a single enzyme may act on pentitols and pentoses, as well as on hexi-tols and hexoses. 

Normally, approximately 10% of glucose is catabolized through the hexose monophosphate pathway, but this fraction may be markedly increased when there is oxidative stress, as is the case when there is an infection or use of certain drugs. The principal function of the hexose monophosphate pathway is to reduce 2 moles of NADP to... 

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