Energy-yielding metabolism

Four of the B vitamins are essential in the citric acid cycle and therefore in energy-yielding metabolism (1) riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor in the a-ketoglutarate dehydrogenase complex and in succinate dehydrogenase (2) niacin, in the form of nicotinamide adenine dinucleotide (NAD),... 

ViTAMiN B2 (RiBOFLAViN) HAS A CENTRAL ROLE iN ENERGY-YIELDING METABOLISM... 

As noted earlier, although the citric acid cycle is central to energy-yielding metabolism its role is not limited to energy conservation. Four- and five-carbon intermediates of the cycle serve as precursors for a wide variety of products. To replace intermediates removed for this purpose, cells employ anaplerotic (replenishing) reactions, which are described below. 

The same intermediates of glycolysis and the citric acid cycle that activate isocitrate dehydrogenase are allosteric inhibitors of isocitrate lyase. When energy-yielding metabolism is sufficiently fast to keep the concentrations of glycolytic and citric acid cycle intermediates low, isocitrate dehydrogenase is inactivated, the inhibition of isocitrate lyase is relieved, and isocitrate flows into the glyoxylate pathway, to be used in the biosynthesis of carbohydrates, amino acids, and other cellular components. 

The thyroid hormones act through nuclear receptors to stimulate energy-yielding metabolism, especially in liver and muscle, by increasing the expression of genes encoding key catabolic enzymes. 

Gancedo C, Serrano R (1989) Energy-yielding metabolism. In Rose AH, Harrisin JS (eds) The yeast, vol. 3. Academic Press, New York, p 205... 

Riboflavin has a central role as a redox coenzyme in energy-yielding metabolism and a more recently discovered role as the prosthetic group of the cryptochromes in the eye - the blue-sensitive pigments that are responsible for day-length sensitivity and the setting of circadian rhythms. 

Because of the central role of flavin coenzymes in energy-yielding metabolism, reference intakes are sometimes calculated on the basis of energy intake 0.6 to 0.8 mg per 1,000 kcal (0.14 to 0.19 mg per MJ). However, in view of the wide range of riboflavin-dependent reactions, in addition to energy-yielding metabolism, it is difficult to justify this basis for the calculation of requirements. 

The other characteristic feature of pellagra is the development of a depressive psychosis, superficially similar to schizophrenia and the organic psychoses, but clinically distinguishable by the sudden lucid phases that alternate with the most florid psychiatric signs. The mental symptoms may be the result of tryptophan depletion, and hence a lower availability of tryptophan for synthesis of the neurotransmitter serotonin (5-hydroxytryptophan). But the role of cADP-ribose and NAADP in controlling calcium release in response to neurotransmitters (Section 8.4.4) and impaired energy-yielding metabolism in the central nervous system as a result of depletion of NAD (P) may also be important. 

In view of the central role of the nicotinamide nucleotides in energy-yielding metabolism, and the fact that, at least in theory, the nicotinamide released by ADP-ribosyltransferase (Section 8.4.2) and poly(ADP-ribose) polymerase (Section 8.4.3) is available to be reutilized for nucleotide synthesis (although this may not occur when these enzymes are significantly activated), niacin requirements are conventionally calculated on the basis of energy expenditure. 

Pantothenic acid has a central role in energy-yielding metabolism as the functional moiety of coenzyme A (CoA), in the biosynthesis of fatty acids as the prosthetic group of acyl carrier protein, and through its role in CoA in the mitochondrial elongation of fatty acids the biosynthesis of steroids, porphyrins, and acetylcholine and other acyl transfer reactions, including postsynthetic acylation of proteins. Perhaps 4% of all known enzymes utilize CoA derivatives. CoA is also bound by disulfide links to protein cysteine residues in sporulating bacteria, where it may be involved with heat resistance of the spores, and in mitochondrial proteins, where it seems to be involved in the assembly of active cytochrome c oxidase and ATP synthetase complexes. 

In general, NAD+ is involved as an election acceptor in energy-yielding metabolism, and the resultant NADH is oxidized by the mitochondrial electron transport chain. The major coenzyme for reductive synthetic reactions is NADPH. An exception here is the pentose phosphate pathway (see Figure 6.4), which reduces NADP+ to NADPH and is the source of about half the reductant for lipogenesis. 

A third kind of sugar-uptake system occurs in bacteria (for reviews, see Refs. 158-162), namely group translocation, and some authorities consider that this occurs in yeasts, too.163 In certain bacteria, uptake of sugar is coupled to its phosphorylation and to energy-yielding metabolism it involves the following overall reaction. 

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