The metabolism of carbohydrates, lipids, nucleic acids and proteins, but we have already underlined the fact that this division is rather arbitrary; it is justified mainly because it facilitates the presentation. In fact — as observed repeatedly — inter-relations between various types of compounds are numerous and the entire cellular metabolism must be regarded as a set of harmoniously integrated reactions.
These inter-relations ought to be presented in a complete overall diagram; this could indeed be done, but the diagram would be extremely complex considering that even partial diagrams comprise a large number of reactions; we will therefore give only a simplified diagram showing the broad lines of the intermediate metabolism (fig. 8-1).
From the energy point of view it is seen that formation of ATP takes place during glycolysis and especially during the electron transfers which accompany photosynthesis (photophosphorylation) and respiration (oxidative phosphorylation), processes. Oxidative phosphorylation follows the Krebs cycle where acetyl-coA resulting from glucose, fatty acids or some amino acids, is oxidized. We have seen that other amino acids can also join the Krebs cycle at various points.
Considering the main possibilities of interconversion, it is observed that:
i. Carbohydrates can be converted into fatty acids through acetyl-coA which is the real pivot of intermediate metabolism;
ii. Carbohydrates can be converted into some amino acids; this merely requires the formation of the corresponding α-keto acid which can then be aminated: pyruvic acid → alanine, oxaloacetic acid → aspartic acid, α- ketoglutaric acid → glutamic acid, 3-phosphoglyceric acid → serine, etc.,
iii. Some amino acids can be the source of carbon atoms of glucose, when their metabolism leads to an intermediate of the Krebs cycle, because oxaloacetate can give phosphoenolpyruvate and the latter leads to glucose by nucleogenesis (glucogenic amino acids);
iv. Some amino acids can also be converted into lipids, in so far as they can lead either to the formation of acetyl-coA and thereby fatty acids, or to neoglucogenesis and therefore formation of glyceraldehyde-3-phosphate which is a precursor of glycerol’,
v. There is no possibility of clear synthesis of carbohydrates from fatty acids through the Krebs cycle because, while two carbon atoms are brought by a molecule of acetyl-coA, two carbon atoms are also eliminated in the form of CO2 before the formation of the compound — oxaloacetic acid — which permits rejoining the neoglucogenesis process. But in some organisms (plants, bacteria, moulds) one finds the glyoxylic acid cycle (see fig. 5-27) which allows the formation of oxaloacetate — and therefore of carbohydrates — from the acetyl-coA resulting from fatty acids;
vi. The fatty acids can also be the precursors of some amino acids, by the same processes: either the Krebs cycle (for amino acids deriving from α-ketoglutaric acid for example), or the glyoxylic acid cycle.
vii. The biosynthesis of nucleotides — and thereby of nucleic acids — requires on the one hand, ribose-5-phosphate formed from glucose, and on the other hand, various amino acids (aspartic acid, glycine, glutamine) and one-carbon units which can also result from the metabolism of some amino acids.