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In this article we will discuss about the interrelations between carbohydrates and lipid metabolisms.
Transformation of Carbohydrates into Lipids:
In most animals and in man, when dietary carbohydrates are in excess, the oxidation of a part of glucose maintains relatively high concentrations of ATP, NADH and NADPH; this will promote:
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i. The synthesis of glycogen from glucose, which requires UTP and therefore ATP;
ii. The reduction of dihydroxyacetone-phosphate to L-α-glycerophosphoric acid, which requires NADH.
iii. The condensation of molecules of acetyl-coenzyme A for the biosynthesis of fatty acids and cholesterol, requiring ATP and NADPH.
When the maximum glycogen storage capacities are reached, the last two pathways are particularly favoured, thus leading to the synthesis of lipids and especially triglycerides which, as seen above, form an extremely convenient energy reserve.
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Transformation of Lipids into Carbohydrates:
In animals, lipids are not converted into carbohydrates, at least not in large proportions. The oxidative decarboxylation of pyruvic acid to acetyl-coenzyme A is indeed not reversible.
It is true that the carbon atoms of acetyl-coenzyme A (originating for example from the oxidation of fatty acids) can, after entering the Krebs cycle, be found in oxaloacetic acid, then in phosphoenolpyruvic acid and be incorporated in glucose thanks to neoglucogenesis, The incorporation takes place only after one complete turn of the cycle, (comprising 2 decarboxylations) and one cannot therefore speak of actual synthesis of glucose.
On the contrary, in plants this conversion takes place, especially in grains rich in lipids, which can be rapidly transformed into carbohydrates during germination; it also takes place in some microorganisms.
The key-reaction of this transformation is the condensation of acetyl-coenzyme A on glyoxylic acid, catalyzed by malic acid synthetase; here also, acetyl-coenzyme A binds by its methyl group to a carbonyl (as it happens during its condensation on oxaloacetic acid for its entry into the Krebs cycle and as in the synthesis of β-hydroxy-β-methyl-glutaryl-coenzyme A). As for glyoxylic acid, it results from the scission of isocitric acid by isocitratase. These two reactions are represented in figure 5-26.
The cycle of glyoxylic acid is diagrammatically shown in figure 5-27. The acetyl-coenzyme A resulting from the oxidation of fatty acids can enter the cycle at two places, either by condensation with glyoxylic acid (reaction described above), or by condensation with oxaloacetic acid (reaction identical to the reaction of entry into the Krebs cycle).
Each turn of the cycle sees the formation of one molecule of succinic acid at the cost of 2 molecules of acetyl-coenzyme A. Succinic acid is oxidized to oxaloacetic acid by a series of reactions which we examined while studying the Krebs cycle, and oxaloacetic acid can give phospho-enol-pyruvic acid by decarboxylation and phosphorylation; the latter acid can be converted into glucose-6-phosphate by the pathway of neoglucogenesis.
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Four molecules of acetyl-coenzyme A (these will give 2 molecules of succinic acid and therefore 2 molecules of oxaloacetic acid which will yield 2CO2 + 2 molecules of phospho-enol-pyruvic acid) are needed to obtain one molecule of glucose.
In plants, the glyoxylic cycle proceeds in the glyoxysomes. These structures also contain all the enzymes of peroxysomal β-oxidation. The acetyl-coenzyme A provided by this mechanism is directly utilized by the glyoxylic cycle.
