Anabolism: Metabolic Pathways and Carbohydrate Biosynthesis

Anabolism: The Constructive Phase of Metabolism

Anabolism is the constructive phase of metabolism. It is the set of metabolic pathways whose objective is to obtain more or less complex organic molecules, sharply reduced, from simpler and relatively oxidized ones. Therefore, these are chemical reduction processes.

Features of Anabolic Pathways

• As we have said, they are basically chemical reduction processes.
• Reactions are strongly endergonic (energy is needed), for which they use the energy released in catabolic reactions as ATP, NADH, and NADPH.
• The anabolic pathways for the synthesis of molecules are different from the catabolic ones, although they often share reversible reactions close to equilibrium. There is always some different way on each route.
• The enzymes that regulate the anabolic and catabolic routes are different.
• Most anabolic routes will take place in the hyaloplasm, although precursor substances come from catabolism generated in different organelles:
  • REL: Phospholipids and cholesterol
  • RER: Protein glycosylation
  • Golgi Apparatus: Glycosylation of proteins and lipids
  • Ribosomes: Proteins
  • Nucleus: Nucleic acids

Anabolism in Autotrophs and Heterotrophs

In autotrophs and heterotrophs, most reactions of anabolism are common. They are the routes of heterotrophic anabolism.

The difference between autotrophic and heterotrophic anabolism lies in the origin of the precursor molecules. In heterotrophic catabolism, they come from the immediate early ingested and cell reserves themselves (this also occurs in cells of autotrophic organisms). Whereas in autotrophic anabolism, in addition, simple precursors are synthesized from inorganic molecules (CO₂, H₂O) through the processes of photosynthesis and chemosynthesis.

Biosynthesis of Carbohydrates: Gluconeogenesis

The basic route for the synthesis of carbohydrates from different non-glucidic precursors is Gluconeogenesis (formation of new glucose). It consists of the biosynthesis of glucose from non-hexose precursors.

It is a key route that occurs in all animals, plants, fungi, and microorganisms. In animals, the precursors include lactate, pyruvate, and glycerol, formed from glycerine in the catabolism of triglycerides and phosphoglycerides.

In plants, besides those mentioned in animals, in photosynthesis by CO₂ fixation in the Calvin cycle, where phosphoglycerate is formed (autotrophic anabolism); in vegetable sprouts from fatty acids and proteins (in glyoxysomes from acetyl-CoA).

The Role of Glyoxysomes in Carbohydrate Synthesis

In glyoxysomes, triacylglycerols stored in seeds are oxidized to acetyl-CoA and dihydroxyacetone phosphate during germination. Acetyl-CoA obtained from the ?-oxidation in glyoxysomes by the so-called glyoxylate cycle is converted into succinate. This succinate goes to the mitochondria and integrates into the Krebs cycle, where it is converted to oxaloacetate. The oxaloacetate leaves the mitochondria to the cytosol, where the path through gluconeogenesis produces glucose and other sugars.