Amphibolic Pathway

Respiration is very often introduced, and thought of, purely as a catabolic pathway -- one whose entire purpose is to break down complex organic molecules such as glucose into simpler ones, releasing usable energy as ATP in the process. While this catabolic, energy-releasing role is certainly central to respiration, describing the pathway as purely catabolic is, on closer examination, an incomplete picture, because glycolysis and the TCA cycle also serve, simultaneously, as a starting point for a great deal of the cell's biosynthetic (anabolic) activity. A pathway that serves both breakdown and biosynthesis in this dual way is described as amphibolic, from the Greek prefix amphi-, meaning 'both' or 'on both sides.'

The respiratory pathway's amphibolic character becomes clear when the substrates it can accept, and the products it can supply, are considered more broadly than glucose oxidation alone. Fats stored in a plant's seeds or other tissues, for instance, are first broken down into their constituent fatty acids and glycerol; the glycerol component is readily converted into dihydroxyacetone phosphate, one of the very intermediates already generated partway through glycolysis, allowing it to enter the respiratory pathway directly at that point, while the fatty acid component undergoes a separate process called beta-oxidation that successively cleaves it into two-carbon fragments, each of which is released as acetyl CoA and can enter the TCA cycle exactly as the acetyl CoA derived from pyruvic acid does. Proteins, similarly, are broken down into their constituent amino acids, which are then deaminated (the nitrogen-containing amino group is removed), leaving behind a carbon skeleton -- a keto acid -- that, depending on the particular amino acid, corresponds to pyruvic acid, oxaloacetic acid, alpha-ketoglutaric acid, or one of several other respiratory intermediates, and so can likewise be fed directly into glycolysis or the TCA cycle for complete oxidation. In this direction, then, the respiratory pathway functions as a common final route through which fats and proteins, not just carbohydrates, can be broken down and their energy released -- underscoring its catabolic role. …