Biochemistry - Chemical Reactions in the Living Cell, Volume 2 - D. Metzler 1980

Organization of Metabolism: Catabolic Pathways
Catabolism of Sugars
ATP Generation Coupled to Substrate Oxidation

The formation of ATP from ADP and P_i is a vital process for all Cells. This process is commonly referred to as "phosphorylation" and is subdivided into: 1) Oxidative Phosphorylation, which is coupled to electron flow along the Electron Transport Chain—typically occurring in Cell/35.html">Mitochondria; 2) photosynthetic phosphorylation, a similar process taking place in METABOLISM/14.html">Chloroplasts driven by light; and 3) substrate-level phosphorylation. The chemical mechanism is fully understood only for this latter case. The best-studied example is The oxidation of glyceraldehyde 3-phosphate coupled with ATP formation (reactions 6 and 7 in Figs. 9-7 and 8-13). The accompanying dehydrogenation of glyceraldehyde 3-phosphate is critically important for Yeast and many other microorganisms that thrive under anaerobic conditions and rely entirely on this reaction as their energy source. Since only two molecules of ATP are generated per hexose molecule¹⁾, it is hardly surprising that yeast must ferment massive amounts of sugar.

In aerobic Tissues, the reoxidation of NADH is carried out by the Components of the Mitochondrial Electron Transport chain (Chapter 10).

¹⁾ The conversion of glucose into lactate, or into ethanol and CO₂, ultimately results in the net synthesis of two ATP molecules. It is most logical to assume these are generated during the oxidation of glyceraldehyde 3-phosphate. Meanwhile, the formation of ATP from PEP and ADP at step 10 (Fig. 9-7) can be viewed as replenishing the ATP "spent" on the priming reactions. Note that the ATP yield from the conversion of Glycogen glucose residues into Pyruvate is three molecules. However, incorporating each glucose residue into glycogen previously required an investment of two ATP molecules (Eq. 11-24). Consequently, the net yield from the Fermentation of stored polysaccharide is only a single ATP molecule per hexose molecule.