16.2Respiration and Fermentation: Glycolysis

Glycolysis*3 (Figure 16-2) takes place in all living organisms. It plays a key role in various metabolic networks in cells (see Selection 4 of Chapter 4, Figure 4-3) and is the most important ATP synthesis pathway. In the pathway, ATP synthesis is catalyzed by two kinase enzymes*1. The entire ATP synthesis reaction uses glucose as the starting material and goes through two kinase reactions that add phosphate groups using ATP, an aldolase reaction that cleaves hexose to two triose molecules, addition of an inorganic phosphoric acid molecule coupled with dehydrogenation, two kinase reactions for ATP synthesis, and interconversion by isomerization (isomerase, mutase). After the hexose is cleaved to triose molecules, only one of the two triose molecules (glyceraldehyde-3-phosphate) is metabolized, whereas the other (dihydroxyacetone phosphate) must undergo isomerization before undergoing metabolism through the same pathway. The respiration and fermentation pathways up to production of pyruvic acid are the same, after which the hydrogen (of NADH*4 ) removed by dehydrogenation is added to synthesize lactic acid or alcohol (lactic acid fermentation or alcohol fermentation). In summary, a total of two ATP molecules can be synthesized from one glucose molecule. The key point of fermentation lies in the fact that ATP can be produced in the absence of oxygen, owing to no net input/output of NADH. The following equation shows changes in free energy during the conversion to two lactic acid molecules. Part of this energy is used for synthesizing two ATP molecules.

*3 Also called the Embden-Meyerhof pathway after its discoverers.

C6H12O6 → 2C3H6O3     ΔG°′ = −196kJ mol-1
(ΔG°′ represents the free-energy change in the standard state: 1 mol, 25°C, and pH 7)

*4 Nicotinamide adenine dinucleotide (reduced form), a coenzyme in redox reactions. See Selection 2 of Chapter 4 (Figure 4-1) for the structural formula.

Figure 16-2 Details of Glycolysis (including fermentation)

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