5 Pyruvate dehydrogenase and the citric acid cycle
As discussed in the preceding chapter, degradation of one mole of glucose to pyruvate via anaerobic glycolysis only yields two moles of ATP. A much higher yield can be obtained by subsequent complete, oxidative degradation of pyruvate to CO2 and H2O, which is accomplished by pyruvate dehydrogenase, the citric acid cycle, and the respiratory chain (Figure 5-1). These transformations all occur in the mitochondria, while glycolysis occurs in the cytosol. Therefore, before pyruvate can be completely degraded, it needs to be transported from the cytosol to the mitochondrial matrix, across the two mitochondrial membranes. While the outer mitochondrial membrane has non-specific pores that allow free permeation of small metabolites (much like the outer membrane of a gram-negative bacterium), the inner membrane is highly selective and only permits passage of those metabolites for which specific carrier systems exist. Pyruvate is shuttled into the mitochondrion by a specific carrier system in exchange for hydroxide (OH-).