levels are high and ATP levels are low, the enzyme works. If ADP levels are low, the enzyme is not active. This glucose-1,6-bisphosphate molecule is highly unstable, so that fructose bisphosphate aldose easily cleaves it into two three-carbon molecules. One is called dihydroxyacetonephosphate, while the other is glycraldehyde-3-phosphate. In the next step, an isomerase (which is a rearrangement enzyme) turns the first molecule into glyceralde-3phosphate, ending the first phase of glycolysis. In the second half of glycolysis, two small three-carbon sugar molecules start the process and energy is created in order to pay back the two ATP molecules used. This leads to a profit of 2 more ATP molecules plus NADH, which has even more energy than ATP. Glyceraldehyde-3-phosphate gets oxidized, giving up two electrons to make NADH from NAD+. A second phosphorus molecule gets added to the glyceraldehyde-3-phosphate but this does not require ATP. This is a rate limiting step because it depends on the availability of NAD+. In the seventh step, phosphoglycerate kinase donates the extra phosphate in order to form ATP from ADP. This is referred to as substrate-level phosphorylation. The end result is 3-phosphoglycerate and an ATP molecule. Then the phosphate group gets moved around to make 2-phosphoglycerate. Enolase is the ninth step. It is a dehydration reaction that forms PEP, also called phosphoenolpyruvate plus water. Finally, PEP is acted on by pyruvate kinase to make another molecule of ATP plus the end-product of glycolysis, which is pyruvate. The pyruvate kinase step is another ratelimiting step.
MITOCHONDRIAL RESPIRATION In glycolysis, ATP is made through substrate-level phosphorylation. As you can see, however, it doesn’t make much ATP and leaves behind pyruvate as an end-product. Most ATP is made in the mitochondria using oxidative phosphorylation. There is a series of reactions that occur after that, through the Krebs cycle, which has oxygen or a non-oxygen inorganic final electron acceptor, depending on the organism. There is a 137
