Glycolysis is a crucial metabolic pathway that converts glucose into pyruvate, yielding energy in the form of ATP and NADH. When starting with one mole of fructose 1,6-bisphosphate, glycolysis produces two moles of pyruvate, resulting in the generation of four moles of ATP and two moles of NADH. However, when glucose is the starting material, the process requires the consumption of two ATP molecules before reaching fructose 1,6-bisphosphate, leading to a net gain of only two moles of ATP and two moles of NADH per glucose molecule. This is due to the initial ATP investment by hexokinase and phosphofructokinase, the first and third enzymes in glycolysis, respectively.
In tracing the carbon atoms during glycolysis, if glucose is labeled with carbon-14 at the C1 and C6 positions, the resulting pyruvate will have its methyl carbon labeled with carbon-14, corresponding to either carbon 1 or carbon 6 from glucose. In the context of ethanol fermentation, pyruvate is converted to acetaldehyde through decarboxylation, releasing carbon dioxide, which comes from either carbon 3 or 4 of glucose. Consequently, the carbon-14 label will be retained in the methyl carbon of ethanol, specifically at the C2 position, as this corresponds to the original carbon-1 or carbon-6 from glucose.
The primary purpose of fermentation is to regenerate NAD+, which is essential for glycolysis to continue, especially in anaerobic conditions where oxygen is limited. Without the regeneration of NAD+, glycolysis would halt due to the depletion of available NAD+ as it is converted to NADH. In glycolysis, the reaction catalyzed by hexokinase is the only one that requires ATP as a substrate, converting glucose into glucose-6-phosphate. Other enzymes, such as pyruvate kinase, produce ATP rather than consume it, while aldolase does not utilize ATP at all.
Additionally, triosephosphate isomerase facilitates the isomerization of dihydroxyacetone phosphate (DHAP) into glyceraldehyde-3-phosphate (G3P), transitioning from a ketose to an aldose form. This reaction is significant in the glycolytic pathway as it allows for the continuation of energy production from glucose.
