Understanding carbon numbering is crucial in biochemical pathways, particularly when discussing the conversion of dihydroxyacetone phosphate (DHAP) to glyceraldehyde-3-phosphate (G3P). In this context, the carbon numbering reflects the original glucose molecule from which these compounds are derived. For instance, the carbons in DHAP correspond to carbons 1, 2, and 3 of glucose, while G3P corresponds to carbons 4, 5, and 6. This is important because the enzyme triosephosphate isomerase facilitates this reversible reaction, which has a delta G close to 0, indicating that it can easily proceed in either direction.
As we progress through the glycolytic pathway, it is essential to remember that each glucose molecule ultimately yields two molecules of G3P. Therefore, all subsequent reactions must be mentally doubled, even if only one molecule is illustrated. In the next step, glyceraldehyde-3-phosphate dehydrogenase catalyzes the conversion of G3P into 1,3-bisphosphoglycerate (1,3-BPG) by adding an inorganic phosphate and reducing NAD+ to NADH. This reaction also has a delta G close to 0, reinforcing its reversible nature.
Following this, phosphoglycerate kinase catalyzes the conversion of 1,3-BPG to 3-phosphoglycerate (3-PG) while generating ATP through substrate-level phosphorylation. This reaction has a negative delta G, indicating it is favorable, although it is still somewhat reversible under cellular conditions. The phosphate group that was added in the previous step is removed in this process.
Next, 3-phosphoglycerate mutase facilitates the conversion of 3-PG to 2-phosphoglycerate (2-PG) by shifting the phosphate group to a different position on the molecule. This reaction also has a delta G close to 0, allowing for reversibility. Throughout these steps, it is vital to keep track of the carbon numbering, as it reflects the original glucose molecule and helps clarify the transformations occurring at each stage.
