<P> Substrate - level phosphorylation is a metabolic reaction that results in the formation of ATP or GTP by the direct transfer of a phosphoryl (PO) group to ADP or GDP from another phosphorylated compound . </P> <P> Unlike oxidative phosphorylation, oxidation and phosphorylation are not coupled in the process of substrate - level phosphorylation, and reactive intermediates are most often gained in the course of oxidation processes in catabolism . Most ATP is generated by oxidative phosphorylation in aerobic or anaerobic respiration while substrate - level phosphorylation provides a quicker, less efficient source of ATP, independent of external electron acceptors . This is the case in human erythrocytes, which have no mitochondria, and in oxygen - depleted muscle . </P> <P> Substrate - level phosphorylation occurs in the cytoplasm of cells during glycolysis and in mitochondria either during the Krebs cycle or by MTHFD1L (EC 6.3. 4.3), an enzyme interconverting ADP + phosphate + 10 - formyltetrahydrofolate to ATP + formate + tetrahydrofolate (reversibly), under both aerobic and anaerobic conditions . In the pay - off phase of glycolysis, a net of 2 ATP are produced by substrate - level phosphorylation . </P> <P> The first substrate - level phosphorylation occurs after the conversion of 3 - phosphoglyceraldehyde and Pi and NAD+ to 1, 3 - bisphosphoglycerate via glyceraldehyde 3 - phosphate dehydrogenase. 1, 3 - bisphosphoglycerate is then dephosphorylated via phosphoglycerate kinase, producing 3 - phosphoglycerate and ATP through a substrate - level phosphorylation . </P>

Where does the most substrate level phosphorylation occur