<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> <P> The second substrate - level phosphorylation occurs by dephosphorylating phosphoenolpyruvate, catalyzed by pyruvate kinase, producing pyruvate and ATP . </P> <P> During the preparatory phase, each 6 - carbon glucose molecule is broken into two 3 - carbon molecules . Thus, in glycolysis dephosphorylation results in the production of 4 ATP . However, the prior preparatory phase consumes 2 ATP, so the net yield in glycolysis is 2 ATP. 2 molecules of NADH are also produced and can be used in oxidative phosphorylation to generate more ATP . </P>

Define the concept of substrate level phosphorylation reactions such as the one found in krebs cycle