Substrate Level Phosphorylation

∞ generated and posted on 2016.12.04 ∞

Donation of inorganic phosphate (Pi) from specific organic compounds to ADP to form ATP.

Substrate-Level Phosphorylation is simply the enzymatic transfer of a phosphate group from an organic molecule to adenosine diphosphate (ADP) to form adenosine triphosphate (ATP).

The production of ATP from ADP requires addition, via a condensation reaction, of a single phosphate (PO43-) to the latter. As with any condensation reaction, this process requires an input of energy. In the case of substrate-level phosphorylation, this energy is supplied by the removal of the same phosphate group from a phosphorylated organic molecule.

The process of substrate-level phosphorylation is conceptually simple. In practice, however, it is complex because specific individual chemical reactions must be catalyzed by specific individual enzymes to produce ATP.

Figure legend: With substrate-level phosphorylation a specific enzyme, other than the ATP synthase involved in chemiosmosis, catalyzes the phosphorylation of ADP to generate ATP. There are two enzymes associated with glycolysis where such reactions take place – phosphoglycerate kinase and pyruvate kinase – and one associated with the Kreb's cycle, succinyl coenzyme A synthase. In the latter it is GTP rather than ATP that is immediately produced though the GTP is then converted to ATP.

Examples of substrate-level phosphorylation are the removal of inorganic phosphates from 1,3-biphosphoglycerate or phosphoenolpyruvate to form 3-phosphoglycerate or pyruvate, respectively, as well as ATP. Those substrate-level phosphorylation steps are seen in glycolysis, and substrate-level phosphorylation is also seen in the Kreb's citric acid cycle.

Key is for the starting, phosphorylated compounds, along with ADP, to possess more free energy than the same compound minus the phosphate group plus ATP.

Substrate-level phosphorylation is simply an enzymatically catalyzed chemical reaction, albeit one that yields ATP. By contrast is phosphorylation that is effected via chemiosmotic processes such as what is described as oxidative phosphorylation. The latter is a process in which ATP production involves far more individual molecular players including an electron transport chain, the generation of a proton-motive force (which requires an intact lipid bilayer and associated cellular compartmentalization), and a complex enzyme known as ATP synthase.