Home >> Science & Nature >> Science

What Is Substrate-Level Phosphorylation?

Adenosine triphosphate, or ATP, is a little like currency for your cells. When the cells carry out activities that require expenditure of energy, they have to spend some of their ATP. Just as you have to make money so you can pay your bills, your cells must produce ATP so they can cover their expenses. They have a couple of different ways they can do so; one of these is substrate-level phosphorylation.

Energetics
- Making ATP takes energy. Your cells get the energy required to make ATP from energy-rich molecules like glucose, breaking down the glucose through an elegant series of reactions. The energy released is harnessed to add a phosphate group to adenosine diphosphate or ADP and turn it into ATP. In substrate-level phosphorylation, ADP and one or more intermediates from the breakdown of glucose or similar compounds are brought together, and a phosphate group is transferred from the intermediate to the ADP.
Enzymes
- This transfer doesn't happen without some help, of course. Enzymes are protein catalysts, giant molecules that speed up important reactions in your cells. The molecule that gets altered by an enzyme is called its substrate. The enzymes that catalyze substrate-level phosphorylation are a little like a real estate broker arranging a sale; they bring buyer and seller together and make sure the deal goes smoothly. In one such reaction, for example, an enzyme called pyruvate kinase catalyzes the transfer of a phosphate from a molecule called phosphoenolpyruvate to another molecule called ADP.
Reversibility
- The reactions catalyzed by enzymes are reversible, meaning they can go both ways. Just as pyruvate kinase can catalyze phosphate transfer from phosphoenolpyruvate to ADP, it can also catalyze phosphate transfer from ATP back to pyruvate again. The direction in which the reaction runs depends on the concentrations of the reactants and the products. That's one reason why it's important for your cells to closely regulate metabolic pathways so it will have the molecules it needs available at the right time.
Oxidative Phosphorylation
- Substrate-level phosphorylation takes place during glycolysis and also during the citric acid cycle, two of the key pathways in metabolic biochemistry. It's not the only way your cells can make ATP, however. Inside the mitochondria of your cells, ATP is also produced through oxidative phosphorylation. Ultimately, most of your ATP comes from this source rather than substrate-level phosphorylation. That's why hydrogen cyanide is such a deadly poison -- it latches onto an important enzyme in your mitochondria so that oxidative phosphorylation can no longer occur. By itself, substrate-level phosphorylation isn't enough to supply all of the ATP your cells need.

Science