What Is Substrate-Level Phosphorylation?

A. Reed

Substrate-level phosphorylation encompasses certain chemical reactions that occur in human cells during glycolysis, the conversion of glucose leading to the production of two high energy molecules, referred to as adenosine triphosphate (ATP). This is accomplished through the chemical transfer of a phosphate group from each of two phosphoenolpyruvate (PEP) molecules, forming ADP, which is then transformed into ATP. In addition to ATP, glycolysis also yields two molecules of nicotinamide adenine dinucleotide (NADH) in reduced form and pyruvate, which is put into the next phase of cellular respiration.

Mitochondria use the by-products of substrate-level phosphorylation to produce ATP.
Mitochondria use the by-products of substrate-level phosphorylation to produce ATP.

ATP is the chief energy molecule used by cells, driving all processes occurring within it, of which substrate-level phosphorylation has a small, although important, role and is actually one of two ways ATP is produced from ADP in humans. Oxidative phosphorylation is the other mechanism necessary for making energy, most of which happens inside of the mitochondria of the cell. Often referred to as the powerhouse of the cell, the mitochondria is the organelle in which all stages of cellular respiration occur, except glycolysis. All steps in glycolysis, including substrate-level phosphorylation, happen in the cell's cytosol, fluid containing all cellular components such as the nucleus and ribosomes.

ATP is the chief energy molecule used by cells, driving all processes occurring within it.
ATP is the chief energy molecule used by cells, driving all processes occurring within it.

Cellular respiration in humans occurs aerobically and consists of four stages of reactions through which food is converted into ATP. Glycolysis is the beginning of the process of which substrate-level phosphorylation is the last step. Next, pyruvate from glycolysis is used to form acetyl coenzyme A, from which the waste product carbon dioxide is released. With the Krebs cycle, part of the coenzyme is used to make yet another chemical called citrate, more carbon dioxide is released as ATP, NADH, and another energy-yielding molecule referred to as flavin adenine dinucleotide (FADH2) are also end products. The last of these stages is the electron transport chain and chemiosmosis, by which energy taken from glucose, NADH, and FADH2 is used in the movement of hydrogen ions across the membrane of the mitochondria, as well as for the production of more ATP.

Pyruvate kinase is the enzyme responsible for catalyzing substrate-level phosphorylation. Other chemical reactions of glycolysis and subsequent cellular respiration phases involve the action of a certain enzyme, a protein necessary for controlling the speed of reactions, which is very important for meeting energy requirements of the human body, as it takes only a minute to consume all available ATP. Once the reaction is complete, the enzyme is recycled and used again.

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Discussion Comments


@burcidi-- Those are good points.

If I remember correctly from bio classes, there are three or so ways that ATP can be made/broken down. Glycolysis and oxidative phosphorylation is two, and then there is also the citric acid cyle (also caled the Kreb's cycle).

Substrate-level phopshorylation happens in both glycolysis and the Kreb's cycle. The only difference is that the Kreb cycle is also in the mitochondria and glycolysis is not (it's in the cytoplasm).

You might also want to specify for those who don't know, that the 2 ATP from glycolysis is per 1 glucose molecule.

It had been a long time since I studied this, it's been a good review!


@ddljohn-- Yes. But keep in mind that substrate-level phosphorylation is not the only way that cells make energy. This is just how glycolysis happens outside of the mitochondria.

Glycolysis (substrate-level phosphorylation) only produces 2 ATP at a time. Aerobic respiration (oxidative phosphorylation) on the other hand produces over 30 at a time. So there is no way that glycolysis could fulfill all of the energy needs of the cell by itself.

The other point is that substrate-level phosphorylation doesn't just turn ADP into ATP; it also transforms ATP into ADP. When you also consider that, substrate-level phosphorylation is going on all the time.


The last paragraph of the article is really interesting. If our cells use up all of the available ATP in only one minute, cellular respiration, substrate-level phosphorylation and ATP production must be going on constantly in the body, right?

Does this process every take a break, when we're sleeping for example? Or are we doing this all the time to maintain our life?

If the latter is true, then it's pretty amazing that our cells can do this without fail from our birth to death, don't you think?

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