Phosphorylation: Definition, Types & Steps

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  • 0:00 What Is Phosphorylation?
  • 0:59 Types of Phosphorylation
  • 1:26 Substrate-Level…
  • 2:28 Oxidative Phosphorylation
  • 3:04 Prepping Glucose for…
  • 3:56 Lesson Summary
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Lesson Transcript
Instructor: Anthony Grattini

Tony has a BA in Biology and has taught secondary Life, Earth, and Physical Science, as well as Honors & AP Chemistry.

Phosphorylation, or the addition of inorganic phosphate, is an important process within a biological system. In this article we will define phosphorylation, explore several types, and go over the steps involved in each.

What Is Phosphorylation?

Before defining phosphorylation, let's briefly discuss metabolism, which is the sum of all the biochemical processes occurring within a living system.

The metabolic process breaks down organic matter to release energy, which is then used to build up cellular components. The biomolecules that facilitate this work of building, balancing, and breaking down things are proteins - the workhorses of the cell.

A number of modifications made to proteins serve to either activate, deactivate, or direct the function of that protein. These are known as post-translational modifications (PTMs), the most common of which is phosphorylation.

Phosphorylation is the process through which a phosphate group is transferred from one molecule to a protein, usually within a biological system. A protein being phosphorylated is like drinking a Red Bull before a workout - it prepares a molecule for some specialized task.

Types of Phosphorylation

There are many types of phosphorylation:

  • Some involve the transfer of phosphate to protein.
  • Others consist of the production of adenosine triphosphate (ATP) by phosphorylation of adenosine diphosphate (ADP).
  • A third phosphorylation type helps to maintain the balance of blood sugar within the body and promote metabolic processes. While there are many more types, we will cover these three in further detail.

Substrate-Level Phosphorylation

Substrate-level phosphorylation includes the transfer of inorganic phosphate via a donor molecule called guanosine triphosphate (GTP) to ADP in order to form ATP. Such a reaction takes place in the 5th step of the Krebs cycle.

In this reaction, an enzyme known as succinyl coenzyme A synthetase facilitates the production of ATP from succinyl coenzyme A, inorganic phosphate, and ADP.

  1. The first step is the displacement of coenzyme A by inorganic phosphate to form succinyl phosphate.
  2. The phosphate is then removed, forming succinate and GTP.
  3. In the final step, the phosphate is transferred to ADP to produce the high-energy ATP.

Only 2 molecules of ATP are formed from each molecule of glucose in substrate-level phosphorylation.

Oxidative Phosphorylation

In contrast to this process, oxidative phosphorylation produces about 34 ATP. Oxidative phosphorylation uses the downward flow of electrons to pump hydrogen ions uphill.

The energy from the flow of these hydrogen ions back downhill is used to stick a phosphate group onto a molecule of ADP, forming ATP. ATP can then prime other molecules within the cell to do work by giving its phosphates away. This process, like substrate-level phosphorylation, occurs within a mitochondrion.

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