Difference Between Substrate Level Phosphorylation and Oxidative Phosphorylation

Main Difference – Substrate Level Phosphorylation vs Oxidative Phosphorylation

Substrate level phosphorylation and oxidative phosphorylation are two types of phosphorylation processes that occur inside living organisms. Phosphorylation refers to the transfer of phosphate group from one compound to another. Generally, the term ‘Phosphorylation’ is used to describe the formation of ATP. Organisms use energy in the form of ATP. In eukaryotes, the organelle which produces ATP is mitochondrium. But, some of the ATPs are produced inside the cytoplasm as well. The main difference between substrate level phosphorylation and oxidative phosphorylation is that substrate level phosphorylation is a direct phosphorylation of ADP with a phosphate group by using the energy obtained from a coupled reaction whereas oxidative phosphorylation is the production of ATP from the oxidized NADH and FADH₂. 

Key Areas Covered

1. What is Substrate Level Phosphorylation
      – Definition, Process, Characteristics
2. What is Oxidative Phosphorylation
      – Definition, Process, Characteristics
3. What are the Similarities Between Substrate Level Phosphorylation and Oxidative Phosphorylation
      – Outline of Common Features
4. What is the Difference Between Substrate Level Phosphorylation and Oxidative Phosphorylation
      – Comparison of Key Differences

Key Terms: Adenosine Diphosphate (ADP), Adenosine Triphosphate (ATP), Cytoplasm, Glycolysis, Krebs Cycle, Mitochondria, Oxidative Phosphorylation, Substrate Level Phosphorylation

What is Substrate Level Phosphorylation

Substrate level phosphorylation refers to a type of phosphorylation in which a phosphate group is transferred from a substrate to ADP. It can also add a phosphate group to guanosine diphosphate (GDP) in order to form a guanosine triphosphate (GTP). The phosphate group is directly removed from a substrate by a coupled reaction and transferred into ADP or GDP. An example reaction of the substrate level phosphorylation is shown in figure 1.

Figure 1: Substrate Level Phosphorylation

Substrate level phosphorylation occurs in glycolysis and Krebs cycle. Glycolysis is the first step of both aerobic respiration and anaerobic respiration. In glycolysis, two substrate level phosphorylation reactions occur, and four ATP molecules are produced. Phosphoglycerate kinase and pyruvate kinase are the two enzymes which are involved in the substrate level phosphorylation in glycolysis. Krebs cycle only occurs in the aerobic respiration. In Krebs cycle, substrate level phosphorylation occurs in the mitochondrial matrix. Two substrate level phosphorylation reactions occur in the Krebs cycle as well. Phosphoenolpyruvate carboxykinase and succinate CoA ligase are the two enzymes involved in the substrate level phosphorylation in Krebs cycle. During the Krebs cycle, 2ATPs are produced by the substrate level phosphorylation. Other than ATP, 6NADHs and 2FADH₂s are produced, and their reduction potentials are used in the generation of ATP by the oxidative phosphorylation in aerobic respiration.  

What is Oxidative Phosphorylation

Oxidative phosphorylation refers to a type of phosphorylation that uses the energy released from the electron transport chain to generate ATP. It is found in the inner membrane of mitochondria in eukaryotes. In prokaryotes, oxidative phosphorylation occurs in the plasma membrane. High energy molecules such as NADH and FADH₂ that are formed in the glycolysis, Krebs cycle, and fatty acid cycle, are oxidized back in the electron transport chain. The energy released by those molecules are used in the generation of ATP in the oxidative phosphorylation. Oxidative phosphorylation only occurs in the aerobic respiration. It produces 26 ATP per one molecule of glucose. The oxidative phosphorylation is shown in figure 2.

Figure 2: Oxidative Phosphorylation

The enzymes which are involved in the oxidative phosphorylation are ATP synthase, cytochrome reductase, cytochrome C oxidase, and NADH-Q reductase. 

Similarities Between Substrate Level Phosphorylation and Oxidative Phosphorylation

• Both substrate level and oxidative phosphorylation add a phosphate group to ADP.
• Enzymes are involved in both substrate level and oxidative phosphorylation.
• Both substrate level and oxidative phosphorylation can occur in the mitochondria.

Difference Between Substrate Level Phosphorylation and Oxidative Phosphorylation

Definition

Substrate Level Phosphorylation: Substrate level phosphorylation refers to a type of phosphorylation in which a phosphate group is transferred from a substrate to ADP.

Oxidative Phosphorylation: Oxidative phosphorylation refers to a type of phosphorylation which uses the energy released from the electron transport chain to generate ATP.

Location

Substrate Level Phosphorylation: Substrate level phosphorylation occurs in the cytoplasm and mitochondria matrix.

Oxidative Phosphorylation: Oxidative phosphorylation occurs on the inner membrane of mitochondria.

Mechanism

Substrate Level Phosphorylation: A phosphate group is directly removed from a substrate by a coupled reaction and transferred into ADP.

Oxidative Phosphorylation: Phosphate groups are added from the energy released in the electron transport chain.

Correlation

Substrate Level Phosphorylation: Substrate phosphorylation is a direct phosphorylation.

Oxidative Phosphorylation: Oxidative phosphorylation is an indirect phosphorylation.

Occurrence

Substrate Level Phosphorylation: Substrate level phosphorylation occurs in the glycolysis and Krebs cycle.

Oxidative Phosphorylation: Oxidative phosphorylation occurs in the electron transport chain.

Oxidation/Reduction of Coenzymes

Substrate Level Phosphorylation: NAD and FAD are reduced during the substrate level phosphorylation.

Oxidative Phosphorylation: NADH⁺ and FADH⁺ are oxidized during the oxidative phosphorylation.

Net ATP Production

Substrate Level Phosphorylation: Four ATPs are produced during the substrate level phosphorylation.

Oxidative Phosphorylation: Thirty four ATPs are produced during the oxidative phosphorylation.

Redox Potential

Substrate Level Phosphorylation: The change of the redox potential of the substrate is less in substrate level phosphorylation.

Oxidative Phosphorylation: The change of the redox potential of the substrate is more in oxidative phosphorylation.

Oxidation Level

Substrate Level Phosphorylation: Partial oxidation of the substrate occurs in substrate level phosphorylation.

Oxidative Phosphorylation: Complete oxidation of the electron donors occurs in the oxidative phosphorylation.

Conclusion

Substrate level phosphorylation and oxidative phosphorylation are two methods of generating ATP inside the living organisms. ATP is the major form of energy molecules used in cellular mechanisms. Substrate level phosphorylation occurs in the glycolysis and Krebs cycle. Oxidative phosphorylation occurs in the electron transport chain. Substrate level phosphorylation is a direct type of phosphorylation in which a phosphate group is directly transferred to an ADP molecule. Oxidative phosphorylation is an indirect method of phosphorylation in which the energy liberated in the electron transport chain is used in generating ATP. The main difference between substrate level phosphorylation and oxidative phosphorylation is the mechanisms of the production of ATP.

Reference:

1.“ Substrate-Level phosphorylation.” Wikipedia, Wikimedia Foundation, 6 Oct. 2017, Available here.
2.Berg, Jeremy M. “Oxidative Phosphorylation.” Biochemistry. 5th edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.

Image Courtesy:

1. “Substrate-level phosphorylation generating ATP” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
2. “Mitochondrial electron transport chain—Etc4” By Fvasconcellos 22:35, 9 September 2007 (UTC) – Vector version of w:Image:Etc4.png by TimVickers, (Public Domain) via Commons Wikimedia