Oxidative phosphorylation pathway
Under normal physiological conditions, oxidative phosphorylation (OXPHOS) produces more than 95% of a cell's energy in the form of ATP. This process involves five different protein complexes, Complex I-V.
A total number of 91 polypeptides, including cytochrome c, are directly involved in OXPHOS. These polypeptides are encoded by both nuclear and mitochondrial (mt) genes. Additional proteins are required for efficient OXPHOS, including those for stability, replication and transcription of mtDNA, the mitochondrial ribosome, those that synthesize the heme, flavin and non-heme iron co-factors and multiple proteins acting in assembly of each of the complexes.
In all, close to 300 proteins are needed for efficient OXPHOS activity.The overall process of oxidative phosphorylation is tightly controlled by transcriptional regulation at the level of DNA, translational effects via RNA levels and stability, by substrate feedback inhibition, and by post-translational modifications including phosphorylation and acetylation.
| Subunits | Polypeptides encoded on mtDNA | Polypetides encoded on nuclear DNA | |
| Complex I NADH dehydrogenase | 45 | 7 | ~40 |
| Complex II Succinate ubiquinone oxidoreductase | 4 | 0 | 4 |
| Complex III Ubiquinol Cytochrome c oxidoreductase | 11 | 1 | 10 |
| Complex IV Cytochrome c oxidoreductase | 13 | 3 | 10 |
| Complex V ATP synthase | 17 | 2 | 15 |
Disease associations with OXPHOS pathway defects
Inefficient electron transfer through complexes I-IV causes human diseases due to loss of energy metabolism and production of toxic reactive oxygen species (ROS) arising from insults to the various enzymes (particularly complexes I, II and III). Defects of complex V are also a cause of mitochondrial dysfunction.
Inborn errors of metabolism due to deficiencies in each of the 5 complexes (OXPHOS disease) account for as many as 1/5000 live births. Mutations in both nuclear and mt genes can cause OXPHOS diseases. Of these, Complex I and Complex IV deficiency are the most common while Complex III deficiencies are rare.
Many late onset human diseases, as well as the aging process itself, are now thought to involve compromised oxidative phosphorylation. Complex I dysfunction is thought to contribute to diabetes as well as several neurological disorders, including Parkinson's disease and schizophrenia. Complex II deficiencies are found in a type of cancer called paraganglioma. Genetic alterations in Complex IV are found in Alzheimer's disease, and there are reports of reduced amounts of this complex in hypoxic cancer cells.
Diverse classes of drugs have been shown to inhibit OXPHOS. The ability to monitor the amount and activities of the 5 OXPHOS complexes is paramount in diagnosing and characterizing diseases presenting with energy deficit. These assays need to be a key part of drug development and drug toxicity studies.
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