|NADH dehydrogenase, subunit 5 (complex I)|
NADH-ubiquinone oxidoreductase chain 5 is a protein that in humans is encoded by the mitochondrial gene MT-ND5. The ND5 protein is a subunit of NADH dehydrogenase (ubiquinone), which is located in the mitochondrial inner membrane and is the largest of the five complexes of the electron transport chain. Variations in MT-ND5 are associated with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) as well as some symptoms of Leigh's syndrome and Leber's hereditary optic neuropathy (LHON).
MT-ND5 is located in mitochondrial DNA from base pair 12,337 to 14,148. The MT-ND5 gene produces a 67 kDa protein composed of 603 amino acids. MT-ND5 is one of seven mitochondrially-encoded subunits of the enzyme NADH dehydrogenase (ubiquinone). Also known as Complex I, it is the largest of the respiratory complexes. The structure is L-shaped with a long, hydrophobic transmembrane domain and a hydrophilic domain for the peripheral arm that includes all the known redox centres and the NADH binding site. MT-ND5 and the rest of the mitochondrially encoded subunits are the most hydrophobic of the subunits of Complex I and form the core of the transmembrane region.
MT-ND5 is a subunit of the respiratory chain Complex I that is believed to belong to the minimal assembly of core proteins required to catalyze NADH dehydrogenation and electron transfer to ubiquinone (coenzyme Q10). Initially, NADH binds to Complex I and transfers two electrons to the isoalloxazine ring of the flavin mononucleotide (FMN) prosthetic arm to form FMNH2. The electrons are transferred through a series of iron-sulfur (Fe-S) clusters in the prosthetic arm and finally to coenzyme Q10 (CoQ), which is reduced to ubiquinol (CoQH2). The flow of electrons changes the redox state of the protein, resulting in a conformational change and pK shift of the ionizable side chain, which pumps four hydrogen ions out of the mitochondrial matrix.
A small percentage of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) are caused by a G>A mutation at base pair 13513 in the MT-ND5 gene. Mutations in the MT-ND5 gene cause impaired Complex I function of the mitochondrial electron transport system, impairing those tissues that require significant energy input, such as the brain and muscles. Those with MT-ND5 mutations can display the major features of MELAS in some patients, as well as symptoms of Leigh's syndrome and/or Leber's hereditary optic neuropathy (LHON) in others.
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