NDUFS4
NADH dehydrogenase  iron-sulfur protein 4, mitochondrial (NDUFS4) also known as NADH-ubiquinone oxidoreductase 18 kDa subunit is an enzyme that in humans is encoded by the NDUFS4 gene. This gene encodes an nuclear-encoded accessory subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (complex I, or NADH:ubiquinone oxidoreductase). Complex I removes electrons from NADH and passes them to the electron acceptor ubiquinone. Mutations in this gene can cause mitochondrial complex I deficiencies such as Leigh syndrome.

# Structure
NDUFS4 is located on the q arm of chromosome 5 in position 11.2 and has 8 exons. The NDUFS4 gene produces a 20.1 kDa protein composed of 175 amino acids. NDUFS4, the protein encoded by this gene, is a member of the complex I NDUFS4 subunit family. It is a peripheral membrane protein located on the matrix side of the inner mitochondrial membrane. NDUFS4 is a component of the iron-sulfur (IP) fragment of the enzyme and contains a transit peptide domain, 4 turns, 6 beta strands, and 4 alpha helixes. Alternative splicing results in multiple transcript variants.

# Function
Complex I, or NADH:ubiquinone oxidoreductase, the first multisubunit enzyme complex of the mitochondrial respiratory chain, plays a vital role in cellular ATP production, the primary source of energy for many crucial processes in living cells. It removes electrons from NADH and passes them by a series of different protein-coupled redox centers to the electron acceptor ubiquinone. In well-coupled mitochondria, the electron flux leads to ATP generation via the building of a proton gradient across the inner membrane. Complex I is composed of at least 41 subunits, of which 7 are encoded by the mitochondrial genome (ND1-6, ND4L) and the remainder by nuclear genes.

# Clinical significance
Mutations in the NDUFS4 gene are associated with Mitochondrial Complex I Deficiency, which is autosomal recessive. This deficiency is the most common enzymatic defect of the oxidative phosphorylation disorders. Mitochondrial complex I deficiency shows extreme genetic heterogeneity and can be caused by mutation in nuclear-encoded genes or in mitochondrial-encoded genes. There are no obvious genotype-phenotype correlations, and inference of the underlying basis from the clinical or biochemical presentation is difficult, if not impossible. However, the majority of cases are caused by mutations in nuclear-encoded genes. It causes a wide range of clinical disorders, ranging from lethal neonatal disease to adult-onset neurodegenerative disorders. Phenotypes include macrocephaly with progressive leukodystrophy, nonspecific encephalopathy, hypertrophic cardiomyopathy, myopathy, liver disease, Leigh syndrome, Leber hereditary optic neuropathy, and some forms of Parkinson disease. Complex I deficiency with autosomal recessive inheritance results from mutation in nuclear-encoded subunit genes, including NDUFV1, NDUFV2, NDUFS1, NDUFS2, NDUFS3, NDUFS6, NDUFS7, NDUFS8, NDUFA2, NDUFA11, NDUFAF3, NDUFAF10, NDUFB3, NDUFB9, ACAD9, FOXRED1, and MTFMT.

# Interactions
NDUFS4 has been shown to have 58 binary protein-protein interactions including 57 co-complex interactions. NDUFS4 appears to interact with UBE2G2.