Biology:GOT2

Short description: Mitochondrial enzyme involved in amino acid metabolism

Generic protein structure example

Aspartate aminotransferase, mitochondrial is an enzyme that in humans is encoded by the GOT2 gene. Glutamic-oxaloacetic transaminase is a pyridoxal phosphate-dependent enzyme which exists in cytoplasmic and inner-membrane mitochondrial forms, GOT1 and GOT2, respectively. GOT plays a role in amino acid metabolism and the urea and Kreb's cycle. Also, GOT2 is a major participant in the malate-aspartate shuttle, which is a passage from the cytosol to the mitochondria. The two enzymes are homodimeric and show close homology. GOT2 has been seen to have a role in cell proliferation, especially in terms of tumor growth.

Structure

GOT2 is a dimer containing two identical subunits that hold overlapping subunit regions. The top and sides of the enzyme are made up of helices, while the bottom is formed by strands of beta sheets and extended hairpin loops. The subunit itself can be categorized into four different parts: a large domain, which binds pyridoxal-P, a small domain, an NH2-terminal arm, and a bridge across two domains, which is formed by residues 48-75 and 301-358. Virtually ubiquitous in eukaryotic cells, GOT2 nucleic acid and protein sequences are highly conserved, and its 5’regulatory regions in genomic DNA resemble those of typical house-keeping genes in that, e.g., they lack a TATA box. The GOT2 gene is also located on 16q21 and has an exon count of 10.

Function

In order to produce the energy needed for everyday activities, our body needs to go through the process of glycolysis, which breaks down glucose into pyruvate. In this pathway, one very important part is the reduction of NAD+ to NADH and then the rapid oxidation of NADH back into NAD+. The oxidation phase mainly occurs in the mitochondria as part of the electron transport chain, but the transfer of NADH into the mitochondria from the cytosol is impossible, due to the impermeability of the inner mitochondrial membrane to NADH. Therefore, the malate-aspartate shuttle is needed to transfer reducing equivalents across the mitochondrial membrane for energy production. GOT2 and another enzyme, MDH, are essential for the functioning of the shuttle. GOT2 converts oxaloacetate into aspartate by transamination. This aspartate as well as alpha-ketoglutarate return into the cytosol, which is then converted back to oxaloacetate and glutamate, respectively.

Another function of GOT2 is that it is believed to transaminate kynurenine into kynurenic acid (KYNA) in the brain. The KYNA made by the GOT2 is thought to be an important factor in brain pathology. It is suggested that KYNA synthesized by GOT2 could constitute a common, and mechanistically relevant, feature of the neurotoxicity caused by mitochondrial poisons, such as rotenone, malonate, 1-methyl-4-phenylpyridinium, and 3-nitropropionic acid.

Clinical Significance

In nearly all cancer cells, glycolysis has been seen to be highly elevated to meet their increased energy, biosynthesis, and redox needs. Therefore, the malate-aspartate shuttle promotes the net transfer of cytosolic NADH into mitochondria to ensure a high rate of glycolysis in diverse cancer cell lines. In a study completed in 2008, inhibiting the malate-aspartate shuttle was found to impair the glycolysis process and essentially decreased breast adenocarcinoma cell proliferation. Furthermore, knocking down GOT2 and GOT1 has also been reported to inhibit cell proliferation and colony formation in pancreatic cancer cell lines, suggesting that the GOT enzyme is essential for maintaining a high rate of glycolysis to support rapid tumor cell growth. Also, both glucose and glutamine increase GOT2 3K acetylation in PANC-1 cells and that GOT2 3K acetylation plays a critical role in coordinating glucose and glutamine uptake to provide energy and support cell proliferation and tumor growth. This implies that inhibiting GOT2 3K acetylation may merit exploration as a therapeutic agent especially for pancreatic cancer.

Mutations in this gene have been associated with an early onset infantile encephalopathy.

Interactions

GOT2 has been seen to interact with:

• oxaloacetate
• kynurenine
• aspartate
• alpha-ketoglutarate

Further reading

• "Structural and genetic relationships between cytosolic and mitochondrial isoenzymes". The International Journal of Biochemistry 16 (12): 1193–9. 1985. doi:10.1016/0020-711X(84)90216-7. PMID 6397370. 
• "Interaction of mitochondrial aspartate aminotransferase with negatively charged lecithin liposomes". Journal of Biochemistry 85 (5): 1157–63. May 1979. PMID 376500. 
• "Assignment of a gene necessary for the expression of mitochondrial glutamic-oxaloacetic transaminase in human-mouse hybrid cells". Cytogenetics and Cell Genetics 22 (1–6): 190–4. 1979. doi:10.1159/000130933. PMID 752471. 
• "Chromosomal localization of human aspartate aminotransferase genes by in situ hybridization". Human Genetics 83 (2): 159–64. Sep 1989. doi:10.1007/BF00286710. PMID 2777255. 
• "Regulation of malate dehydrogenase activity by glutamate, citrate, alpha-ketoglutarate, and multienzyme interaction". The Journal of Biological Chemistry 263 (22): 10687–97. Aug 1988. doi:10.1016/S0021-9258(18)38026-8. PMID 2899080. 
• "Nucleotide sequence and tissue distribution of the human mitochondrial aspartate aminotransferase mRNA". Biochemical and Biophysical Research Communications 157 (3): 1309–15. Dec 1988. doi:10.1016/S0006-291X(88)81017-9. PMID 3207426. 
• "Regulation of aminotransferase-glutamate dehydrogenase interactions by carbamyl phosphate synthase-I, Mg2+ plus leucine versus citrate and malate". The Journal of Biological Chemistry 260 (10): 6069–79. May 1985. doi:10.1016/S0021-9258(18)88939-6. PMID 3997814. 
• "The primary structure of mitochondrial aspartate aminotransferase from human heart". Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology 832 (1): 46–51. Nov 1985. doi:10.1016/0167-4838(85)90172-4. PMID 4052435. 
• "Genetic polymorphisms of human mitochondrial glutamic oxaloacetic transaminase". Science 169 (3943): 391–2. Jul 1970. doi:10.1126/science.169.3943.391. PMID 5450376. Bibcode: 1970Sci...169..391D. 
• "Three-dimensional structure of a pyridoxal-phosphate-dependent enzyme, mitochondrial aspartate aminotransferase". Proceedings of the National Academy of Sciences of the United States of America 77 (5): 2559–63. May 1980. doi:10.1073/pnas.77.5.2559. PMID 6930651. Bibcode: 1980PNAS...77.2559F. 
• "Mapping studies on human mitochondrial glutamate oxaloacetate transaminase". Annals of Human Genetics 46 (Pt 2): 145–52. May 1982. doi:10.1111/j.1469-1809.1982.tb00705.x. PMID 7114792. 
• "Assignment to chromosome 16 of a gene necessary for the expression of human mitochondrial glutamate oxaloacetate transaminase (aspartate aminotransferase) (E.C. 2.6.1.1.)". Biochemical Genetics 18 (9–10): 947–54. Oct 1980. doi:10.1007/BF00500127. PMID 7225087. 
• "Structural features of the precursor to mitochondrial aspartate aminotransferase responsible for binding to hsp70". The Journal of Biological Chemistry 270 (42): 24732–9. Oct 1995. doi:10.1074/jbc.270.42.24732. PMID 7559589. 
• "Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides". Gene 138 (1–2): 171–4. Jan 1994. doi:10.1016/0378-1119(94)90802-8. PMID 8125298. 
• "Construction and characterization of a full length-enriched and a 5'-end-enriched cDNA library". Gene 200 (1–2): 149–56. Oct 1997. doi:10.1016/S0378-1119(97)00411-3. PMID 9373149.