Biology:IDH3G

Short description: Protein-coding gene in the species Homo sapiens

Generic protein structure example

Isocitrate dehydrogenase [NAD] subunit gamma, mitochondrial is an enzyme that in humans is encoded by the IDH3G gene.

Isocitrate dehydrogenases (IDHs) catalyze the oxidative decarboxylation of isocitrate to 2-oxoglutarate. These enzymes belong to two distinct subclasses, one of which utilizes NAD(+) as the electron acceptor and the other NADP(+). Five isocitrate dehydrogenases have been reported: three NAD(+)-dependent isocitrate dehydrogenases, which localize to the mitochondrial matrix, and two NADP(+)-dependent isocitrate dehydrogenases, one of which is mitochondrial and the other predominantly cytosolic. NAD(+)-dependent isocitrate dehydrogenases catalyze the allosterically regulated rate-limiting step of the tricarboxylic acid cycle. Each isozyme is a heterotetramer that is composed of two alpha subunits, one beta subunit, and one gamma subunit. The protein encoded by this gene is the gamma subunit of one isozyme of NAD(+)-dependent isocitrate dehydrogenase. This gene is a candidate gene for periventricular heterotopia. Several alternatively spliced transcript variants of this gene have been described, but only some of their full length natures have been determined. [provided by RefSeq, Jul 2008]

Structure

IDH3 is one of three isocitrate dehydrogenase isozymes, the other two being IDH1 and IDH2, and encoded by one of five isocitrate dehydrogenase genes, which are IDH1, IDH2, IDH3A, IDH3B, and IDH3G. The genes IDH3A, IDH3B, and IDH3G encode subunits of IDH3, which is a heterotetramer composed of two 37-kDa α subunits (IDH3α), one 39-kDa β subunit (IDH3β), and one 39-kDa γ subunit (IDH3γ), each with distinct isoelectric points. Alignment of their amino acid sequences reveals ~40% identity between IDH3α and IDH3β, ~42% identity between IDH3α and IDH3γ, and an even closer identity of 53% between IDH3β and IDH3γ, for an overall 34% identity and 23% similarity across all three subunit types. Notably, Arg88 in IDH3α is essential for IDH3 catalytic activity, whereas the equivalent Arg99 in IDH3β and Arg97 in IDH3γ are largely involved in the enzyme's allosteric regulation by ADP and NAD. Thus, it is possible that these subunits arose from gene duplication of a common ancestral gene, and the original catalytic Arg residue were adapted to allosteric functions in the β- and γ-subunits. Likewise, Asp181 in IDH3α is essential for catalysis, while the equivalent Asp192 in IDH3β and Asp190 in IDH3γ enhance NAD- and Mn²⁺-binding. Since the oxidative decarboxylation catalyzed by IDH3 requires binding of NAD, Mn²⁺, and the substrate isocitrate, all three subunits participate in the catalytic reaction. Moreover, studies of the enzyme in pig heart reveal that the αβ and αγ dimers constitute two binding sites for each of its ligands, including isocitrate, Mn²⁺, and NAD, in one IDH3 tetramer.

Function

As an isocitrate dehydrogenase, IDH3 catalyzes the irreversible oxidative decarboxylation of isocitrate to yield α-ketoglutarate (α-KG) and CO₂ as part of the TCA cycle in glucose metabolism. This step also allows for the concomitant reduction of NAD+ to NADH, which is then used to generate ATP through the electron transport chain. Notably, IDH3 relies on NAD+ as its electron acceptor, as opposed to NADP+ like IDH1 and IDH2. IDH3 activity is regulated by the energy needs of the cell: when the cell requires energy, IDH3 is activated by ADP; and when energy is no longer required, IDH3 is inhibited by ATP and NADH. This allosteric regulation allows IDH3 to function as a rate-limiting step in the TCA cycle. Within cells, IDH3 and its subunits have been observed to localize to the mitochondria.

Clinical Significance

The IDH3G gene may be involved in drug resistance in gastric cancer.

See also

• IDH1
• IDH2
• IDH3A
• IDH3B

Further reading

• "The genomic organization of a human creatine transporter (CRTR) gene located in Xq28.". Genomics 35 (2): 383–5. 1996. doi:10.1006/geno.1996.0373. PMID 8661155. 
• "Identification and functional characterization of a novel, tissue-specific NAD(+)-dependent isocitrate dehydrogenase beta subunit isoform.". J. Biol. Chem. 274 (52): 36866–75. 2000. doi:10.1074/jbc.274.52.36866. PMID 10601238. 
• "Bovine NAD+-dependent isocitrate dehydrogenase: alternative splicing and tissue-dependent expression of subunit 1.". Biochemistry 39 (7): 1807–16. 2000. doi:10.1021/bi991691i. PMID 10677231. 
• "DNA cloning using in vitro site-specific recombination.". Genome Res. 10 (11): 1788–95. 2001. doi:10.1101/gr.143000. PMID 11076863. 
• "Systematic subcellular localization of novel proteins identified by large-scale cDNA sequencing.". EMBO Rep. 1 (3): 287–92. 2001. doi:10.1093/embo-reports/kvd058. PMID 11256614. 
• "Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences.". Proc. Natl. Acad. Sci. U.S.A. 99 (26): 16899–903. 2003. doi:10.1073/pnas.242603899. PMID 12477932. Bibcode: 2002PNAS...9916899M. 
• "The status, quality, and expansion of the NIH full-length cDNA project: the Mammalian Gene Collection (MGC).". Genome Res. 14 (10B): 2121–7. 2004. doi:10.1101/gr.2596504. PMID 15489334. 
• "From ORFeome to biology: a functional genomics pipeline.". Genome Res. 14 (10B): 2136–44. 2004. doi:10.1101/gr.2576704. PMID 15489336. 
• "Towards a proteome-scale map of the human protein-protein interaction network.". Nature 437 (7062): 1173–8. 2005. doi:10.1038/nature04209. PMID 16189514. Bibcode: 2005Natur.437.1173R. 
• "The LIFEdb database in 2006.". Nucleic Acids Res. 34 (Database issue): D415–8. 2006. doi:10.1093/nar/gkj139. PMID 16381901. 
• "Identification of Mn2+-binding aspartates from alpha, beta, and gamma subunits of human NAD-dependent isocitrate dehydrogenase.". J. Biol. Chem. 281 (30): 21073–81. 2006. doi:10.1074/jbc.M602956200. PMID 16737955. 
• "Role of alpha-Asp181, beta-Asp192, and gamma-Asp190 in the distinctive subunits of human NAD-specific isocitrate dehydrogenase.". Biochemistry 46 (18): 5391–7. 2007. doi:10.1021/bi700061t. PMID 17432878.