Catalytic subunit of the enzyme which catalyzes the decarboxylation of isocitrate (ICT) into alpha-ketoglutarate. The heterodimer composed of the alpha (IDH3A) and beta (IDH3B) subunits and the heterodimer composed of the alpha (IDH3A) and gamma (IDH3G) subunits, have considerable basal activity but the full activity of the heterotetramer (containing two subunits of IDH3A, one of IDH3B and one of IDH3G) requires the assembly and cooperative function of both heterodimers. D-threo-isocitrate + NAD(+) = 2-oxoglutarate + CO2 + NADH Divalent metal cations; Mn(2+) or Mg(2+). Activity higher in presence of Mn(2+) than of Mg(2+). Binds 1 Mg(2+) or Mn(2+) ion per subunit. The heterotetramer and the heterodimer composed of IDH3A and IDH3G subunits can be allosterically activated by citrate (CIT) or/and ADP, and the two activators can act independently or synergistically. The heterodimer composed of IDH3A and IDH3B subunits cannot be allosterically regulated and the allosteric regulation of the heterotetramer is through the IDH3G subunit and not the IDH3B subunit. The IDH3G subunit contains the allosteric site which consists of a CIT-binding site and an ADP-binding site, and the binding of CIT and ADP causes conformational changes at the allosteric site which are transmitted to the active site in the catalytic subunit (IDH3A) through a cascade of conformational changes at the heterodimer interface, leading to stabilization of the isocitrate-binding at the active site and thus activation of the enzyme. ATP can activate the heterotetramer and the heterodimer composed of IDH3A and IDH3G subunits at low concentrations but inhibits their activities at high concentrations, whereas ATP exhibits only inhibitory effect on the heterodimer composed of IDH3A and IDH3B subunits. kcat is 26.7 sec(-1) for the heterotetramer with isocitrate as substrate. kcat is 14.6 sec(-1) for the heterodimer composed of IDH3A and IDH3B subunits with isocitrate as substrate. kcat is 9.72 sec(-1) for the heterodimer composed of IDH3A and IDH3G subunits with isocitrate as substrate. kcat is 23.4 sec(-1) for the heterotetramer with isocitrate as substrate in the presence of citrate and ATP. kcat is 11.9 sec(-1) for the heterodimer composed of IDH3A and IDH3G subunits with isocitrate as substrate in the presence of citrate and ATP. kcat is 28.4 sec(-1) for the heterotetramer with isocitrate as substrate in the presence of citrate and ADP. kcat is 15.8 sec(-1) for the heterodimer composed of IDH3A and IDH3B subunits with isocitrate as substrate in the presence of citrate and ADP. kcat is 17.6 sec(-1) for the heterodimer composed of IDH3A and IDH3G subunits with isocitrate as substrate in the presence of citrate and ADP. Heterooligomer of subunits alpha (IDH3A), beta (IDH3B), and gamma (IDH3G) in the apparent ratio of 2:1:1. The heterodimer containing one IDH3A and one IDH3B subunit and the heterodimer containing one IDH3A and one IDH3G subunit assemble into a heterotetramer (which contains two subunits of IDH3A, one of IDH3B and one of IDH3G) and further into the heterooctamer. The disease is caused by variants affecting the gene represented in this entry. Belongs to the isocitrate and isopropylmalate dehydrogenases family.