On Jan. 31st, 2017, Scientific Reports published online a work entitled “The β and γ subunits play distinct functional roles in the α2βγ heterotetramer of human NAD-dependent isocitrate dehydrogenase” by Prof. Jianping Ding’s group at National Center for Protein Science Shanghai, State Key Laboratory of Molecular Biology, Center for Excellence in Molecular Cell Science, Institute of Biochemistry and Cell Biology, Chinese Academy of Sciences. This work reveals that the β and γ subunits play distinct functional roles in the α2βγ heterotetramer of human NAD-dependent isocitrate dehydrogenase.
Isocitrate dehydrogenases (IDHs) are a family of enzymes existing in all organisms, which catalyze the oxidative decarboxylation of isocitrate (ICT) into α-ketoglutarate (α-KG) using NADP or NAD as coenzyme. Eukaryotes contain both NADP-IDHs and NAD-IDHs. NADP-IDHs, localized in the cytosol, mitochondria and peroxisomes, are demonstrated to play important roles in cellular defenses against oxidative damage, detoxification of reactive oxygen species, and synthesis of fat and cholesterol. NAD-IDHs, localized in the mitochondria, exert the catalytic activity in the Krebs cycle.
In the past works, researchers from Prof. Ding’s group have revealed the molecular mechanism of the biological function of human cytosolic and yeast mitochondrial NADP-IDHs and the molecular basis of the pathogenesis of human cytosolic NADP-IDH mutants (J. Biol. Chem., 2004; Protein Sci., 2008; Cell Res., 2010). Mammalian NAD-IDHs are composed of three types of subunits in the ratio of 2α:1β:1γ, which can be activated by CIT and ADP through decreasing the Km for substrate without significantly affecting the Vmax. The α subunit is essential to the catalysis of the enzyme and contains the binding sites for the substrate, coenzyme and metal ion. The β and γ subunits are the regulatory subunits, which are believed to have the ability to bind different regulators. It is also found that the α and β subunits and the α and γ subunits form the basic structural units of the holoenzyme, which exhibit some enzymatic activities. However, the structural and functional differences between the two regulatory subunits of NAD-IDHs and the molecular mechanism of the allosteric regulation are largely unknown.
In this work, Dr. Tengfei Ma and his colleagues in Prof. Ding’s group systematically characterized the enzymatic properties of the α2βγ heterotetramer and the αβ and αγ heterodimers of human NAD-IDH in the absence or presence of regulators. The biochemical data show that the αβ and αγ enzymes alone have considerable basal activities but exhibit different enzymatic properties: the αγ enzyme shares very similar kinetic characteristics and allosteric regulation patterns as the α2βγ enzyme, whereas the ab enzyme does not. ATP could activate the α2βγ and αγ enzymes at low concentration and inhibit the enzymes at high concentration, but has only inhibitory effect on the αβ enzyme. Moreover, they found that the αβ and αγ heterodimers contribute equally to the full activity of the holoenzyme in both the absence and presence of the activators, and the positive regulation is through the γ subunit but not the β subunit. This work reveals for the first time the functional differences of the two regulatory subunits of human NAD-IDH: the γ subunit plays the regulatory role, and the β subunit mainly the structural role in the α2βγ heterotetramer.
This study was supported by grants from the National Natural Science Foundation of China, the Ministry of Science and Technology of China and the Chinese Academy of Sciences.
AUTHOR CONTACT:
Jianping Ding, Ph.D., Professor
National Center for Protein Science Shanghai, State Key Laboratory of Molecular Biology, Center for Excellence in Molecular Cell Science, Institute of Biochemistry and Cell Biology, Chinese Academy of Sciences
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