Isocitrate dehydrogenase regulation

It may seem surprising that isocitrate dehydrogenase is strongly regulated, because it is not an apparent branch point within the TCA cycle. However, the citrate/isocitrate ratio controls the rate of production of cytosolic acetyl-CoA, because acetyl-CoA in the cytosol is derived from citrate exported from the mitochondrion. (Breakdown of cytosolic citrate produces oxaloacetate and acetyl-CoA, which can be used in a variety of biosynthetic processes.) Thus, isocitrate dehydrogenase activity in the mitochondrion favors catabolic TCA cycle activity over anabolic utilization of acetyl-CoA in the cytosol. 

D. C. LaPorte and D. E. Koshland, Jr., Phosphorylation of isocitrate dehydrogenase as a demonstration of enhaced sensitivity in covalent regulation. Nature, 305, 286-290 (1983). 

Citrate synthase, isocitrate dehydrogenase and oxogluta-rate dehydrogenase are key enzymes regulating the flux through the cycle all three catalyse non-equilibrium reactions (Chapter 3). 

The most important factor in the regulation of the cycle is the NADH/NAD ratio. In addition to pyruvate dehydrogenase (PDH) and oxoglu-tarate dehydrogenase (ODH see p. 134), citrate synthase and isodtrate dehydrogenase are also inhibited by NAD deficiency or an excess of NADH+HT With the exception of isocitrate dehydrogenase, these enzymes are also subject to product inhibition by acetyl-CoA, suc-cinyl-CoA, or citrate. 

Isocitrate dehydrogenase catalyzes the NAD-dependent reduction of isocitrate to a-ketoglutarate. The dimeric enzyme is regulated via phosphorylation. Phosphorylation on SerllS leads to a complete inactivation of the enzyme. 

An understanding of the molecular basis for regulation of isocitrate dehydrogenase by phosphorylation was facilitated by X-ray crystallography of the phosphorylated enzyme in complex with isocitrate. The crystal structures of mutants of the enzyme in which SerllS had been exchanged for aspartate or glutamate were also solved (Hurley et al., 1990). The structure of the enzyme in complex with the substrate isocitrate revealed the phophorylation site to be localized near isocitrate. SerllS itself binds the substrate directly via a H-bond with the O of isocitrate (fig. 2.13). 

The partitioning of isocitrate between the citric acid cycle and the glyoxylate cycle is controlled at the level of isocitrate dehydrogenase, which is regulated by reversible phosphoiylation. 

Isocitrate Dehydrogenase Is Regulated by the NADH-to-NAD+ Ratio and the Energy Charge... 

To understand why isocitrate dehydrogenase is so intensely regulated we must consider reactions beyond the TCA cycle, and indeed beyond the mitochondrion (fig. 13.15). Of the two compounds citrate and isocitrate, only citrate is transported across the barrier imposed by the mitochondrial membrane. Citrate that passes from the mitochondrion to the cytosol plays a major role in biosynthesis, both because of its immediate regulatory properties and because of the chain of covalent reactions it initiates. In the cytosol citrate undergoes a cleavage reaction in which acetyl-CoA is produced. The other cleavage product, oxaloacetate, can be utilized directly in various biosynthetic reactions or it can be converted to malate. The malate so formed can be returned to the mitochondrion, or it can be converted in the cytosol to pyruvate, which also results in the reduction of NADP+ to NADPH. The pyruvate is either utilized directly in biosynthetic processes, or like malate, can return to the mitochondrion. 

ADP, acetyl-CoA, succinyl-CoA, and citrate. The major known sites for regulation of the cycle include two enzymes outside the cycle (pyruvate dehydrogenase and pyruvate carboxylase) and three enzymes inside the cycle (citrate synthase, isocitrate dehydrogenase, and a-ketoglutarate dehydrogenase). All of these sites of regulation represent important metabolic branchpoints. 

RD Chen, P Gadal. Structure, function and regulation ofNAD- and NADP-isocitrate dehydrogenases in higher plants and in other organisms. Plant Physiol Biochem 28 411-427, 1990. 

JH Hurley, P Thorsness, V Ramalingham, N Helmers, DE Koshland Jr, RM Stroud. Structure of a bacterial enzyme regulated by phosphorylation, isocitrate dehydrogenase. Proc Natl Acad Sci (USA) 86 8635-8639, 1989. 

JR Cupp, L McAlister-Henn. Kinetic analysis of NAD+-isocitrate dehydrogenase with altered isocitrate binding site contribution of IDH1 and IDH2 subunits to regulation and catalysis. Biochemistry 32 9323-9328, 1993. 

The enzyme has been isolated from many tissues, the best source being a heart muscle or yeast. The isocitrate dehydrogenase requires the presence of cofactors Mg2+ or Mn2+. As an allosteric enzyme, it is regulated by a number of modulators. ADP, adenosine diphosphate, is a positive modulator and therefore stimulates enzyme activity. The... 

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