Isodtrate dehydrogenase

The diagram looks very promising in terms of citric acid formation in that a-oxoglutarate dehydrogenase is inactive, isodtrate dehydrogenase has veiy low activity and aconitase equilibrates 90% towards dtric add. 

Figure 9.2 Summary of reactions of the Krebs cycle. The names of the enzymes are dtrate synthase, aconitase, isodtrate dehydrogenase (there are two enzymes, one ubTizes NAD as the cofactor, the other NADPT it is assumed that the NAD -specific enzyme is that involved in the cycle), oxoglutarate dehydrogenase, sucdnyl CoA synthetase, succinate dehydrogenase, fumarate hydratase, malate dehydrogenase.

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. 

Interconversion processes (see p. 120) also play an important role. They are shown here in detail using the example of the PDH complex (see p. 134). The inactivating protein kinase [la] is inhibited by the substrate pyruvate and is activated by the products acetyl-CoA and NADH+H. The protein phosphatase [Ibj—like isodtrate dehydrogenase [3] and the ODH complex [4j-is activated by Ca. This is particularly important during muscle contraction, when large amounts of ATP are needed. Insulin also activates the PDH complex (through inhibition of phosphorylation) and thereby promotes the breakdown of glucose and its conversion into fatty acids. 

Fig. 2.13. Substrate binding site of isocitrate dehydrogenase E. coli). The interactions involved in the binding of Mg Msodtrate at the active site of isodtrate dehydrogenase are shown. After Hnrley et al., (1990), with permission.

T. Yaoi, K. Miyazaki, T. Oshima, Y. Komukai, and M. Go, Conversion of the coenzyme specificity of isodtrate dehydrogenase by module replacement, J. Biochem. (Tokyo) 1996, 119, 1014-1018. 

Phosphorylation in bacteria. A bacterial enzyme whose activity is controlled by phosphorylation is isodtrate dehydrogenase. Transfer of a phospho group to the -OF4 of Ser 113 completely inactivates the... 

Oxidative decarboxylations. Isocitrate, a secondary alcohol, is oxidized by NAD in step 3 to give a ketone, which loses COj to give -ketoglutarate. Catalyzed by the enzyme isodtrate dehydrogenase, the decarboxylation is a typical reaction of a /3-keto acid, just like that in the acetoacetic ester synthesis (Section 22.8). 

Dbstyl 2149843 Isodtrate dehydrogenase 1 (NADP+) 96413 Enzyme activity not changed in 32 patients with diabetes relative to controls (1)... 

Rgure 5.3 Major omtrol points of glycolysis and the TCA cycle. Enzymes I, hexoidnase II, phosphofructokinase Hi. pyruvate kinase IV, pyruvate dehydrogenase V, citrate synthase VI, aconitase VII, isodtrate dehydrogenase VIII, a-oxoglutarate dehydrogenase. 

The anion (oxalosuccinate) is an intermediate in the isocitrate dehydrogenase reaction of the Tricarboxylic add cycle (see). Isodtrate dehydrogenase catalyses both the oxidation of isodtrate to oxalosuccinate, and its decarboxylation to 2-oxoglutarate. 

Isocitrate dehydrogenases have been studied in extreme thermophiles mainly from the point of view of elucidation of mechanisms of thermostability. This has included the studies of Hibino et al. [32] on the enzyme from a Bacillus stearothermophilus strain grown at 65 °C. In the presence of substrate the enzyme was markedly stabilized as judged by retention of tertiary conformation, reflected by the dramatically improved resistance to denaturation by urea. The thermostability of the partially purified isodtrate dehydrogenase from Thermus aquaticus was shown to be dependent upon the buffer used and on the addition of isodtrate [135]. This enzyme has also been purified from Thermus thermophilus HB8 [136]. The enzyme had a dimeric structure with identical 57.5 kDa subunits as judged by SDS-polyacrylamide gel electrophoresis, whilst gel filtration of the native enzyme on two different matrices gave values of 95 kDa and 120 kDa. The enzyme had a pH optimum of 7.8, required a divalent metal ion for activity (Mn " > Mg " ) and was NADP dependent, although NAD" was shown to replace NADP" with low efficiency. 

Henke, B., Girzalsky, W., Berteaux-Lecelher, V. Erdmann, R. (1998) J. Biol. Chem., 273, 3702-3711, IDP3 encodes a peroxisomal NADP-dependent isodtrate dehydrogenase required for the beta-oxidation of unsaturated fatty adds. 

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