Isocitrate transport, mitochondrial

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. 

X1B-X14), and NADP-linked isocitrate dehydrogenase SIS, S16). Numerous studies have been carried out with isolated mitochondria to identify the source of NADPH for 11 -hydroxylation by studying the effect of inhibitors and uncouplers of oxidative phosphorylation in the presence of different hydrogen donors (83, 19S, SIO, SIS, SI4-SSS). Part of this work was inconclusive since several complicating features in the metabolism of adrenocortical mitochondria were insufficiently taken into account such as secondary inhibitory effects of the substrates, inhibitors, or uncouplers used 83, SI4, SSO, SSS) the requirement of transport of substrates across the mitochondrial membrane SS3, SS4) and the possibility of intramitochondrial dismutation reactions SS3). 

The second metabolic pathway which we have chosen to describe is the tricarboxylic acid cycle, often referred to as the Krebs cycle. This represents the biochemical hub of intermediary metabolism, not only in the oxidative catabolism of carbohydrates, lipids, and amino acids in aerobic eukaryotes and prokaryotes, but also as a source of numerous biosynthetic precursors. Pyruvate, formed in the cytosol by glycolysis, is transported into the matrix of the mitochondria where it is converted to acetyl CoA by the multi-enzyme complex, pyruvate dehydrogenase. Acetyl CoA is also produced by the mitochondrial S-oxidation of fatty acids and by the oxidative metabolism of a number of amino acids. The first reaction of the cycle (Figure 5.12) involves the condensation of acetyl Co and oxaloacetate to form citrate (1), a Claisen ester condensation. Citrate is then converted to the more easily oxidised secondary alcohol, isocitrate (2), by the iron-sulfur centre of the enzyme aconitase (described in Chapter 13). This reaction involves successive dehydration of citrate, producing enzyme-bound cis-aconitate, followed by rehydration, to give isocitrate. In this reaction, the enzyme distinguishes between the two external carboxyl groups... 

As fatty acids are produced under conditions of high energy, the high NADH/NAD" ratio in the mitochondria inhibits isocitrate dehydrogenase, which leads to citrate accumulation within the mitochondrial matrix. As the citrate accumulates, it is transported out into the cytosol to donate carbons for fatty acid synthesis. 

Citrate that is not oxidized by isocitrate dehydrogenase can be transported from the mitochondrial matrix into the cytoplasm. In the cytoplasm of adipocytes and hepatocytes, oxaloacetate and acetyl-CoA are formed from citrate, not by the reversal of the citrate synthase-catalyzed reaction, bnt by ATP-dependent citrate lyase. As the name indicates, the free energy of ATP hydrolysis drives this reaction in the degradative direction. 

In spite of the absence of integral mitochondria, the red cell contains some of the enzymes (fumerase, isocitric dehydrogenase, malic dehydrogenase, and cytochrome oxidase) functioning in the Krebs cycle and electron transport. These enzymes probably represent mitochondrial remnants, and their presence in the mature erythrocyte may be a consequence of their greater stability. Similarly, enzymes concerned with... 

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