Aconitase, enzyme system

NADH dehydrogenase is an enormous 42-subunit complex that contains a binding site for NADH, several FMN and iron-sulfur (Fe-S) center binding proteins, and binding sites for CoQ (see Fig 21.5). An FMN accepts two electrons from NADH and is able to pass single electrons to the Fe-S centers (Fig. 21.6). Fe-S centers, which are able to delocalize single electrons into large orbitals, transfer electrons to and from CoQ. Fe-S centers are also present in other enzyme systems, such as other proteins, which transfer electrons to CoQ, in the cytochrome b-Ci complex, and in aconitase in the TCA cycle. 

The various catalytic properties of aconitase, for example, might be due to the presence of several proteins so similar in many respects that separation is difficult. Nevertheless, as we have already pointed out, the two catalytic properties of aconitase have been separated in A. niger. Furthermore, the claim that these various catalytic properties are due to a single protein because the preparation is declared pure on the basis of ultracentrifugal and electrophoretic analysis is unjustified, because these methods demonstrate only that the protein components of the material under study are similar in size and mobility. Most protein chemists would require more severe criteria of purity, such as end-group analyses. This line of reasoning applies a fortiori to the multiple-enzymes systems. 

It was then that the very important discovery of aconitase (Martius and Knoop) was announced and it was demonstrated that the so-called dtric dehydrogenase was in fact a mixture of aconitase, isocitric dehydrogenase, and oxalosuccinic decarboxylase. This multi-enzyme system explained the passage of dtrate to succinate. 

Regulation of Calcium Level. Parathormone controls the metabolism of Ca++. It activates the osteoclasts and thereby causes a demineralization of bone and a rise of the Ca++ level in blood. It is not yet clear how these minerals are dissolved. Citrate probably plays a key role at least, it occurs in relatively high concentrations in bony tissue. Bone tissue contains the enzyme system for forming citrate, but little aconitase and isocitrate dehydrogenase. 

Aconitase, an unstable enzyme,4 is concerned with the reversible conversion of cis-aconitate to either citric acid or isocitric acid. It may be noted that the entire system of tricarboxylic cycle enzymes are present in the mitochondria separated from cells, and, furthermore, it has been found that the mitochondrial enzymes differ from the isolated enzymes in that the former require no addition of D.P.N. (co-enzyme I) or T.P.N. (co-enzyme II) for activity. Peters suggests that the citrate accumulation is caused by the competitive reaction of the fluorocitrate with aconitase required for the conversion of citrate to isocitrate. This interference with the tricarboxylic acid... 

The aconitase system presents yet another fascinating story.Aconitase is a key enzyme in the Krebs cycle, catalyzing the conversion of citrate and isocitrate through the intermediacy of cE-aconitate, as shown in Equation (7.6). 

Questions of detailed mechanism for aconitase remain open. ENDOR spectroscopy shows that both substrate and water (or OH ) can bind at the cluster. Does one of its Fe-atom vertices play the Lewis-acid role necessary for aconitase activity Is the Fe3S4 Fe4S4 conversion a redox- or iron-activated switch, which works as a control system for the activity of aconitase These and other questions will continue to be asked. If aconitase is indeed an Fe-S enzyme with an iron-triggered control mechanism, it may be representative of a... 

In addition, it has become increasingly evident that there is significant mitochondrial dysfunction and impairment of the oxidative phosphorylation system [29, 41, 66-69]. This impairment is felt to be secondary to inhibition of the Krebs cycle enzymes citrate synthase, aconitase, and isocitrate dehydrogenase by methylcitrate, inhibition of pyruvate carboxylase by methylmalonic acid, and inhibition of pyruvate dehydrogenase complex. 

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