Aconitase catalytic properties

The ability of aconitase to form both isocitrate and c/.y-aconitate is not well understood. It should be pointed out that, here again, the enzyme is capable of two different reactions an intramolecular conversion in the process of transforming citrate to isocitrate and a dehydration in the reaction transforming citrate into cw-aconitate. All attempts to separate these two catalytic properties have failed except in Aspergillus niger, in which two separate aconitases have been found. 

Various relationships between enzymes of the Krebs cycle and mitochondria are possible. For instance, all enzymes could be enclosed within mitochondrial structures or the enzymes could take part in the structural build-up of the cell. There is no evidence demonstrating that all enzymes of the Krebs cycle are part of the mitochondria. The existence of enzymes with multiple catalytic properties (isocitric dehydrogenase, aconitase, and malic dehydrogenase) and the failure to separate the multiple steps of an overall reaction (pyruvic and a-ketoglutarate oxidation) are sometimes taken as evidence for the participation of the enzyme in the building-up of the mitochondrial structure, but these arguments do not take into account the limitations of the actual biochemical methods, and, therefore, conclusions based upon them are premature. 

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. 

Aconitase was the first protein to be identified as containing a catalytic iron-sulfur cluster [24-26]. It was also readily established that the redox properties of the [4Fe-4S](2+ 1+) cluster do not play a role of significance in biological functioning the 1 + oxidation state has some 30% of the activity of the 2+ state [25], Since then several other enzymes have been identified or proposed to be nonredox iron-sulfur catalysts. They are listed in Table 2. It appears that all are involved in stereospecific hydration reactions. However, these proteins are considerably less well characterized than aconitase. In particular, no crystal structural information is available yet. Therefore, later we summarize structural and mechanistic information on aconitase, noting that many of the basic principles are expected to be relevant to the other enzymes of Table 2. 

---