Iron clusters catalytic reactions involving

In view of the demonstrated ease in which the tetrameric cluster cleaves (equation 1) and the redox-induced decomposition of dimeric 5, the integrity of the complexes at the end of the catalytic reaction and the nature of the iron species under a reducing environment are ambiguous. The possibility that mononuclear species are involved in the... 

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. 

Two clusters in CO dehydrogenase are required for oxidation of CO or reduction of CO 2 (Figure 1). The catalytic site is a nickel iron sulfur cluster called Cluster C. The two electrons involved in this redox reaction are transferred to or from a ferredoxin-like [4Fe64S] cluster called Cluster B. Cluster C is a NiFeS center whereas. Cluster B is most certainly a typical [4Fe64S] cluster (Ragsdale et ah, 1982 Lindahl et al., 1990 Lindahl et ah, 1990). 

Fig. (7). Hypothetical consequences of NO-mediated inhibition of plant cytosolic aconitase [208]. The interaction between NO and cytosolic aconitase triggers a cluster dissassembly and subsequently the inhibition of the catalytic activity of the enzyme. By analogy to mammalian studies, the resulting apoprotein may act as iron regulatory protein (1RP) and may modulate the translation of mRNA encoding proteins involved in the cellular iron homeostasis. The elevated free iron concentration promotes the Fenton reaction leading to hydroxyl radical (HO ) production. Both HO- and high concentrations of iron create a killing environment for host and pathogen.

Fig. 6 Putative catalytic cycle of CODH involving a Ni - Fe bridging hydride. The bridging hydride is assumed to be stable enough to prevent its fast reaction with a proton. The reaction can be divided into several steps 1) CO2 binds end-on at the axial El site 2) the hydride attacks the bound CO2 forming a transient formate complex 3) the C - OH bond is broken, generating an intermediate that has El occupied by CO and E2 by OH" 4) CO dissociates from Ni as a Ni(II)-bound axial CO is not very stable [135], generating the Credi state that has a bridging hydroxide 5) addition of two electrons via the D- and B-clusters and of two protons via a proton channel leads to dehydration and regeneration of the Cj-ed2 state. The hypothetical Cint form is depicted as having Ni(III). However, an alternative would be a species with Ni(II) and one of the cluster irons oxidized to Fe(III).

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