Mononuclear sites

There are hundreds of iron-containing enzymes. In general, the iron can exist as (a) a mononuclear site, in which it is coordinated by a tetrapyrrole structure (hemes) or strictly by amino acid residues that donate oxo, nitrogen, or sulfur ligands (b) a dinuclear site in which the irons are bridged by oxo, nitrogen, or sulfur coordination (c) a trinuclear site as in the 3Fe-4S clusters or (d) a tetranuclear site as in the [4Fe-4S] clusters. 

Zinc is a constituent of over 300 enzymes with much research into the coordination of zinc to the protein backbone, and how its chemistry is modulated by the donor set and environment.2 As well as the Lewis acid catalysis properties in enzymes, the structural role in zinc finger proteins has been a major area of research since the late 1990s. A number of reviews on zinc physiology, enzymology, and proteins in general have been published.978-981 There is extensive analysis available to classify the mononuclear sites in zinc proteins and identification of structural relationships of the extended environment.982,983... 

The formal transfer of an oxygen atom is one way of describing the function of the Mo site in molybdoenzymes.5 The formation of dinuclear reduction products is a complication that causes difficulty in trying to model the mononuclear site.5,165 This difficulty can be overcome by the use of sterically demanding ligands that prevent the formation of the dinuclear complex,73 79,125 176 177 For example, the cycle shown in Scheme 3 can be effected without dimerization. Further, in this case DMSO and the enzyme substrate biotin sulfoxide, can serve as the oxo donor to form the Movl dioxo complex during the catalytic cycle.79,177 The Movl complex involved is discussed structurally above (Figure 7). 

Figure 18 Proposed conversion of dinitrogen to ammonia at a mononuclear site, based on structurally characterized complexes.

An additional crucial piece of information emerges from the alloxan-thine study (24). Thus, it was shown that one alloxanthine binds to the enzyme per active molybdenum site. This result clearly implies that the molybdenum site is mononuclear. If a dinuclear site were involved, then it would be unlikely to require two alloxanthine molecules for inhibition and would be expected to be at least partially inhibited with one alloxan-thine/two molybdenum. Also, a difference in binding constant would be expected for the second compared with the first bound alloxanthine, but none is found. This result, coupled with the lack of evidence for Mo(V)-Mo(V) spin-spin interactions in the EPR spectra, clearly implicates a mononuclear site, and it would seem that xanthine oxidase possesses two full catalytic units, each containing one molybdenum, one flavin, and two Fe2S2 units (20). Other molybdenum oxidases also contain paired prosthetic groups and subunits, and it is likely that they each have two catalytic units per molecule. 

There are relatively few characterized metaUoproteins having mononuclear Mn sites. By far the best characterized mononuclear Mn site is shown in Superoxide Dismutase, although there are several other proteins that appear to have tightly bound mononuclear sites, including an extradiol dioxygenase, and several members of the cupin family. 

Costas et al. have reported spectroscopic evidence for an Fe Fe complex that can be considered a structural model for the putative Fe Fe (/x-0)2 core of methane monooxygenase intermediate The synthetic complex was prepared at —80 °C in CH2CI2 by decay of a mononuclear low-spin Fe peroxo precursor. The Mossbauer spectra showed that all iron in the sample is intermediate spin (5 = 1) Fe, but the data were compatible with either a mononuclear site or a weakly coupled ( J <5cm ) symmetric dimer. Combination of the Mossbauer technique with resonance Raman and EXAFS spectroscopies provided evidence for a bis-/x-oxo bridged diiron(IV) complex. The complex of Costas et al however, is not an electronic model for intermediate Q, as the latter contains high-spin Fe sites. 

The nature of the active site of ACS has recently been the subject of much debate [131,132,170,171]. It has now been established that it contains two nickel atoms [131], although the mechanism is still under discussion [172]. For a review of the current situation, see Hegg [134, 173]. Still to be resolved issues are, inter alia, the order of introduction of CO and CH3, the mechanism of the CH3 migration, the nature of the electron reservoir, the oxidation state of (Ni) (Scheme 1.19), and the question as to the involvement of a mononuclear or binuclear Ni site for the reaction. Fontecilla suggests a Ni(0) species and a mononuclear site [131]. 

---