Dinuclear metal centers

Scheme 1 Catalase mechanism showing schematically the bis(carboxylato) bridged dinuclear metal centers. DH/Dy hydron donor/acceptor species. (After Refs. 7, 96.)...

Phosphates bridge dinuclear metal centers in the active sites of some phosphoes-terases [1-3]. Dinuclear metal complexes should be able to provide double Lewis acid activation for hydrolyzing phosphates. To quantify double Lewis acid activation for cleaving phosphate diesters, we studied the reaction of 24 (Figure 6.16), which has... 

Figure 2. Possible 02 (peroxide) binding modes to a dinuclear metal center.

Figure 11. Mechanistic features that may be important in catalysis of phosphate ester or amide hydrolysis by a dinuclear metal center. See text for further discussion. (Adapted from references 74 and 80).

W U-H fOPr1) will isomerize olefins it selectively takes 1-butene to cis-2-butene, for example (54). While one can but speculate about the ultimate impact of dinuclear transition metal chemistry, it is surely fair to say that the elucidation of the intimate mechanisms of reactions at dinuclear metal centers will prove more challenging and fascinating than did their analogues at mononuclear centers. 

Leucine aminopeptidase is interesting in that its active site contains two zinc atoms which together bind and activate the water molecule [74]. Despite this enzyme containing a dinuclear metal center at its active site, its mechanism, and specifically its mode of proton transfers reactions, appear to follow the general theme established by thermolysin and carboxypeptidase Adenosine deaminase and other members of the family of nucleoside and nucleotide deaminases utilize zinc-bound water as the catalytic nucleophile to displace ammonia from the 6-position of purines or the 4-position of pyrimidines and in all cases display inverse solvent deuterium isotope effects ranging from 0.3 to 0.8 on fec/Kni [75-80]. These effects are reminiscent of those observed for metallopro-teases and have their origins, like those of the proteases, in fractionation factors for the protons of the bound water that are less than one. 

All phosphatases catalyze the same net reaction, the hydrolysis of a phosphate monoester to inorganic phosphate and the alcohol or phenol from the ester group. As already mentioned, despite the thermodynamic favorability of this reaction, the kinetic barrier is formidable. A number of the enzymes that catalyze this reaction have been characterized. Phosphatases vary in their preference for the charge state of the substrate (either the monoanion or the dianion), in the presence or absence of a metal center, and in the utilization of a phosphoenzyme intermediate versus direct attack by water. Even among metallophosphatases, there are variations in the means by which the dinuclear metal center participates in binding and catalysis. 

Fig. 9. Proposed DNA phosphate hydrolysis by enzymes containing dinuclear metal centers, via two-metal-ion assisted phosphoryl transfer. Solid lines indicate the geometry around the central phosphorus atom. Charges on oxygen atoms and metals have been omitted.

Figure 14-1 Schematic representations of the dinuclear metal centers of bacterioferritin constructed from the X-ray coordinates reported by Frolow et al. [10]. Although the chemical nature and redox level of the occupying metal ions were not stated it is likely that Ml and M2 are both Mn ", since the crystals were obtained from solutions of apo-protein and lOOmM MnCl2-...

Figure 15-3 Dinuclear metal centers in (A) HuHF (B) EcFtna (C) EcBfr (D) DvRr.

Figure 15-4 Schematic diagrams comprising dinuclear metal centers in HuHF, EcFtna and EcBfr with those of ribonucleotide reductase R2 subunit (RNR R2), methane monooxygenase hydroxylase component (MMOH) and DvRr. The third metal sites in HuHF and EcFtna are also indicated.

Figure 17-2 A comparison of consensus amino acid sequences of serine/threonine protein phosphatases with bacteriophage A protein phosphatase. Consensus sequences within the phosphoesterase domain [23-25] of calcineurin, PPl, and PP2A were generated from multiple sequence alignments for each member. The standard amino acid codes are used. The letter X refers to a residue that is not conserved within the consensus sequence for each member while spaces represent introduced gaps. Letters noted in bold represent ligands to the metal ions of the dinuclear metal center (see Figure 17-3).

Figure 17-3 Active site dinuclear metal centers of (A) purple acid phosphatase [29, 30], (B) PPl [31, 32], (C) calcineurin [15, 16], and (D) protein phosphatase 2C [27] adapted from the X-ray structures. Figures in A, B, and C are reproduced with permission from [18],...

Despite the absence of sequence homology with PPl, calcineurin, or purple acid phosphatase, protein phosphatase 2C also contains a dinuclear metal center [27]. In the case of the recombinant PP2C, crystals were obtained in the presence of MnCl2, thus placing two Mn ions in the active site (Figure 17-3D). The metal coordination... 

Figure 17-10 An alternate mechanism for substrate binding to the dinuclear metal center, in which the phosphate ester coordinates to both Fe and Fe ions. Subsequent nucleophilic attack ensues from a /i-hydroxo ligand.

Two other well-characterized hydrolytic enzymes are known that also possess a very similar j3aj3a(3 protein structure, together with the phosphoesterase signature motif and a dinuclear metal center the phage A protein phosphatase (APP) and the 5 -nucleotidase (5 -NT) from... 

As noted in the introduction, the effects of multiple modes of catalysis are often multiplicative rather than simply additive. Consequently, it is not surprising that a number of hydrolytic metalloenzymes have evolved that utilize a constellation of three metal ions in catalysis. Perhaps not coincidentally, all well-characterized examples of this class catalyze the hydrolytic cleavage of phosphate ester or phosphoric acid anhydride bonds, which represent a difficult and long-standing chemical problem. In every case but one, the metal ions in the trimetal centers are all zinc. As we shall see, alkaline phosphatase utilizes a Zn2Mg trinuclear center. It should be pointed out that in the older literature many of the enzymes discussed in this section have been described as containing dinuclear metal centers. Only in the last few years has it become clear that three metal ions are present and participate in catalysis by these systems. 

Purple Acid Phosphatases. Purple acid phosphatases (PAPs) utilize a dinuclear metal center to catalyze the hydrolysis of phosphate monoesters. The characteristic purple color of these enzymes arises from a charge transfer absorption at about 560 nm, between a tyrosinate ligand and the conserved Fe + found in all PAPs. The second metal ion varies with the source of the enzyme and is always divalent. Mammalian PAPs are monomeric and have Fe -Fe " centers, whereas most plant PAPs are dimeric with Fe " -Zn + centers. A PAP isolated from sweet potato contains an Fe +-Mn + center, the first of its kind in any enzyme (26,27). This novel PAP also differs from others by its greater catalytic efficiency toward both activated and unactivated substrates (27), as well as in its strict requirement for manganese in the divalent site (26). 

Although structurally CODHj o, from the aerobic organism Oligotropha carboxy-dovorans, is a member of the XO family, its unusual dinuclear metal center suggests a unique catalytic mechanism. Like its anaerobic nickel-containing counterpart (see the section on nickel) the enzyme catalyzes the following reaction ... 

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