Methane hydroxylase component MMOH

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.

A structural module comprising tetra(carboxylato)di-iron(II) centers with two additional A-donor groups has been identified at the active sites of the hydroxylase component (MMOH) of soluble methane monooxygenase, the R2 subunit of class I ribonucleotide reductase (RNR-R2), " " and stearoyl-acyl carrier protein (AGP) A desaturase (A9D). d8 structural comparisons of these enzymes reveal striking architectural similarities involving an approximately 18 A segment of a pseudo-222-symmetric four-helix bundle that encapsulates their di-iron active... 

The multiprotein complex methane monooxygenase (MMO) serves meth-anotrophs to convert methane to methanol. It can be either soluble (sMMO) or membrane bound ( particulate , pMMO) and it typically consists of three components, a reductase (MMOR), a component termed protein B (MMOB) and a hydroxylase denoted MMOH. The nature of the metal cofactors in the latter component are reasonably well understood for sMMO as will be discussed in the non-heme iron section. For the pMMO of Methylococcus capsulatus an obligate requirement for copper was shown. As reported in reference 1 a trinuclear Cu(II) cluster was discussed128 but the number and coordination of coppers still is a matter of continuing investigation since then. 

The presentation in 1993 of the structure of the hydroxylase component of methane monooxygenase (MMOH) by Rosenzweig et al. (15) is the third published three-dimensional structure of a diiron-oxygen protein (Fig. 1). The previous two are from hemerythrin (Hr) (16,17) and protein R2 of E. coli ribonucleotide reductase (RNR-R2) (18, 19). Some other dinuclear iron proteins with known fi-oxo or p.-hydroxo bridges are purple acid phosphatases (PAP) [(e.g., uteroferrin (Uf)] (20, 21), ferritins (in early stages of nucleation) (22), rubrerythrin (Rr) (23-26), nigerythrin (26), and soluble stearoyl-acyl carrier protein A desaturase (A-AGP) (27, 28). 

ACP = acyl carrier protein ACPA D = ACPA desat-urase AlkB = octane 1-monooxygenase AOX = alternative oxidase DMQ hydroxylase = 5-demethoxyquinone hydroxylase EXAFS = extended X-ray absorption fine structure spectroscopy FMN = flavin mononucleotide FprA = flavoprotein A (flavo-diiron enzyme homologue) Hr = hemerythrin MCD = magnetic circular dichroism MME hydroxylase = Mg-protophorphyrin IX monomethyl ester hydroxylase MMO = methane monooxygenase MMOH = hydroxylase component of MMO NADH = reduced nicotinamide adenine dinucleotide PAPs = purple acid phosphatases PCET = proton-coupled electron transfer, PTOX = plastid terminal oxidase R2 = ribonucleotide reductase R2 subunit Rbr = rubrerythrin RFQ = rapid freeze-quench RNR = ribonucleotide reductase ROO = rubredoxin oxygen oxidoreductase XylM = xylene monooxygenase. 

A number of other metalloenzymes have M 2(His)2(02CR)4 active sites (see Chapter 8.13). Most prominent of these are the di-iron enzymes, including the hydroxylase component of methane monooxygenase (MMOH, Figure 15a) and class 1 ribonucleotide reductase R2 proteins (Figure ISb). " These enzymes have two conserved Asp/Glu(Xaa)nGluXaaXaaHis sequence motifs in a four-helix bundle that provide the six amino-acid ligands for the di-iron... 

Proteins with dinuclear iron centres comprise some prominent and well studied representatives like ribonucleotide reductase (RNR), purple acid phosphatase (PAP), methane monooxygenase hydroxylase (MMOH), ruberythrin and hemerythrin. The last of these is an oxygen carrier in some sea worms it has been well characterized within this group and has thus laid the foundation to this class of iron coordination motif. Ruberythrin is found in anaerobic sulfate-reducing bacteria. Its name implies that, in addition to a hemerythrin-related diiron site another iron is coordinated in a mononuclear fashion relating to rubredoxin, which is an iron-sulfur centre. The latter will not be treated here. The hydroxylase component of methane monooxygenase is one of the three components in soluble methane monooxygenase (MMO) and contains the active diiron site it is found in methanotropic bacteria. Purple add phosphatase (PAP) occurs mainly in plants and animals, and catalyses the hydrolysis of monophosphate esters. Finally, ribonucleotide reductase reduces ribonucleotides to deoxyribonucleotides and thus has a key position in DNA synthesis. 

The best characterized of the BMMs are the sMMOs (Figure 13.24), which are the only members of the family capable of activating the inert C-H bond of methane, one of the most difficult reactions in nature to achieve. Like most members of the BMM superfamily, sMMO requires three protein components, the hydroxylase MMOH, which contains the carboxylate-bridged diiron centre, a regulatory protein MMOB and a [2Fe-2S]- and FAD-containing reductase (MMOR) which shuttles electrons from NADH to the diiron centre. 

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