Methane monooxygenase sMMO

Two types of methane monooxygenases have been studied (1) soluble methane monooxygenase (sMMO) and (2) particulate (membrane-bound) methane monooxygenase (pMMO). The well-studied sMMO is produced by methanotrophs under copper-limiting conditions. All methanotrophs produce pMMO—found in intracytoplasmic membranes— but it is the less well-studied enzyme. 

Scheme 2.6 Examples of reactions catalyzed byenzymesthat carry a dinudear iron active site (a) hydroxylation of methane by soluble methane monooxygenase (sMMO) [7] (b) reduction of ribonucleotides by class I ribonucleotide reductase (RNR)...

Soluble methane monooxygenase (sMMO) is the best studied binuclear non-heme iron oxygenase enzyme, largely due... 

Methane is oxidized under aerobic conditions by a group of bacteria called methanotrophs. These widespread bacteria play an important role in the global cycling of methane. Two types of methane oxidation systems are known, a ubiquitous particulate methane monooxygenase (pMMO) and a cytoplasmic soluble methane monooxygenase (sMMO) found in only a few strains. These enzymes have different catalytic characteristics, and so it is important to know the conditions under which each is expressed. In those strains containing both sMMO and pMMO, the available copper concentration controls which enzyme is expressed. However, the activity of the pMMO is also affected by copper. Data on methane oxidation in natural samples suggest that methanotrophs are not copper-limited in nature and express the pMMO predominantly. 

However, a few species of methanotrophs also have the ability to produce a second cytoplasmic enzyme, called the soluble methane monooxygenase (sMMO) (1, 2) (Table I). When the sMMO is present, the complex internal membrane systems are absent. The sMMO has been intensively studied in the past few years, and much is known about this enzyme (5, 6) (Table II). It was purified and characterized from three strains, and in all three cases it consists of three components containing a total of five polypeptides. The genes for these polypeptides, were cloned and sequenced from two strains and show... 

Although soluble methane monooxygenases (sMMO) located in the cytoplasm of the bacterial cells have been well characterized in terms of their structure and reaction mechanism, " those of pMMO and AMO are poorly understood at present. This is due to difficulty in obtaining a pure enzyme that retains reactivity. It has so far been demonstrated that the enzymes contain up to 15 copper ions per enzyme, but only a type 2 copper(II) supported by three or four imidazole ligands can be detected by The X-ray absorption and EPR studies... 

Methane monooxygenase may exist in either soluble (sMMO) or particulate (pMMO) forms. These display different substrate ranges and different rates of transformation rates, and most methanotrophs express only the latter form of the enzyme (Hanson and Hanson 1996). The particulate form of methane monooxygenase contains copper, or both copper... 

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. 

We consider briefly the structure and mechanism of action of soluble MMO (sMMO), the best characterized of the BMMs (bacterial methane monooxygenases) (Figure 13.27), which is able to activate the inert C—H bond of... 

Methanotrophs synthesize both soluble (sMMO) and particulate (pMMO) monooxygenases and the synthesis of the particulate methane monooxygenase is determined by the concentration of Cu in the medium this is noted again in Chapter 5, Section 5.2.4. 

Cometabolism observed at methanotrophs is a result of nonspecific methane monooxygenase activity towards organic compounds that do not serve as carbon or energy sources (Brigmon, 2001). A soluble form of monooxygenase (sMMO), which is present in the type II methanotrophs, has low substrate specificity. It is able to oxidize several alkanes and alkenes, cychc hydrocarbons, aromatics, and halogenic aromatics (Grosse et al. 1999) for instance, trans-dichloroethylene, vinyl chloride (Yoon Semrau, 2008), dichloromethane (DCM) (Chiemchaisri et al. 2001), and 1,1,1-trichloroethane (TCA) (Kjeldsen et al. 1997). 

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