Methane monooxygenase pMMO

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. 

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. 

To understand the role of these bacteria in methane cycling, the methane oxidation system must be studied. In methanotrophs, methane is oxidized to methanol by an enzyme called the methane monooxygenase (MMO) (I), which uses methane, molecular oxygen, and reducing equivalents to produce methanol and water. All known methanotrophs contain a membrane-bound MMO, called the particulate methane monooxygenase (pMMO). The presence of this enzyme system is correlated with the complex internal membrane system found in all known methanotrophs. 

Himes, R. A., Karlin, K. D. (2009). Copper-dioxygen complex mediated C—H bond oxygenation relevance for particulate methane monooxygenase (pMMO). Current Opinion in Chemical Biology, 13, 119—131. 

Finally, membrane-bound ( particulate ) methane monooxygenase (pMMO) is a bacterial enzyme that effects the oxidation of methane to methanol (Equation (5)) ... 

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... 

Itoh et al. used Cu yd-diketiminato complexes with general formula 4, and their reactivity has been described as a functional model for pMMO (particulate methane monooxygenase). Initially, the Hgands were reacted with both Cu and Cu precursors, with a variety of species formed, depending on the specific conditions employed [111, 112]. It was then shown that both Cu and Cu complexes ultimately led to bis(/z-oxo)(Cu )2 species upon reaction with O2 and H2O2, respectively. Use of these Cu complexes as the pre-catalysts for the oxidation of alkanes (cyclohexane and adamantane) in the presence of H2O2 resulted in low yields ( 20%). 

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. 

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. 

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... 

It is also worth mentioning the biological significance of these studies and the inspiration of biology on their development. In fact, pMMO (particulate methane monooxygenase), a main enzyme in the metabolic pathway of methanotrophs, is a membrane multicopper enzyme that catalyzes the oxidation of alkanes to the corresponding alcohols [66-68], which is mimicked by the multinuclear copper systems. 

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