Methane monooxygenase copper enzyme

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... 

H. H. T. Nguyen, S. J. Elliott, J. H. K. Yip, S. I. Chan (1998) The particulate methane monooxygenase from Methylococcus capsulatus (Bath) is a novel copper-containing three-subunit enzyme - Isolation and characterization. J. Biol. Chem., 273 7957-7966... 

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. 

Usually, these metalloproteins contain both type 2 and type 3 copper centers, together forming a triangular-shaped trinuclear active site, such as found in laccase (polyphenol oxidase) [38-41] and ascorbate oxidase (3) [42]. Recent evidence for a related arrangement has been reported for the enzyme particulate methane monooxygenase as well [43], but in this case the Cu Cu distance of the type 2 subunit (2.6 A) appears to be unusually short and the third Cu ion is located far from the dinuclear site. 

Figure 3 Illustration of possible partial reaction cycles of some oxygenase enzymes. Water molecules and protein ligands have sometimes been omitted for clarity, (a) P450 (18) (b) intradiol dioxygenase (7) (c) lipoxygenase (7) (d) a-KG-dependent non-heme Iron enzymes (14) (e) soluble methane monooxygenase (15) (f) uncoupled blnuclear copper (16) (g) coupled blnuclear copper (h) flavin monooxygenases (17).

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. 

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. 

Following the discovery that the active site of particulate methane monooxygenase contains a dicopper site [ 142], evidence was obtained for a mixed-valent copper(I,II) species in the isolated form of this enzyme and for the lengthening of the metal-metal distance upon reduction to the Cu(I,I)... 

Methane is regularly used as a fuel in microbial fuel cells and traditional metal-catalyzed fuel cells, but it has yet to be studied with enzymatic BFC systems. Methane monooxygenase is a membrane-bound enzyme that reduces methane to methanol with low redox potentials (< 100 mV) [44]. It is weU characterized, and one source of the enzyme has a copper center that would enable DET at the eleetrode [45]. This enzyme could be added to a methanol anode, thus allowing the use of methane as a fuel in a gas-permeable fuel eell. 

Xin J-Y, Cui J-R, Hu X-X, Li S-B, Xia C-G, Zhu L-M, Wang Y-Q. Particulate methane monooxygenase from Methylosinus trichosporium is a copper-containing enzyme. Biochem Biophys Res Commun 2002 295 182-186. 

Monooxygenase activity has been characterized for a couple of enzymes other than tyrosinase that use different numbers of copper ions to activate dioxygen. While tricopper sites have been proposed to act in concert in particular copper containing methane... 

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