The Biotransformation of Propylene to Propylene Oxide byMethylococcus Capsulatus(Bath): 1. Optimization of Rates

Facultative methanotrophs - diversity, genetics, molecular ecology and biotechnological potential: a mini-review

Methane-oxidizing bacteria (methanotrophs) play a vital role in reducing atmospheric methane emissions, and hence mitigating their potent global warming effects. A significant proportion of the methane released is thermogenic natural gas, containing associated short-chain alkanes as well as methane. It was one hundred years following the description of methanotrophs that facultative strains were discovered and validly described. These can use some multi-carbon compounds in addition to methane, often small organic acids, such as acetate, or ethanol, although Methylocella strains can also use short-chain alkanes, presumably deriving a competitive advantage from this metabolic versatility. Here, we review the diversity and molecular ecology of facultative methanotrophs. We discuss the genetic potential of the known strains and outline the consequent benefits they may obtain. Finally, we review the biotechnological promise of these fascinating microbes.

Mutagenesis and expression of methane monooxygenase to alter regioselectivity with aromatic substrates

Soluble methane monooxygenase (sMMO) from methane-oxidising bacteria can oxygenate more than 100 hydrocarbons and is one of the most catalytically versatile biological oxidation catalysts. Expression of recombinant sMMO has to date not been achieved in Escherichia coli and so an alternative expression system must be used to manipulate it genetically. Here we report substantial improvements to the previously described system for mutagenesis of sMMO and expression of recombinant enzymes in a methanotroph (Methylosinus trichosporium OB3b) expression system. This system has been utilised to make a number of new mutants and to engineer sMMO to increase its catalytic precision with a specific substrate whilst increasing activity by up to 6-fold. These results are the first 'proof-of-principle' experiments illustrating the feasibility of developing sMMO-derived catalysts for diverse applications.

Alkane Oxidation: Methane Monooxygenases, Related Enzymes, and Their Biomimetics

Methane monooxygenases (MMOs) mediate the facile conversion of methane into methanol in methanotrophic bacteria with high efficiency under ambient conditions. Because the selective oxidation of methane is extremely challenging, there is considerable interest in understanding how these enzymes carry out this difficult chemistry. The impetus of these efforts is to learn from the microbes to develop a biomimetic catalyst to accomplish the same chemical transformation. Here, we review the progress made over the past two to three decades toward delineating the structures and functions of the catalytic sites in two MMOs: soluble methane monooxygenase (sMMO) and particulate methane monooxygenase (pMMO). sMMO is a water-soluble three-component protein complex consisting of a hydroxylase with a nonheme diiron catalytic site; pMMO is a membrane-bound metalloenzyme with a unique tricopper cluster as the site of hydroxylation. The metal cluster in each of these MMOs harnesses O₂ to functionalize the C-H bond using different chemistry. We highlight some of the common basic principles that they share. Finally, the development of functional models of the catalytic sites of MMOs is described. These efforts have culminated in the first successful biomimetic catalyst capable of efficient methane oxidation without overoxidation at room temperature.

Methanol improves methane uptake in starved methanotrophic microorganisms

Methanotrophs in enrichment cultures grew and sustained atmospheric methane oxidation when supplied with methanol. If they were not supplied with methanol or formate, their atmospheric methane oxidation came to a halt, but it was restored within hours in response to methanol or formate. Indigenous forest soil methanotrophs were also dependent on a supply of methanol upon reduced methane access but only when exposed to a methane-free atmosphere. Their immediate response to each methanol addition, however, was to shut down the oxidation of atmospheric methane and to reactivate atmospheric methane oxidation as the methanol was depleted.

Degradation of organic pollutants by methane grown microbial consortia

Microbial consortia were enriched from various environmental samples with methane as the sole carbon and energy source. Selected consortia that showed a capacity for co-oxidation of naphthalene were screened for their ability to degrade methyl-tert-butyl-ether (MTBE), phthalic acid esters (PAE), benzene, xylene and toluene (BTX). MTBE was not removed within 24 h by any of the consortia examined. One consortium enriched from activated sludge ("AAE-A2"), degraded PAE, including (butyl-benzyl)phthalate (BBP), and di-(butyl)phthalate (DBP). PAE have not previously been described as substrates for methanotrophic consortia. The apparent Km and Vmax for DBP degradation by AAE-A2 at 20 degrees C was 3.1 +/- 1.2 mg l(-1) and 8.7 +/- 1.1 mg DBP (g protein x h)(-1), respectively. AAE-A2 also showed fast degradation of BTX (230 +/- 30 nmol benzene (mg protein x h)(-1) at 20 degrees C). Additionally, AAE-A2 degraded benzene continuously for 2 weeks. In contrast, a pure culture of the methanotroph Methylosinus trichosporium OB3b ceased benzene degradation after only 2 days. Experiments with methane mono-oxygenase inhibitors or competitive substrates suggested that BTX degradation was carried out by methane-oxidizing bacteria in the consortium, whereas the degradation of PAE was carried out by non-methanotrophic bacteria co-existing with methanotrophs. The composition of the consortium (AAE-A2) based on polar lipid fatty acid (PLFA) profiles showed dominance of type II methanotrophs (83-92% of biomass). Phylogeny based on a 16S-rRNA gene clone library revealed that the dominating methanotrophs belonged to Methylosinus/Methylocystis spp. and that members of at least 4 different non-methanotrophic genera were present (Pseudomonas, Flavobacterium, Janthinobacterium and Rubivivax).

The Leeuwenhoek Lecture 2000 the natural and unnatural history of methane-oxidizing bacteria

Methane gas is produced from many natural and anthropogenic sources. As such, methane gas plays a significant role in the Earth's climate, being 25 times more effective as a greenhouse gas than carbon dioxide. As with nearly all other naturally produced organic molecules on Earth, there are also micro-organisms capable of using methane as their sole source of carbon and energy. The microbes responsible (methanotrophs) are ubiquitous and, for the most part, aerobic. Although anaerobic methanotrophs are believed to exist, so far, none have been isolated in pure culture. Methanotrophs have been known to exist for over 100 years; however, it is only in the last 30 years that we have begun to understand their physiology and biochemistry. Their unique ability to use methane for growth is attributed to the presence of a multicomponent enzyme system-methane monooxygenase (MMO)-which has two distinct forms: soluble (sMMO) and membrane-associated (pMMO); however, both convert methane into the readily assimilable product, methanol. Our understanding of how bacteria are capable of effecting one of the most difficult reactions in chemistry-namely, the controlled oxidation of methane to methanol-has been made possible by the isolation, in pure form, of the enzyme components.The mechanism by which methane is activated by sMMO involves abstraction of a hydrogen atom from methane by a high-valence iron species (FeIV or possibly FeV) in the hydroxylase component of the MMO complex to form a methyl radical. The radical combines with a captive oxygen atom from dioxygen to form the reaction product, methanol, which is further metabolized by the cell to produce multicarbon intermediates. Regulation of the sMMO system relies on the remarkable properties of an effector protein, protein B. This protein is capable of facilitating component interactions in the presence of substrate, modifying the redox potential of the diiron species at the active site. These interactions permit access of substrates to the hydroxylase, coupling electron transfer by the reductase with substrate oxidation and affecting the rate and regioselectivity of the overall reaction. The membrane-associated form is less well researched than the soluble enzyme, but is known to contain copper at the active site and probably iron. From an applied perspective, methanotrophs have enjoyed variable successes. Whole cells have been used as a source of single-cell protein (SCP) since the 1970s, and although most plants have been mothballed, there is still one currently in production. Our earlier observations that sMMO was capable of inserting an oxygen atom from dioxygen into a wide variety of hydrocarbon (and some non-hydrocarbon) substrates has been exploited to either produce value added products (e.g. epoxypropane from propene), or in the bioremediation of pollutants such as chlorinated hydrocarbons. Because we have shown that it is now possible to drive the reaction using electricity instead of expensive chemicals, there is promise that the system could be exploited as a sensor for any of the substrates of the enzyme.

Molecular characterization of structural genes coding for a membrane bound hydrogenase in Methylococcus capsulatus (Bath)

The first gene cluster encoding for a membrane bound [NiFe] hydrogenase from a methanotroph, Methylococcus capsulatus (Bath), was cloned and sequenced. The cluster consisted of the structural genes hupS and hupL and accessory genes hupE, hupC and hupD. A DeltahupSL deletion mutant of Mc. capsulatus was constructed by marker exchange mutagenesis. Membrane associated hydrogenase activity disappeared. The membrane associated hydrogenase appeared to have a hydrogen uptake function in vivo.

Production of enantiopure styrene oxide by recombinant Escherichia coli synthesizing a two-component styrene monooxygenase

A whole cell biocatalytic process was developed to enable the efficient oxidation of styrene to chiral (S)-styrene oxide with an enantiomeric excess better than 99%. Recombinant Escherichia coli cells were employed to express the genes styAB encoding the styrene monooxygenase of Pseudomonas sp. strain VLB120 from an expression plasmid utilizing the alk regulatory system of P. oleovorans GPo1. The strains reached specific activities of up to 70 U* (g cell dry weight)(-1) in shake-flask experiments with glucose as the carbon source. An efficient two-liquid phase fed-batch process was established for the production of (S)-styrene oxide with hexadecane as an apolar carrier solvent and a nutrient feed consisting of glucose, magnesium sulfate, and yeast extract. Engineering of the phase fraction and the composition of organic phase and feed led to a 2-L scale process with maximal volumetric productivities of 2.2 g (S)-styrene oxide per liter liquid volume per hour. This optimized process was based completely on defined medium and used bis(2-ethylhexyl)phthalate as the apolar carrier solvent, which together with substrate and inducer consisted of 50% of the total liquid volume. Using this system, we were able to produce per liter liquid volume 11 g of enantiopure (S)-styrene oxide in 10 h.