Abstract
Particulate methane monooxygenase (pMMO) is a characteristic membrane-bound metalloenzyme of methane-oxidizing bacteria that can catalyze the bioconversion of methane to methanol. However, in order to achieve pMMO-based continuous methane-to-methanol bioconversion, the problems of reducing power in vitro regeneration and pMMO stability need to be overcome. Methanobactin (Mb) is a small copper-chelating molecule that functions not only as electron carrier for pMMO catalysis and pMMO protector against oxygen radicals, but also as an agent for copper acquisition and uptake. In order to improve the activity and stability of pMMO, methanobactin–Cu (Mb–Cu)-modified gold nanoparticle (AuNP)–pMMO nanobiohybrids were straightforwardly synthesized via in situ reduction of HAuCl4 to AuNPs in a membrane fraction before further association with Mb–Cu. Mb–Cu modification can greatly improve the activity and stability of pMMO in the AuNP–pMMO nanobiohybrids. It is shown that the Mb–Cu-modified AuNP–pMMO nanobiohybrids can persistently catalyze the conversion of methane to methanol with hydroquinone as electron donor. The artificial heterogeneous nanobiohybrids exhibited excellent reusability and reproducibility in three cycles of catalysis, and they provide a model for achieving hydroquinone-driven conversion of methane to methanol.
Highlights
Green conversion of methane is a growing focus of energy and sustainable development
Consists of three components: a hydroxylase (MMOH, 251 kDa) component composed of three polypeptides and a hydroxo-bridged binuclear iron cluster, a reductase (MMOR, 38.6 kDa) component composed of one polypeptide containing both flavin adenine dinucleotide (FAD) and [Fe2 S2 ] cofactors, and a regulatory polypeptide (MMOB, 15.9 kDa) [3]. particulate methane monooxygenase (pMMO) is a copper-containing enzyme composed of three polypeptides with molecular masses of approximately 45 kDa, 26 kDa, and 23 kDa [4]
The preparation of the AuNP–pMMO nanobiohybrids in aqueous medium was carried out by adding the pMMO-enriched membrane fractions to an aqueous solution of HAuCl4 and gentle stirring at room temperature
Summary
Green conversion of methane is a growing focus of energy and sustainable development. The only selective catalyst for methane conversion to methanol under normal temperature and pressure conditions is methane monooxygenase (MMO) found in methane-oxidizing bacteria (methanotrophic bacteria) [1]. Methane-oxidizing bacteria can express two different MMOs regulated by copper concentration during growth [2], which are soluble methane monooxygenase (sMMO) and particulate methane monooxygenase (pMMO). In cells cultured under lower copper concentration, the sMMO located in the intercellular space is predominately expressed. In cells cultured under higher copper concentration, the pMMO present in the inner cytoplasmic membrane is exclusively expressed. SMMO consists of three components: a hydroxylase (MMOH, 251 kDa) component composed of three polypeptides and a hydroxo-bridged binuclear iron cluster, a reductase (MMOR, 38.6 kDa) component composed of one polypeptide containing both flavin adenine dinucleotide (FAD) and [Fe2 S2 ] cofactors, and a regulatory polypeptide (MMOB, 15.9 kDa) [3]. The metal content of pMMO reported by different laboratories is controversial, with reported values of Molecules 2019, 24, 4027; doi:10.3390/molecules24224027 www.mdpi.com/journal/molecules
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