Research Areas

Biological Methane Oxidation

Biological Methane Oxidation

Left, transmission electron micrograph of Methylosinus trichosporium OB3b cells. The bundled membrane structures at the periphery house pMMO. Right, crystal structure of the Methylococcus capsulatus (Bath) pMMO trimer. The pmoB subunits are shown in gray, the pmoA subunits are shown in teal, and the pmoC subunits are shown in wheat. Copper ions are shown as cyan spheres and zinc ions are shown as gray spheres.

Methanotrophic bacteria oxidize methane to methanol in the first step of their metabolic pathway. Methane is a potent greenhouse gas, with a global warming potential more than 20 times that of carbon dioxide. Global warming presents a significant threat to human health, and as the only biological methane sink, methanotrophs have attracted much attention as a means of mitigating methane emissions. Moreover, increasing natural gas reserves combined with an ongoing price spread between natural gas and gasoline have led to renewed interest in bioconversion of methane.

Whereas current catalysts that can selectively activate the 105 kcal mol-1 C-H bond in methane require high temperatures and pressures, methanotrophs perform this chemistry under ambient conditions using methane monooxygenase (MMO) enzymes. The primary MMO in nature, particulate MMO (pMMO), is a three-subunit, integral membrane protein. Despite extensive research and the availability of multiple crystal structures, the active site structure and chemical mechanism of pMMO remain one of the major unsolved problems in bioinorganic chemistry. Current efforts in the laboratory are directed at elucidating the atomic details of the copper active site, understanding the mechanisms of dioxygen activation and methane oxidation, including how substrates, products, electrons, and protons access the active site, and probing the function of pMMO within the larger context of methanotroph physiology.

To learn more about this project:

Sirajuddin, S.; Rosenzweig, A. C. Enzymatic oxidation of methane. Biochemistry 2015, 54, 2283-2294.

Sirajuddin, S.; Barupala, D.; Helling, S.; Marcus, K.; Stemmler, T. L.; Rosenzweig, A. C. Effects of zinc on particulate methane monooxygenase activity and structure. J. Biol. Chem. 2014, 289, 21782-21794.

Balasubramanian, R.; Smith, S. M.; Rawat, S.; Yatsunyk, L. A.; Stemmler, T. L.; Rosenzweig, A. C. Oxidation of methane by a biological dicopper center.  Nature 2010, 465, 115-119.

Copper Acquisition by Methanotrophic Bacteria

Copper acquisition by methanotrophic bacteria

Top, biosynthesis of Ms. trichosporium OB3b Mbn (right) from its precursor peptide, MbnA (left). Bottom, Mbn operon from Ms. trichosporium OB3b. The proteins encoded by this operon function in Mbn transport and biosynthesis.

Copper acquisition is particularly important for methanotrophs because their primary metabolic enzyme, particulate methane monooxygenase (pMMO), requires copper for activity. Some methanotrophs meet their high requirement for copper by secreting and re-internalizing methanobactin (Mbn), a peptide-derived, copper-chelating natural product. Beyond its role in methanotrophy, Mbn also has antibiotic properties and is a potential therapeutic for diseases of copper overload

All Mbns characterized thus far bind Cu(I) with two nitrogen-containing heterocycles and two adjacent thioamide groups. Mbn is biosynthesized by post-translational modification of a precursor peptide called MbnA, and bioinformatics analyses revealed that the mbnA gene is located within an operon that also includes genes encoding machinery for Mbn transport as well as putative biosynthetic enzymes. Importantly, these operons are found in a range of non-methanotrophic bacteria, including gram-positive pathogens, suggesting the Mbn-like natural products may be widespread in nature. Current efforts in the laboratory are focused on discovering new Mbns and related natural products, characterizing the proteins involved in Mbn export and import, and unraveling the mechanisms of its biosynthesis.

To learn more about this project:

Kenney, G. E.; Sadek, M.; Rosenzweig, A. C. Copper-responsive gene expression in the methanotroph Methylosinus trichosporium OB3b. Metallomics 2016, DOI: 10.1039/C5MT00289C.

Kenney, G. E. and Rosenzweig, A. C. Genome mining for methanobactins. BMC Biol. 2013, 11, 17.

Metal Transport

Metal transport

Left, Architecture of a typical P1B-ATPase, including 8 transmembrane helices. MBD, metal binding domain; AD, actuator domain; ATPBD, ATP binding domain. Right, structure of the N-terminal MBD from the Cupriavidus metallidurans CH34 Cd(II), Co(II), and Zn(II)-transporting P1B-ATPase CzcP. Bound Cd(II) ions are shown as light yellow spheres.

Metalloproteins comprise close to one third of all proteins and almost half of all enzymes. Acquisition and management of metal ions is therefore a critical part of metabolism for all forms of life. Metals must be handled such that the correct ions are provided to essential enzymes and proteins, but do not accumulate to deleterious levels. Membrane transporters, metallochaperones, and metal sensors maintain metal ion concentrations in cells and cellular compartments, and aberrant handling of metal ions is linked to numerous human diseases. Understanding metal homeostasis on the molecular level is thus a central problem in cell biology and bioinorganic chemistry.

Current efforts in the laboratory are focused on P1B-ATPases, integral membrane proteins that couple the energy of ATP hydrolysis to translocation of transition metal ions across membranes. One feature unique to the P1B-ATPases is the presence of soluble metal binding domains that are believed to regulate metal transport. Current efforts in the laboratory are directed at determining the metal substrates for previously uncharacterized P1B-ATPases, elucidating the molecular details of metal binding by both the soluble domains and the transmembrane region, and investigating the molecular details of the transport mechanism.

To learn more about this project:

Smith, A. T.; Barupala, D.; Stemmler, T. L.; Rosenzweig, A. C. A new metal binding domain involved in cadmium, cobalt, and zinc transport. Nat. Chem. Biol. 2015, 11, 678-684.

Smith, A. T.; Smith, K. P.; Rosenzweig, A. C. Diversity of the metal-transporting P1B-type ATPases. J. Biol. Inorg. Chem. 2014, 6, 947-960.