2010 Annual Science Report
Arizona State University Reporting | SEP 2009 – AUG 2010
Stoichiometry of Life - Task 1f - Concept Studies - Nickel-Molybdenum Co-Limitation and Evolution of Mo-Nitrogenase
The element molybdenum (Mo) is critical for key processes in the cycling of nitrogen (N); for example, it is essential for the enzyme nitrogenase which bacteria use to convert gaseous N to “fixed” N that can be used in biological processes. However, this process costs a lot of energy. In some microbes, this energy can be captured and used via enzymes that involve both Mo and nickel (Ni). This project investigates the role of these Ni-Fe enzymes in making nitrogenase-driven processes more energetically efficient and how these enzymes may have evolved in the deep past when Ni concentrations were lower.
Graduate Student Jennifer Glass has recently written a hypothesis paper (in prep. for Nature Geoscience) in collaboration with Eric Boyd (MSU Team), Steve Romaniello and Ariel Anbar on an idea she first presented at a 2010 AbSciCon Lightning Talk, which is summarized here: In modern biological systems, molybdenum (Mo)-nitrogenase is the primary enzyme responsible for the conversion of dinitrogen to ammonia. However, in the low-Mo Archean ocean, it is thought that nitrogen fixation was catalyzed by nitrogenase enzymes that contained vanadium (V) or iron (Fe) in place of Mo. During dinitrogen reduction these “alternative” forms of nitrogenase expend significant energy in the form of hydrogen gas, resulting in the need for an additional ~8 (V-nitrogenase) and ~32 (Fe-nitrogenase) mol ATP per catalytic cycle when compared to Mo-nitrogenase. A fraction of this energy can be recovered if the hydrogen produced by nitrogenase is recycled by Ni-Fe hydrogenases. We propose that a drop in marine Ni concentrations 2.7 billion years ago (Konhauser et al., 2009, Nature) may have impaired the activity of these hydrogenase enzymes, leading to a reduced ability of organisms to recycle the hydrogen produced by alternative nitrogenases. In combination with transient pulses of Mo into the ocean as a result of increased oxidative weathering during this period, this marine Ni famine could have provided the evolutionary impetus for the evolution of the more efficient Mo-containing form of nitrogenase.
PROJECT INVESTIGATORS:Ariel Anbar
PROJECT MEMBERS:Eric Boyd
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere.
Environment-dependent, molecular evolution in microorganisms
Co-evolution of microbial communities
Effects of environmental changes on microbial ecosystems