Atmospheric carbon dioxide levels are rising, triggering global climate change, scientists agree. Researchers have been searching for ways to scrub some of this damaging gas from the atmosphere, and the answer may have been right in front of them. "We actually have taken our inspiration from nature itself," says Tobias Erb, a biochemist at the Max Planck Institute for Terrestrial Microbiology in Germany, in a phone interview with The Christian Science Monitor. Plants and other photosynthesizing organisms can turn carbon dioxide into biomass. And now Dr. Erb and his team have built a synthetic pathway to do that more efficiently – at least in a test tube, and perhaps someday in plants or other organisms.
Almost every living creature on Earth relies on photosynthesis for its survival, but the process is far from efficient. Now some clever genetic engineering that gets around one of the process's stumbling blocks has been shown to boost crop productivity by 40 percent in the field. The world is crying out for these kinds of transformative gains. The Green Revolution of the 1960s saw global crop yields rise dramatically as fertilizers, pesticides, and industrial agriculture became widespread. But we've more or less optimized these methods, and current productivity improvements are ticking up at around two percent per year.
Rising atmospheric carbon dioxide (CO2) concentration as a result of extensive use of fossil fuel resources is one of the main causes of global warming. Natural photosynthesis converts 100 billion tons of CO2 into biomass annually (1). Although natural photosynthesis plays a vital role in absorbing CO2 emitted from fossil fuel use, it cannot prevent the net increase of atmospheric CO2 concentration since the Industrial Revolution. Natural CO2 fixation is mainly achieved by a CO2 fixation pathway called the Calvin cycle, in which ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is the key enzyme. To date, six CO2 fixation pathways, including the Calvin cycle, have been discovered (2). On page 900 of this issue, Schwander et al. (3) report a synthetic CO2 fixation pathway that is more energy efficient than the Calvin cycle, expanding the capabilities for recapturing atmospheric CO2 for use as a carbon feedstock.
A set of chemical reactions occurring spontaneously in Earth's early chemical environments could have provided the foundations upon which life evolved. The discovery that a version of the Krebs cycle, which occurs in most living cells, can proceed in the absence of cellular proteins called enzymes suggests that metabolism is older than life itself. Metabolism describes the fiendishly complex network of reactions that enable organisms to generate energy and the molecules they need to survive, grow and reproduce. The Krebs cycle – also known as the tricarboxylic acid (TCA) cycle – is at the heart of this network. It describes a circular chain of reactions that generates precursors of amino acids and lipids used to build proteins and membranes, and molecules that help the cell to produce its energy.
Lipoyl synthase (LipA) catalyzes the last step in the biosynthesis of the lipoyl cofactor, which is the attachment of two sulfhydryl groups to C6 and C8 of a pendant octanoyl chain. The appended sulfur atoms derive from an auxiliary [4Fe-4S] cluster on the protein that is degraded during turnover, limiting LipA to one turnover in vitro. We found that the Escherichia coli iron-sulfur (Fe-S) cluster carrier protein NfuA efficiently reconstitutes the auxiliary cluster during LipA catalysis in a step that is not rate-limiting. We also found evidence for a second pathway for cluster regeneration involving the E. coli protein IscU. These results show that enzymes that degrade their Fe-S clusters as a sulfur source can nonetheless act catalytically.