Current demand for plastics and chemical raw materials is met through large-scale production of ethylene from fossil fuels. This makes it necessary to search for new, renewable processes.
Using bacterial enzymes as catalysts could be the key, but only a few naturally occurring enzymes have the ability to form ethylene. These enzymes typically require energy-rich substrates and produce CO as a by-product.
Thus, a few years ago, the scientific community was genuinely excited when the enzyme methylthio-alkane reductase was discovered in the bacterium Rhodospirillum rubrum. This enzyme enables the bacterium to produce ethylene under oxygen-free conditions without releasing CO.
The oxygen-free nature of this process posed a problem. Due to the considerable challenges involved in purifying and handling these oxygen-sensitive metalloenzymes, methylthio-alkane reductase could only be studied in cell cultures and there was no detailed understanding of their inner workings. Many important questions regarding its biotechnological potential remain unanswered: How can the enzymes catalyze this reaction, and what properties determine it?
Researchers at the Max Planck Institute for Terrestrial Microbiology in Marburg, led by Dr. Johannes Rebelein, have now succeeded in purifying the enzyme and elucidating its structure in collaboration with RPTU Kaiserslautern. The catalytic, spectroscopic and structural characterisation revealed an exciting discovery: 'The reaction is driven by large, complex iron-sulfur clusters, which were previously thought to occur only in nitrogenases, some of the oldest enzymes on Earth,' explains Ana Lago-Maciel, a doctoral student and the study's first author. The methylthio-alkane reductase is the first non-nitrogenase enzyme known to contain these metal clusters.
Nitrogenases emerged billions of years ago as the only enzymes in nature that can reduce gaseous nitrogen from the atmosphere, making it available for life by enabling the incorporation of nitrogen into biomolecules such as DNA and proteins. This unique ability is based on the big and complex iron-sulfur clusters. Due to their structural complexity and geochemical significance, nitrogenase clusters are classified as one of the "great clusters of biology".
The research provides the biochemical and structural basis for a geochemically significant source of hydrocarbons. 'In fact, the enzyme has remarkable versatility,' explains Johannes Rebelein. 'It can produce sustainably a range of hydrocarbons including ethylene, ethane and methane."
The enzyme's substrate spectrum is very different from that of nitrogenases and opens new doors for understanding how the reactivity of metal clusters is determined by the protein scaffold. 'Our study provides the in-depth structural knowledge we need to tame these reductases biotechnologically and adapt their product spectrum to our needs,' says Johannes Rebelein.
He adds, the results provide clues about the past evolution of the 'great clusters of biology'. 'Our results suggest that structurally similar enzymes were using these clusters for reductive catalysis long before nitrogenases evolved. "This is a significant shift in our understanding of this crucial part of Earth's history."