Methylthio-alkane reductases convert methylated sulfur compounds to methanethiol and small hydrocarbons, a process with important environmental and biotechnological implications.1 These enzymes are classified as nitrogenase-like enzymes, despite lacking the ability to convert dinitrogen to ammonia, raising fundamental questions about the factors controlling their activity and specificity. To elucidate this reactivity, we have just comprehensively characterized the in vitro activity and structure of the methylthio-alkane reductase by biochemical, spectroscopic and structural means.2 These studies revealed that the methylthio-alkane reductase contains large nitrogenase metalloclusters, specifically the [Fe8S7]-cluster (P-cluster) and an [Fe8S9C]-cluster (L-cluster) in the active site. These large nitrogenase metalloclusters have never been found outside nitrogenase enzymes. Our findings suggest that distinct metallocluster coordination, surroundings, and substrate channels, determine the activity difference between nitrogenases and methylthio-alkane reductase, preventing the reduction of N2 by methylthio-alkane reductases. Furthermore, our findings indicate that P- and [Fe8S9C]-cluster-containing reductases likely predated the emergence of bona fide nitrogenases. Thus, the investigation of methylthio-alkane reductases provided new insights into nitrogen fixation, sulfur-compound reduction, and hydrocarbon production.