The invention relates to a genetically engineered bacterium comprising an energy-generating fermentation pathway and methods related thereto. In particular, the invention provides a bacterium comprising a phosphate butyryltransferase (Ptb) and a butyrate kinase (Buk) (Ptb-Buk) that act on non-native substrates to produce a wide variety of products and intermediates. In certain embodiments, the invention relates to the introduction of Ptb-Buk into a C1-fixing microoorgansim capable of producing products from a gaseous substrate.