Hydrogenase and fumarate reductase isolated fromWoli-nella succinogeneswere incorporated into liposomes con-taining menaquinone. The two enzymes were found to be oriented solely to the outside of the resulting proteolipo-somes. The proteoliposomes catalyzed fumarate reduction by H2 which generated an electrical proton potential (Dw V, negative inside) in the same direction as that gen-erated by fumarate respiration in cells ofW. succinogenes.