2000 Annual Science Report
Pennsylvania State University Reporting | JUL 1999 – JUN 2000
Enzymes of Ancient Metabolic Pathways - Ferry 2
The origin of life occurred sometime between 3.8 (end of heavy bombardment of Earth) and 3.5 (presence of microfossils in ancient sediments) billion years ago. In the absence of a volcanic source of methane and ammonia, the Earth’s atmosphere was a weakly reduced mixture of CO2 and N2 combined with lesser amounts of CO, H2, and reduced S gases (Kasting, 1993). The yield of complex organic matter through abiotic reactions under these conditions is believed to have been extremely low (Kasting, 1997). However, experiments by W@chtersh@user (1997) found that an aqueous slurry of coprecipitated NiS and FeS converted CO and CH3SH into acetic acid. This opened up the possibility of a chemoautotrophic origin of life in which early organisms could fix CO2 and CO enzymatically. Carbon monoxide dehydrogenase/ acetyl CoA synthase (CODH/ACS), the central enzyme of the reductive acetyl CoA pathway of many diverse modern-day anaerobes, fixes CO and CO2 non-photosynthetically. CODH/ACS utilizes substrates (CO, H2, and CO2) proposed to be present in the earth’s atmosphere around the time life originated. Its active site contains a NiFeS metal center, similar to that demonstrated in the experiments by W@chtersh@user
Formation of this important ancient enzyme and its NiFeS active site requires cysteine. Through genetic, physiological and biochemical studies we determined that in Methanosarcina thermophila cysteine biosynthesis occurs through the serine pathway. The two genes required for this pathway form an operon in M. thermophila. O-acetyl-serine sulfhydrylase (OASS), which catalyzes the final step of the pathway, exhibits positive cooperativity and is expressed under growth conditions requiring cysteine biosynthesis. The last two genes of the homoserine pathway do not seem to be present in the genome, and the enzymes are not expressed under growth conditions requiring cysteine biosynthesis.