Global climate change and the energy crisis have led to a renewed focus on the biology and chemistry of one-carbon compounds, including the greenhouse gas carbon dioxide and toxic pollutant carbon monoxide. Anthropogenic CO and CO₂ continue to be added to the atmosphere and the oceans at an alarming rate and are major threats to human and environmental health. The last decade has seen dramatic advances in our understanding of the structures and mechanisms employed by carbon monoxide dehydrogenase (CODH) and acetyl coenzyme A synthase (ACS), evolutionarily ancient nickel-containing enzymes that catalyze CO and CO₂ chemistry.¹  These studies have not only provided fascinating insights into environmentally essential biological processes in the global carbon cycle, they have also inspired the development of biomimetic catalysts for CO₂ and CO utilization.