Nickel and the carbon cycle.

This article, dedicated to Edward Stiefel, reviews three nickel enzymes that play important roles in the carbon cycle: CO dehydrogenase, acetyl-CoA synthase, and methyl-coenzyme M reductase. After a short discussion of the carbon cycle, the structures of the active centers of the proteins and their proposed mechanisms are discussed. A brief description of future research areas is presented for each enzyme system. A short perspective on future research on nickel enzymes ends this contribution.