Redox-dependent rearrangements of the NiFeS cluster of carbon monoxide dehydrogenase
Résumé
Life relies on countless chemical reactions, almost all of which need to be sped up by enzymes. About half of all enzymes carry metal ions that expand the range of the reactions that they can catalyze. In some enzymes these metal ions assemble with sulfur ions to form so-called metalloclusters. These structures can carry out many different types of reactions, including converting simple forms of elements like nitrogen and carbon into other forms that can be used to make more complicated biological molecules.
One enzyme that contains metalloclusters is carbon monoxide dehydrogenase. Known as CODH for short, this enzyme uses a metallocluster called the “C-cluster” to interconvert two gases: the pollutant carbon monoxide and the greenhouse gas carbon dioxide. CODH enzymes are found inside certain bacteria, but they are also of interest for humans, who wish to use them to remove the harmful gases from the environment. But this is not as simple as it may at first seem: CODH enzymes usually become inactive when exposed to air because the metalloclusters fall apart in the presence of oxygen. One CODH enzyme from a widespread bacterium called Desulfovibrio vulgaris, however, is an attractive target for industrial use because it can tolerate oxygen better. Yet, it is still unclear why this enzyme does not get inactivated the way other CODHs do.
Wittenborn et al. have now characterized the CODH enzyme from D. vulgaris in more depth via a technique called X-ray crystallography, which can reveal the location of individual atoms within a molecule. By a happy accident, the structures revealed that the C-cluster can adopt a dramatically different arrangement of metal and sulfur ions after being exposed to oxygen. This rearrangement is fully reversible; when oxygen is removed, the metal and sulfur ions move back to their normal positions. This ability to flip between different arrangements appears to protect the metallocluster from losing its metal ions when exposed to oxygen.
By providing structural snapshots of how CODH responds to oxygen these results provide a more complete understanding of an enzyme that plays a key role in the global carbon cycle. This understanding could help scientists to develop bioremediation tools to remove carbon monoxide and carbon dioxide from the atmosphere and to engineer bacteria to capture carbon to make biofuels.