Binding of a second CO molecule observed (IMAGE) University of Freiburg Caption The iron-vanadium (FeV) cofactor in vanadium-dependent nitrogenase was made to react with carbon monoxide (CO) and then gassed under pressure, allowing two molecules of the substrate to be visualized in bound form. FeV cofactor is one of the largest and most complex metal centers in proteins currently known. It consists of seven iron ions (gray), 9 sulfur ions (yellow), a central carbon (black), and a vanadium ion (green), and also carries a carbonate ion and a molecule of homocitrate as organic ligands. Credit Graphic: Oliver Einsle Usage Restrictions The graphic may only be used with appropriate caption or credit. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.