THE MOLYBDENUM IRON PROTEIN OF NITROGENASE - STRUCTURAL AND FUNCTIONAL FEATURES OF METAL CLUSTER PROSTHETIC GROUPS

Current proposals for structures of the two recognized types of metal-sulfur clusters in nitrogenase are compared to chemical and spectroscopic evidence in hand. It appears that the P-clusters are pairs of linked Fe4S4 cubes, and the most reasonable interpretation of the oxidation state of the resting enzyme, prior to ATP- coupled injection of electrons from the Fe protein, suggests that the Fe atoms are all ferrous, making it appear that electrons added to such clusters would create a strongly reducing entity in the protein, and further suggesting that the P-clusters donate reducing equivalents to the M (cofactor) clusters, the presumed locale of a dinitrogen reduction. The reported trigonal geometry of six of the Fe atoms in the M centers immediately suggests that these have bound hydride unseen in the crystal structure. Enzyme turned over with ATP and reductant, in D2O, failed in initial experiments to yield EPR evidence for the expected strongly coupled deuterons. If the resting state has a powerful reductant (native P-clusters) and a H+-reducing site (M-centers) one has to suppose that ATP facilitates the transfer of e- between these centers, in addition to the already observed binding of ATP to the Fe protein component of nitrogenase.