New insights into structure-function relationships in nitrogenase: A 1.6 angstrom resolution X-ray crystallographic study of Klebsiella pneumoniae MoFe-protein

The X-ray crystal structure of Klebsiella pneumoniae nitrogenase component 1 (Kp1) has been determined and refined to a resolution of 1.6 Angstrom, th e highest resolution reported for any nitrogenase structure. Models derived from three 1.6 Angstrom resolution X-ray data sets are described; two repr esent distinct oxidation states, whilst the third appears to be a mixture o f both oxidized and reduced states (or perhaps an intermediate state). The structures of the protein and the iron-molybdenum cofactor (FeMoco) appear to be largely unaffected by the redox status, although the movement of Ser beta 90 and a surface helix in the beta subunit may be of functional signif icance. By contrast, the 8Fe-7S beta-cluster undergoes discrete conformatio nal changes involving the movement of two iron atoms. Comparisons with know n component 1 structures reveal subtle differences in the FeMoco environmen t, which could account for the lower midpoint potential of this cluster in Kp1. Furthermore, a non-proline-cis peptide bond has been identified in the a subunit that may have a functional role. It is within 10 Angstrom of the FeMoco and may have been overlooked in other component 1 models. Finally, metal-metal and metal-sulphur distances within the metal clusters agree wel l with values derived from EXAFS studies, although they are generally longe r than the values reported for the closely related protein from Azotobacter vinelandii. A number of bonds between the clusters and their ligands are d istinctly longer than the EXAFS values, in particular, those involving the molybdenum atom of the FeMoco. (C) 1999 Academic Press.