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Crystal Structure of Azotobachter vinelandii Nitrogenase Iron Protein at 2.2 Å Resolution


Schlessman, Jamie L. (1997) Crystal Structure of Azotobachter vinelandii Nitrogenase Iron Protein at 2.2 Å Resolution. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8bav-4095.


Biological nitrogen fixation by the two-component metalloenzyme nitrogenase provides an elegant solution to the problem of reducing abundant, but relatively inert, dinitrogen to the biologically usable ammonia needed by all organisms. This oxygen sensitive enzyme, consisting of the separately purifiable nitrogenase iron protein and molybdenum iron protein, couples nucleotide hydrolysis to electron transfer to catalyze the ATP-dependent reaction. Iron protein acts as the sole known biological reductant to molybdenum iron protein, which contains the actual site of substrate reduction. MgATP binding to iron protein induces dramatic conformational changes in the protein's structure required for docking with molybdenum iron protein. Complex formation and dissociation are essential for nucleotide hydrolysis, electron transfer, and substrate reduction.

We have determined the crystal structure of Azotobacter vinelandii nitrogenase iron protein at 2.2 Å resolution in the absence of nucleotide. Crystals grew in space group P212121, and represent a new crystal form compared with that of the structure previously determined at 2.9 Å resolution. X-ray diffraction data were collected from a single crystal using cryocrystallographic techniques. The structure was solved by molecular replacement, followed by solvent flattening and noncrystallographic averaging. The current model contains 575 of 578 possible amino acid residues and 372 solvent molecules, and has been refined to R-value of 22.3 % (R-free = 29.0 %) for all data to 2.2 Å, with good stereochemistry.

The overall topology of nitrogenase iron protein consists of an Fe4S4 cluster symmetrically ligated by two identical subunits of doubly wound α/β structure similar to those of other nucleotide binding proteins. A detailed description is provided of those structural features important for iron protein function, including nucleotide binding regions, the Fe4S4 cluster environment, intersubunit interactions, and the molybdenum iron protein binding surface. Comparisons are made between the current model and that of C. pasteurianum iron protein, as well as those of two A. vinelandii nitrogenase complexes. Analysis of the various iron protein structures provides a framework for considering the wealth of relevant nitrogenase spectroscopic, biochemical, and genetic information.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Rees, Douglas C.
Thesis Committee:
  • Unknown, Unknown
Defense Date:9 June 1997
Funding AgencyGrant Number
General ElectricUNSPECIFIED
Record Number:CaltechTHESIS:09272017-143047601
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:10463
Deposited By: Benjamin Perez
Deposited On:27 Sep 2017 22:21
Last Modified:19 Apr 2021 22:36

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