2NIP:
reference: Conformational variability in structures of the nitrogenase iron proteins from Azotobacter vinelandii and Clostridium pasteurianum., Schlessman JL, Woo D, Joshua-Tor L, Howard JB, Rees DC, J Mol Biol 1998 Jul 24;280(4):669-85. PMID: 9677296

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