6CDK:
reference: Structural characterization of the P(1+) intermediate state of the P-cluster of nitrogenase., Keable SM, Zadvornyy OA, Johnson LE, Ginovska B, Rasmussen AJ, Danyal K, Eilers BJ, Prussia GA, LeVan AX, Raugei S, Seefeldt LC, Peters JW, J Biol Chem. 2018 May 2. pii: RA118.002435. doi: 10.1074/jbc.RA118.002435. PMID: 29720402

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