1K23:
reference: The "open" and "closed" structures of the type-C inorganic pyrophosphatases from Bacillus subtilis and Streptococcus gordonii., Ahn S, Milner AJ, Futterer K, Konopka M, Ilias M, Young TW, White SA, J Mol Biol 2001 Nov 2;313(4):797-811. PMID: 11697905

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