1QSI:
reference: Magnesium(II) and zinc(II)-protoporphyrin IX's stabilize the lowest oxygen affinity state of human hemoglobin even more strongly than deoxyheme., Miyazaki G, Morimoto H, Yun KM, Park SY, Nakagawa A, Minagawa H, Shibayama N, J Mol Biol 1999 Oct 8;292(5):1121-36. PMID: 10512707

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