2IN4:
reference: Dynamic docking of cytochrome b5 with myoglobin and alpha-hemoglobin: heme-neutralization "squares" and the binding of electron-transfer-reactive configurations., Wheeler KE, Nocek JM, Cull DA, Yatsunyk LA, Rosenzweig AC, Hoffman BM, J Am Chem Soc. 2007 Apr 4;129(13):3906-17. Epub 2007 Mar 8. PMID: 17343378

This OCA Structure page
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