4NAB:
reference: SERCA mutant E309Q binds two Ca(2+) ions but adopts a catalytically incompetent conformation., Clausen JD, Bublitz M, Arnou B, Montigny C, Jaxel C, Moller JV, Nissen P, Andersen JP, le Maire M, EMBO J. 2013 Dec 11;32(24):3231-43. doi: 10.1038/emboj.2013.250. Epub 2013 Nov, 22. PMID: 24270570

This OCA Structure page
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