3QI3:
reference: Structural asymmetry of phosphodiesterase-9, potential protonation of a glutamic Acid, and role of the invariant glutamine., Hou J, Xu J, Liu M, Zhao R, Luo HB, Ke H, PLoS One. 2011 Mar 31;6(3):e18092. PMID: 21483814

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