2K31:
reference: Solution structure of the cGMP binding GAF domain from phosphodiesterase 5: insights into nucleotide specificity, dimerization, and cGMP-dependent conformational change., Heikaus CC, Stout JR, Sekharan MR, Eakin CM, Rajagopal P, Brzovic PS, Beavo JA, Klevit RE, J Biol Chem. 2008 Aug 15;283(33):22749-59. Epub 2008 Jun 4. PMID: 18534985

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