1KOU:
reference: Structure of the photoactive yellow protein reconstituted with caffeic acid at 1.16 A resolution., van Aalten DM, Crielaard W, Hellingwerf KJ, Joshua-Tor L, Acta Crystallogr D Biol Crystallogr 2002 Apr;58(Pt 4):585-90. PMID: 11914481

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