Cartoon structure of GFP. The fluorophore structure originates from an internal Ser-Tyr-Gly sequence which is post-translationally modified to a 4-(p-hydroxybenzylidene)- imidazolidin-5-one. The fluorophore is generated by sequential auto-catalysis process. The reaction is initiated by a rapid cyclization between Ser65 and Gly67 to form an imidazolin-5-one intermediate which is followed by a much slower rate-limiting oxygenation of the Tyr66 side chain by O2 on a timescale of hours. Gly67 is required for formation of the fluorophore, no other amino acid can replace Gly in this role.
For more detail regarding the mechanism of GFP and GFP derivatives check out this paper:
http://www.pnas.org/content/100/21/12111.full
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