Phototransformation of green fluorescent protein with UV and visible light leads to decarboxylation of glutamate 222

Wild type green fluorescent protein (GFP) from Aequorea victoria absorbs predominantly at 398 nm. Illumination with UV (254 nm) or visible (390 nm) light transforms this state (GFP(398)) into one absorbing at 483 nm (GFP(483)). Here we show that this photoconversion of GFP is a one-photon process that is paralleled by decarboxylation of Glu 222. We propose a mechanism in which decarboxylation is due to electron transfer between the gamma-carboxylate of Glu 222 and the p-hydroxybenzylidene-imidazolidinone chromophore of GFP, followed by reverse transfer of an electron and a proton to the remaining carbon side chain atom of Glu 222. Oxidative decarboxylation of a gamma-carboxylate represents a new type of posttranslational modification that may also occur in enzymes with high-potential reaction intermediates.