4dxq

Publication Abstract from PubMed

In green-to-red photoconvertible fluorescent proteins, a three-ring chromophore is generated by the light-activated incorporation of a histidine residue into the conjugated pi-system. We have determined the pH-rate profile and high- and low-pH X-ray structures of a Least Evolved Ancestral (LEA) protein constructed in the laboratory based on statistical sequence analysis. LEA incorporates the minimum number of substitutions necessary and sufficient for facile color conversion, and exhibits a maximal photoconversion quantum yield of 0.0015 at pH 6.1. The rate measurements provide a bell-shaped curve, indicating that the reaction is controlled by the two apparent pKa values 4.5 (+/- 0.2) and 7.5 (+/- 0.2) flanking the chromophore pKa of 6.3 (+/- 0.1). These data demonstrate that the photoconversion rate of LEA is not proportional to the A-form of the GFP-like chromophore, as previously reported for Kaede-type proteins. We propose that the observed proton dissociation constants arise from the internal quadrupolar charge network consisting of Glu222, His203, Glu148 and Arg69. Increased active site flexibility may facilitate twisting of the chromophore upon photoexcitation, thereby disrupting the charge network and activating the Glu222 carboxylate for proton abstraction from a carbon acid. Subsequently, the proton may be delivered to the Phe64 carbonyl by a hydrogen bonded network involving Gln42, or by means of His65 side chain rotations promoted by protein breathing motions. A structural comparison of LEA with the non-photoconvertible LEA-Q42A variant supports a role for Gln42 either in catalysis, or in the coplanar preorganization of the green chromophore with the His65 imidazole ring.

Acid-base catalysis and crystal structures of a least-evolved ancestral GFP-like protein undergoing green-to-red photoconversion.,Kim H, Grunkemeyer TJ, Modi C, Chen L, Fromme R, Matz MV, Wachter RM Biochemistry. 2013 Oct 17. PMID:24134825^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.