BINDING OF THE OXIDIZED, REDUCED, AND RADICAL FLAVIN SPECIES TO CHORISMATE SYNTHASE - AN INVESTIGATION BY SPECTROPHOTOMETRY, FLUOROMETRY, AND ELECTRON-PARAMAGNETIC-RESONANCE AND ELECTRON-NUCLEAR DOUBLE-RESONANCE SPECTROSCOPY
P. Macheroux et al., BINDING OF THE OXIDIZED, REDUCED, AND RADICAL FLAVIN SPECIES TO CHORISMATE SYNTHASE - AN INVESTIGATION BY SPECTROPHOTOMETRY, FLUOROMETRY, AND ELECTRON-PARAMAGNETIC-RESONANCE AND ELECTRON-NUCLEAR DOUBLE-RESONANCE SPECTROSCOPY, Biochemistry, 35(5), 1996, pp. 1643-1652
Chorismate synthase (EC 4.6.1.4) binds oxidized ribonavin-5'-phosphate
mononucleotide (FMN) with a K-D of 30 mu M at 25 degrees C, but in th
e presence of 5-enolpyruvylshikimate-3-phosphate (EPSP), the K-D decre
ases to ca. 20 nM. Similar effects occur with the substrate analogue (
6R)-6-fluoro-EPSP (K-D = 36 nM) and chorismate (K-D = 540 nM). Fluores
cence of oxidized FMN is slightly quenched in the presence of chorisma
te synthase. Addition of EPSP or the (6R)6-fluoro analogue causes a sh
ift of the fluorescence from 520 to 495 nm. Chorismate causes no shift
in, but a quenching of, the fluorescence emission maximum. In the pre
sence of EPSP, (6R)-6-fluoro-EPSP, or chorismate, the neutral flavinse
miquinone is generated. The electron paramagnetic resonance (EPR) Line
width of the flavin radical is indicative of a neutral flavinsemiquin
one. Frozen solution electron nuclear double resonance (ENDOR) of the
radical with (6R)-6-fluoro-EPSP shows a number of proton ENDOR line pa
irs. The largest splitting is assigned to a hyperfine coupling to the
methyl group beta-protons at position 8 of the isoalloxazine ring. The
hyperfine-coupling (hfc) components have values of A(perpendicular to
) = 8.07 MHz and A(parallel to) = 9.60 MHz, giving A(iso) of 8.58 MHz,
consistent with a neutral semiquinone form. The isotropic hfc couplin
g of the 8-methyl protons with (6R)6-fluoro-EPSP decreases by about 0.
5 MHz when chorismate is bound, indicating that the spin density distr
ibution within the isoalloxazine ring system depends critically on the
nature of the ligand. The redox potential of FMN in the presence of c
horismate synthase was 95 mV more positive than that of free FMN (at p
H 7.0), equivalent to a 1660-fold tighter binding of reduced FMN. The
pH dependence of the redox potential of chorismate synthase-bound FMN
exhibits a slope of -30 mV per pH unit between pH 6 and 9, indicating
that the two-electron reduction of the flavin is associated with the u
ptake of one proton; this, and the UV-visible spectrum, is consistent
with the reduced flavin being bound to chorismate synthase in its mono
anionic form.