Av. Veselov et al., Q-band ENDOR (electron nuclear double resonance) of the high-affinity ubisemiquinone center in cytochrome bo(3) from Escherichia coli, BIOCHEM, 39(11), 2000, pp. 3169-3175
Electron nuclear double resonance (ENDOR) was performed on the protein-boun
d, stabilized, high-affinity ubisemiquinone radical, Q(H)(.-), of bo(3) qui
nol oxidase to determine its electronic spin distribution and to probe its
interaction with its surroundings. Until this present work, such ENDOR stud
ies of protein-stabilized ubisemiquinone centers have only been done on pho
tosynthetic reaction centers whose function is to reduce a ubiquinol pool.
In contrast, Q(H)(.-) serves to oxidize a ubiquinol pool in the course of e
lectron transfer from the ubiquinol pool to the oxygen-consuming center of
terminal bos oxidase. As documented by large hyperfine couplings (>10 MHz)
to nonexchangeable protons on the Q(H)(.-) ubisemiquinone ring, we provide
evidence for an electronic distribution on Q(H)(.-) that is different from
that of the semiquinones of reaction centers. Since the ubisemiquinone itse
lf is physically nearly identical in both Q(H)(.-) and the bacterial photos
ynthetic reaction centers, this electronic difference is evidently a functi
on of the local protein environment. Interaction of Q(H)(.-) with this loca
l protein environment was explicitly shown by exchangeable deuteron ENDOR t
hat implied hydrogen bonding to the quinone and by weak proton hyperfine co
uplings to the local protein matrix.