T. Lazarova et al., Fourier transform infrared evidence for early deprotonation of Asp(85) at alkaline pH in the photocycle of bacteriorhodopsin mutants containing E194Q, BIOPHYS J, 78(4), 2000, pp. 2022-2030
The role of the extracellular Glu side chains of bacteriorhodopsin in the p
roton transport mechanism has been studied using the single mutants E9Q, E7
4Q, E194Q, and E204Q; the triple mutant E9Q/E194Q/E204Q; and the quadruple
mutant E9Q/E74Q/E194Q/E204Q. Steady-state difference and deconvoluted Fouri
er transform infrared spectroscopy has been applied to analyze the M- and N
-like intermediates in membrane films maintained at a controlled humidity,
at 243 and 277 K at alkaline pH. The mutants E9Q and E74Q gave spectra simi
lar to that of wild type, whereas E194Q, E9Q/E194Q/ E204Q, and E9Q/E74Q/E19
4Q/E204Q showed at 277 K a N-like intermediate with a single negative peak
at 1742 cm(-1) indicating that Asp(85) and Asp(96) are deprotonated. Under
the same conditions E204Q showed a positive peak at 1762 cm(-1) and a negat
ive peak at 1742 cm(-1), revealing the presence of protonated Asp(85) (in a
n M intermediate environment) and deprotonated Asp96. These results indicat
e that in E194Q-containing mutants, the second increase in the Asp(85) pK(a
) is inhibited because of lack of deprotonation of the proton release group
. Our data suggest that Glu(194) is the group that controls the pK(a) of As
p(85).