Mechanism of base-catalyzed Schiff base deprotonation in halorhodopsin

Journal Article · Tue Oct 21 00:00:00 EDT 1986 · Biochemistry; (United States)

DOI:https://doi.org/10.1021/bi00369a057· OSTI ID:6526636

Lanyi, J K

It has been shown earlier that the deprotonation of the Schiff base of illuminated halorhodopsin proceeds with a much lower pK/sub a/ than that of the unilluminated pigment and the reversible protonation change is catalyzed by azide and cyanate. The authors have studied the kinetics of the proton-transfer events with flash spectroscopy and compared a variety of anionic bases with different pK/sub a/ with regard to the apparent binding constants and the catalytic activities. The results suggest a general base catalysis mechanism in which the anionic bases bind with apparently low affinity to halorhodopsin, although with some degree of size- and/or shape-dependent specificity. The locus of the catalysis is accessible from the cytoplasmic side of the membrane and is not at site I, where various anions bind and shift the pK/sub a/ of the deprotonation. Neither is it at site II, where a few specific anions (like chloride) bind to the all-trans pigment. It may be concluded that while the all-trans pigment loses its Schiff base proton very rapidly at its pK/sub a/, there is a kinetic barrier to this deprotonation in the 13-cis photointermediate that can be partially overcome by the reversible protonation of an extrinsic anionic base, which shuttles protons between the interior of the protein and the aqueous medium. The need for an extrinsic proton acceptor for efficient deprotonation of the Schiff base of halorhodopsin is one of the main differences between this pigment and the analogous retinal protein, bacteriorhodopsin.

• Research Organization: Univ. of California, Irvine
• OSTI ID: 6526636
• Journal Information: Biochemistry; (United States), Vol. 25:21
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
RHODOPSIN
BIOCHEMICAL REACTION KINETICS
CHARGE EXCHANGE
VISIBLE RADIATION
BIOLOGICAL EFFECTS
ABSORPTION SPECTROSCOPY
PHOTOSYNTHETIC BACTERIA
PROTONS
SCHIFF BASES
BARYONS
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
FERMIONS
HADRONS
IMINES
KINETICS
NUCLEONS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PIGMENTS
PROTEINS
RADIATIONS
REACTION KINETICS
SPECTROSCOPY
550200* - Biochemistry