posted on 2023-01-06, 22:03authored byKõu Timpmann, Liina Kangur, Arvi Freiberg
Much
of the thermodynamic parameter values that support life are
set by the properties of proteins. While the denaturing effects of
pressure and temperature on proteins are well documented, their precise
structural nature is rarely revealed. This work investigates the destabilization
of multiple Ca2+ binding sites in the cyclic LH1 light-harvesting
membrane chromoprotein complexes from two Ca-containing sulfur purple
bacteria by hydrostatic high-pressure perturbation spectroscopy. The
native (Ca-saturated) and denatured (Ca-depleted) phases of these
complexes are well distinguishable by much-shifted bacteriochlorophyll a exciton absorption bands serving as innate optical probes
in this study. The pressure-induced denaturation of the complexes
related to the failure of the protein Ca-binding pockets and the concomitant
breakage of hydrogen bonds between the pigment chromophores and protein
environment were found cooperative, involving all or most of the Ca2+ binding sites, but irreversible. The strong hysteresis observed
in the spectral and kinetic characteristics of phase transitions along
the compression and decompression pathways implies asymmetry in the
relevant free energy landscapes and activation free energy distributions.
A phase transition pressure equal to about 1.9 kbar was evaluated
for the complexes from Thiorhodovibrio strain 970
from the pressure dependence of biphasic kinetics observed in the
minutes to 100 h time range.