Hysteretic Pressure Dependence of Ca²⁺ Binding in LH1 Bacterial Membrane Chromoproteins

Abstract

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 Ca 2+ binding sites in the cyclic LH1 light-harvesting membrane chromoprotein complexes from two Ca-containing sulfur purple bacteria by hydrostatic highpressure 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 Ca 2+ 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.

Evaluation of the relationship between color-tuning of photosynthetic excitons and thermodynamic stability of light-harvesting chromoproteins

“…The spectral changes seen in these experiments are consistent with the depletion of LH1 from Ca, which characterizes the denaturation of the core complexes (as seen in Fig. 8E,F, and in Timpmann et al 2023).…”

“…After a sufficiently long waiting time, however, initial states of the chromoproteins appear to almost perfectly recover (as can be seen in Fig. 8C,D, and in Timpmann et al 2023).…”

“…970 and Tch. tepidum (Timpmann et al 2023). By using the Qy exciton absorption band as an optical probe, it was possible to distinguish between the native (Ca-saturated, Ca+) and denatured (Ca-depleted, Ca-) phases of these complexes.…”

Dominant role of excitons in photosynthetic color-tuning and light-harvesting

“…As seen from spectral responses, the effect of ions exchange is much less dramatic than depletion of the ions. This is reasonable, because the ion replacement is expected only slightly distort the native Ca 2+ binding sites, while the removal of ions is prone to collapse the sites that may initiate a large-scale restructuring of the protein ( Timpmann et al, 2023 ). The widely separate positions of the blue anisotropy dips observed in case of Trv.…”