On the Red Antenna States of Photosystem I Mutants from Cyanobacteria Synechocystis PCC 6803

Abstract

To identify the molecular composition of the low-energy states in cyanobacterial Photosystem I (PSI) of Synechocystis PCC6803, we focus on highresolution (low-temperature) absorption, emission, resonant, and nonresonant holeburned spectra obtained for wild-type (WT) PSI and three PSI mutants. In the Red_a mutant, the B33 chlorophyll (Chl) is added to the B31-B32 dimer; in Red_b, histidine 95 (His95) on PsaB (which coordinates Mg in the B7 Chl within the His95-B7-A31-A32cluster) is replaced with glutamine (Gln), while in the Red_ab mutant, both mutations are made. We show that the C706 state (B31-B32) changes to the C710 state (B31-B32-B33) in both Red_a and Red_ab mutants, while the C707 state in WT Synechocystis (localized on the His95-B7-A31-A32 cluster) is modified to C716 in both Red_b and Red_ab. Excitation energy transfer from C706 to the C714 trap in the WT PSI and Red_b mutant is hampered as reflected by a weak emission at 712 nm. Large electron−phonon coupling strength (exposed via resonant hole-burned spectra) is consistent with a strong mixing of excited states with intermolecular charge transfer states leading to significantly red-shifted emission spectra. We conclude that excitation energy transfer in PSI is controlled by fine-tuning the electronic states of a small number of highly conserved red states. Finally, we show that mutations modify the protein potential energy landscape as revealed by different shapes and shifts of the blue-and red-shifted antiholes.

The structure of photosystem I from a high-light-tolerant cyanobacteria

“…The difference spectrum of the absorption, C. aponinum–Synechocystis , shows a strong negative peak at 701 nm revealing that C. aponinum PSI contains less LWC than Synechocystis ( Figure 2D ). Based on the Gaussian deconvolution of this peak three components were identified at 699, 704, and 711 nm, the latter two in agreement with the site energy of previously reported LWC in Synechocystis that would be altered in C. aponinum ( Toporik, 2020 ; Khmelnitskiy et al, 2020 ; Figure 2E ).…”

“…A similar effect of Ca 2+ binding was shown previously in the light harvesting complex-1 of purple bacteria ( Kimura et al, 2008 ; Swainsbury et al, 2017 ). The Ca 2+ ion in PsaL of WT Synechocystis is located near known LWCs, chlorophylls B7 and A32 ( Khmelnitskiy et al, 2020 ; Jordan et al, 2001 ), and our results support its contribution to the low-energy states of this chlorophyll pair ( Figure 4D,E ). We do not resolve any changes in the 77 K emission between WT Synechocystis and Red_c or Red_d , this is in agreement with previous results showing that red shifting the B7-A32 dimer contributes very little to the maximum emission from the PSI trimer in WT Synechocystis, even at 4 K ( Khmelnitskiy et al, 2020 ).…”

“…This is a seemingly contradictory result, as C. aponinum contains less LWC, which are the main emitters of the 77 K emission. We suggest that the elimination of some LWC in C. aponinum cause a greater emission from a different either C706 or C714 ( Toporik, 2020 ; Khmelnitskiy et al, 2020 ; Riley et al, 2007 ) LWC with a lower energy. This is consistent with the findings that there is less red absorption in C. aponinum together with the red shifted emission at 77 K.…”

“…The absorbance properties of specific chlorophylls are highly dependent on its coordinating residues as well as excitonic coupling between neighboring chlorophylls; and changing either can alter the spectroscopic properties of a chlorophyll. ( Wientjes et al, 2011 ; Toporik, 2020 ; Khmelnitskiy et al, 2020 ). These characteristics have led to several suggestions to the physiological role of LWC such as directing energy to P700, extending light-harvesting capabilities into the far red, and photoprotective mechanisms ( Valkunas et al, 1995 ; Trissl, 1993 ; Rivadossi et al, 1999 ; Schlodder et al, 2005 ; Herascu et al, 2016 ).…”

Comparison between the Light-Harvesting Mechanisms of Type-I Photosynthetic Reaction Centers of Heliobacteria and Photosystem I: Pigment Site Energy Distribution and Exciton State

“…The Chl- a ’s labeled as C706, C708, or C714 in PSI were experimentally identified as the low-energy Chl- a (so-called red-Chls). ,− Exciton states of these experimentally assigned red-Chls need to be determined theoretically by considering short-range interactions. However, the putative 37th and 41th Chl- a s do not belong to the previously identified candidates of red-Chls. ,− This may be due to the use of Poisson-TrESP/CDC methods that do not include the short-range electrostatic interactions, as shown in Figure S5, in this study.…”