FTIR spectroscopy shows structural similarities between photosystems II from cyanobacteria and spinach

FTIR evidence that the PsbP extrinsic protein induces protein conformational changes around the oxygen-evolving Mn cluster in photosystem II

Extrinsic proteins of photosystem II (PSII) regulate the oxygen-evolving reaction performed at the Mn cluster by controlling the binding properties of the indispensable cofactors Ca(2+) and Cl(-). However, the molecular mechanism underlying this regulation is not yet understood. We have investigated the structural couplings of the extrinsic proteins PsbO, PsbP, and PsbQ of higher plants with the Mn cluster using Fourier transform infrared (FTIR) spectroscopy. Light-induced FTIR difference spectra upon the S(1) --> S(2) transition were measured using spinach PSII membranes, and the effects of the selective depletion of extrinsic proteins were examined. Depletion of the PsbP and PsbQ proteins by NaCl washing revealed clear changes in the amide I bands with no appreciable changes in the bands of carboxylate and imidazole groups, whereas the depletion of all three proteins by CaCl(2) washing did not cause further changes. The original amide I features were recovered by reconstitution of the NaCl-washed PSII with PsbP, and the same recovery was observed with (13)C-labeled PsbP. These results indicate that the PsbP protein, but not PsbQ and PsbO, affects the protein conformation around the Mn cluster in the intrinsic proteins without changing the ligand structure. Reconstitution with Delta15-PabP, in which the 15 N-terminal residues were truncated, did not restore the amide I bands, indicating that the interaction of the N-terminal region induces the conformational changes. This observation correlates well with a previous finding that Delta15-PabP did not restore the Ca(2+) and Cl(-) retention ability upon rebinding to PSII [Ifuku, K., et al. (2005) Photosynth. Res. 84, 251-255]. Therefore, the evidence strongly suggests that protein conformational changes around the Mn cluster induced by PsbP through its N-terminal region affect the binding properties of Ca(2+) and Cl(-) and enhance their retention.

The Unusually Strong Hydrogen Bond between the Carbonyl of QAand His M219 in theRhodobacter sphaeroidesReaction Center Is Not Essential for Efficient Electron Transfer from QA-to QB

In native reaction centers (RCs) from photosynthetic purple bacteria the primary quinone (QA) and the secondary quinone (QB) are interconnected via a specific His-Fe-His bridge. In Rhodobacter sphaeroides RCs the C4=O carbonyl of QA forms a very strong hydrogen bond with the protonated Npi of His M219, and the Ntau of this residue is in turn coordinated to the non-heme iron atom. The second carbonyl of QA is engaged in a much weaker hydrogen bond with the backbone N-H of Ala M260. In previous work, a Trp side chain was introduced by site-directed mutagenesis at the M260 position in the RC of Rb. sphaeroides, resulting in a complex that is completely devoid of QA and therefore nonfunctional. A photochemically competent derivative of the AM260W mutant was isolated that contains a Cys side chain at the M260 position (denoted AM260(W-->C)). In the present work, the interactions between the carbonyl groups of QA and the protein in the AM260(W-->C) suppressor mutant have been characterized by light-induced FTIR difference spectroscopy of the photoreduction of QA. The QA-/QA difference spectrum demonstrates that the strong interaction between the C4=O carbonyl of QA and His M219 is lost in the mutant, and the coupled CO and CC modes of the QA- semiquinone are also strongly perturbed. In parallel, a band assigned to the perturbation of the C5-Ntau mode of His M219 upon QA- formation in the native RC is lacking in the spectrum of the mutant. Furthermore, a positive band between 2900 and 2400 cm-1 that is related to protons fluctuating within a network of highly polarizable hydrogen bonds in the native RC is reduced in amplitude in the mutant. On the other hand, the QB-/QB FTIR difference spectrum is essentially the same as for the native RC. The kinetics of electron transfer from QA- to QB were measured by the flash-induced absorption changes at 780 nm. Compared to native RCs the absorption transients are slowed by a factor of about 2 for both the slow phase (in the hundreds of microseconds range) and fast phase (microseconds to tens of microseconds range) in AM260(W-->C) RCs. We conclude that the unusually strong hydrogen bond between the carbonyl of QA and His M219 in the Rb. sphaeroides RC is not obligatory for efficient electron transfer from QA- to QB.

Vibrational spectroscopy to study the properties of redox-active tyrosines in photosystem II and other proteins

Tyrosine radicals play catalytic roles in essential metalloenzymes. Their properties--midpoint potential, stability...--or environment varies considerably from one enzyme to the other. To understand the origin of these properties, the redox tyrosines are studied by a number of spectroscopic techniques, including Fourier transform infrared (FTIR) and resonance Raman (RR) spectroscopy. An increasing number of vibrational data are reported for the (modified-) redox active tyrosines in ribonucleotide reductases, photosystem II, heme catalase and peroxidases, galactose and glyoxal oxidases, and cytochrome oxidase. The spectral markers for the tyrosinyl radicals have been recorded on models of (substituted) phenoxyl radicals, free or coordinated to metals. We review these vibrational data and present the correlations existing between the vibrational modes of the radicals and their properties and interactions formed with their environment: we present that the nu7a(C-O) mode of the radical, observed both by RR and FTIR spectroscopy at 1480-1515 cm(-1), is a sensitive marker of the hydrogen bonding status of (substituted)-phenoxyl and Tyr*, while the nu8a(C-C) mode may probe coordination of the Tyr* to a metal. For photosystem II, the information obtained by light-induced FTIR difference spectroscopy for the two redox tyrosines TyrD and TyrZ and their hydrogen bonding partners is discussed in comparison with those obtained by other spectroscopic methods.