Thermodynamics of Charge Separation of Photosystem I in the menA and menB Null Mutants of Synechocystis sp. PCC 6803 Determined by Pulsed Photoacoustics

Abstract

When the biosynthesis of phylloquinone is inhibited in Synechocystis sp. PCC 6803 by interrupting the menA or the menB gene, photosystem I (PS I) recruits plastoquinone-9 (A(P)) to occupy the A(1) sites. In PS I from the menA and menB null mutants, forward electron transfer from the quinone to the FeS clusters occurs approximately 1000 times slower than in wild-type PS I [Semenov, A. Yu., Vassiliev, I. R., van der Est, A., Mamedov, M. D., Zybailov, B., Shen, G., Stehlik, D., Diner, B. A., Chitnis, P. R., and Golbeck, J. H. (2000) J. Biol. Chem. 275, 23429-23438]. To investigate the effect on thermodynamics, the enthalpy and volume changes of charge separation in PS I in the menA and menB mutants were measured using pulsed time-resolved photoacoustics on the nanosecond and microsecond time scales. The observed thermodynamic data are the same for the menA and menB mutants. This is expected because the recruited quinone (A(P)) is the same in both mutants. The volume change of PS I from the mutants following charge separation on both time scales was -17 +/- 2 A(3), less than that of the wild type, -21 A(3). The quantum yield of charge separation was found to be slightly lower (85 +/- 9%) than that of wild-type PS I (96 +/- 10%). The observed reaction is assigned to the formation of P(700)(+)A(P)(-) from P(700)*A(P). An enthalpy change (DeltaH) of -0.69 +/- 0.07 eV was obtained for this reaction. In contrast, a larger enthalpy change -0.8 eV for the formation of P(700)(+)A(1)(-) from P(700)* and an apparent entropy change (TDeltaS, T = 25 degrees C) of -0.2 eV were obtained in wild-type PS I [Hou, H. J. M., and Mauzerall, D. (2006) J. Am. Chem. Soc. 128, 1580-1586]. Taking the free energy to be -0.70 eV in PS I of the mutants, the apparent entropy is close to zero in the mutants. Since the apparent entropy change for the overall reaction of the production of P(700)(+)F(A/B)(-) from P(700)* is very likely the same as that of the wild type, +0.35 eV, this implies that the reaction of P(700)(+)A(P)(-)F(A/B) --> P(700)(+)A(P)F(A/B)(-) in the mutants is almost completely entropy driven (DeltaG = -0.07 eV and TDeltaS = +0.40 eV). These results show that not only the kinetics but also the thermodynamics of electron transfer reactions in PS I are significantly affected by the recruitment of the foreign plastoquinone-9 into the A(1) site.

Enthalpy, Entropy, and Volume Changes of Electron Transfer Reactions in Photosynthetic Proteins

“…In contrast, a large enthalpy change of ~ −1.0 eV for the formation of P 700 + A 1 -from P 700 * in the wild-type PS I was observed. These results strongly suggest that not only the kinetics but also the thermodynamics of electron transfer reactions in PS I is significantly affected by the recruitment of the foreign plastoquinone-9 into the A 1 site (Hou et al, 2009). …”

Probing the Thermodynamics of Photosystem I by Spectroscopic and Mutagenic Methods

“…The observed reaction on the microsecond time scale is attributed to the formation of P700 + FA/B -from excited P700*for the wild-type PS I and of P700 + AP -for menA/B PS I. The enthalpy changes in menA and menB null mutants were determined to be 0.640.1 eV and 0.740.1 eV from the difference in slopes in Figure 9, respectively, according to the method described previously Hou et al, 2009). Figure 10 is the typical photoacoustic wave on the fast nanosecond time scale reaction.…”

“…By use of the electron transfer theory and kinetic data, the redox potential of plastoquinone at the A1 site was estimated to be -0.61 V (Hou et al, 2009). However, the error to be at least 0.1 V. The ΔG for producing P700 + AP -from P700* is then -0.71 eV.…”

Methodology of Pulsed Photoacoustics and Its Application to Probe Photosystems and Receptors

“…In contrast, a large enthalpy change of ∼ −1.0 eV for the formation of P 700 + A 1 − from P 700 * in the wild-type PS I was observed. These results strongly suggest that not only the kinetics but also the thermodynamics of electron transfer reactions in PS I is significantly affected by the recruitment of the foreign plastoquinone-9 into the A 1 site [8]. …”

“…Pulsed time-resolved photoacoustics provides an unique way to probe the molecular mechanism of electron transfer and proton transfer events in chemical reactions [1–7] and in biological systems, including photosynthetic complexes [8–16] and retinal-based pigments such as halorhodopsins [17,18], bacteriorhodopsins [19–24] and rhodopsins [25,26]. For a given photoreaction, the accessible parameters include the molecular volume change secondary to conformational change or electrostriction, and enthalpy and entropy changes.…”