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

Abstract

Photosynthesis involves a series of electron transfer steps to convert the sunlight energy into the chemical energy in green plants, algae, and cynobacteria. The three–dimensional structures of photosynthetic reaction centers at the high resolution uncover the binding sites and precise orientation of cofactors and their interaction with proteins and provided an excellent model to investigate the mechanism of electron transfer reaction. To understand fully an electron transfer reaction, it is necessary to understand not just its kinetics, but also thermodynamics. However, in contrast to the kinetics of electron transfer mechanisms, thermodynamic information is far less accessible. Thermodynamics reveals the energy levels of reactants and products, as well as the driving forces in the reaction. The driving force of the chemical reaction is the Gibbs free energy, which is composed of enthalpic and entropic components. Pulsed photoacoustics provides direct measurements of enthalpy and volume changes of electron transfer. Using pulsed photoacoustics, the volume change and enthalpy of electron transfer reaction were measured in the photosynthetic reaction center complex of Rb. sphaeroides. A large entropy was calculated based on these measurements. Further photoacoustic measurements indicated that the entropy change of electron transfer in photosystem I from Synechocystis sp. PCC 6803 is similar to that in bacterial center. The deconvolution fit of the photoacoustic waves distinguished thermodynamic parameters of a large negative enthalpy change and large volume change for P700* →A1 step and a positive enthalpy change and a small volume change for A1 → FA/B step. To explore the specific role of protein matrix, the menA and menB genes were inactivated by molecular genetics and showed the altered thermodynamics of electron transfer. Inactivating the menG gene causes 2–phytyl–1,4–naphthoquinone (Q) to be presented as a quinone acceptor. The fit by convolution of menG photoacoustic waves resolved a large volume contraction for the P700* → Q step and a positive volume change for the Q– → FA/B step. The photoacoustic data of the bacterial reaction center, menA/B mutant, menG mutant, and wild type phtosystem I show significant positive entropy. In contrast, electron transfer in photosystem II is accompanied by a small negative entropy change. In vivo photoacoustic measurements confirmed the difference in entropy between photosysmte I and photosystem II. We conclude that apparent entropy may play a vital role in photosynthetic electron transfers.