Light-induced proton-pump

Ultrafast continuum pump-probe spectroscopy has led to the discovery of subpicosecond processes in bacteriorhodopsin. This protein contains a retinal chromophore and acts as a light-induced proton pump that creates a potential difference across a membrane in Halobacterium halobium, which can be used as a free-energy source. Changes in the bleaching, SE, and ESA were observed and were used in concert to describe, assign, and model the observed spectral changes and kinetics. 

A. Warshel, Correlation between the structure and efficiency of light-induced proton pumps. Methods Enzymol. 127 578 (1986). 

Lukashev, E.P. and Robertson, B., Bacteriorhodopsin retains its light-induced proton-pumping function after being heated to 140"C, Bioelectrochem. Bioenerg., 37,157-160, 1995. 

Photosynthetic reaction centers from Rhodobacter sphaeroides and bacteri-orhodopsin (BR) from purple membrane (PM) have been used for their unique optoelectronic properties and for their capability of providing light-induced proton and electron pumping. Once assembled they display extremely high thermal and temporal stability... 

A crucially important finding is that submitochon-drial particles or vesicles from broken chloroplasts will synthesize ATP from ADP and P , when an artificial pH gradient is imposed.172186 Isolated purified FjF0 ATPase from a thermophilic Bacillus has been coreconstituted into liposomes with the light-driven proton pump bacteiiorhodopsin (Chapter 23). Illumination induced ATP synthesis.187 These observations support Mitchell s proposal that the ATP synthase is both spatially separate from the electron carriers in the membrane and utilizes the protonmotive force to make ATP. Thus, the passage of protons from the outside of the mitochondria back in through the ATP synthase induces the formation of ATP. What is the stoichiometry of this process ... 

Photoprotonic signals can be generated by light induced proton release, this might happen for the quaternary N-R+ derivatives of 117, for instance [8.233]. The photoproduction of proton gradients across membranes could serve as a light-powered proton pump for inducing vectorial processes such as the transport of protons [8.233, 8.234] or H+-ATPase model reactions [8.274]. 

Very recently first SERRS results about bacteriorhodopsin have been communicated by Nabiev et al. Bacteriorhodopsin is a membrane protein found in bacteria which functions as a light driven proton pump. Using the short-range mechanism of SERS (Chapt. 4.1) the active site (retinal chromophore) position of the protein in the membrane has been estimated with high accuracy It is interesting to note, that adsorption of bacteriorhodopsin on silver colloids seems to fix light-induced cyclic transformations in the protein active sites. 

FIGURE 22.27 Light-induced pH changes in chloroplast compartments. Illumination of chloroplasts leads to proton pumping and pH changes in the chloroplast, such that the pH within the thylakoid space falls and the pH of the stroma rises. These pH changes modulate the activity of key Calvin cycle enzymes. 

As discussed in Section 22.7, illumination of chloroplasts leads to light-driven pumping of protons into the thylakoid lumen, which causes pH changes in both the stroma and the thylakoid lumen (Figure 22.27). The stromal pH rises, typically to pH 8. Because rubisco and rubisco activase are more active at pH 8, COg fixation is activated as stromal pH rises. Fructose-1,6-bisphosphatase, ribulose-5-phosphate kinase, and glyceraldehyde-3-phosphate dehydrogenase all have alkaline pH optima. Thus, their activities increase as a result of the light-induced pH increase in the stroma. 

In this review, we will focus on the energy landscape of a light activated protein, bacteriorhodopsin (bR), and the point mutations we have induced in bR to enhance its thermal, photochemical, and proton-pumping characteristics. 

Figure 19.25. Comparison of Photosynthesis and Oxidative Phosphorylation. The light-induced electron transfer in photosynthesis drives protons into the thylakoid lumen. The excess protons flow out of the lumen through ATP synthase to generate ATP in the stroma. In oxidative phosphorylation, electron flow down the electron-transport chain pumps protons out of the mitochondrial matrix. Excess protons from the intermembrane space flow into the matrix through ATP synthase to generate ATP in the matrix.

FIGURE 22.13 The mechanism of photophosphorylation. Photosynthetic electron transport establishes a proton gradient that is tapped by the CF,CFq— ATP synthase to drive ATP synthesis. Critical to this mechanism is the fact that the membrane-bound components of light-induced electron transport and ATP synthesis are asymmetric with respect to the thylakoid membrane so that directional discharge and uptake of H ensue, generating the proton-motive force. The number of protons pumped through the ATP synthase varies by species and is the subject of active research. 

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