2. Membrane protein assembly and application.

 

Membrane proteins (MPs) are the nanoscopic entities associated with cellular membranes that act as “gate-keepers” to manage matter, energy and information transport across the membrane boundaries. They are among the earliest complex biological functionalities we know of that defined life activities. For instance, studies of fossilized cyanobacteria 2.8 billion years ago revealed that the photosynthesis centers were used to produce oxygen [1]. This process drastically changed the composition of primitive atmosphere and provoked an explosion of biodiversity on earth. During the course of evolution, various MPs are evolved and refined to carry out tremendous amount of specific functions with high efficiency. Recent studies estimated that MPs represent ~30% of the currently sequenced genomes [2] and are targets for ~70% of all drugs in the market [3]. Among MPs the photosynthetic reaction center [4], Cytochrome c oxidase [5] and ATP synthase [6] are well-known for their roles on solar energy harvesting and chemical energy metabolism that powers nearly all life forms; G protein-coupled receptors (GPCRs) are a large family of MPs that sense stimuli ranging from small molecules to large proteins [7]. Other examples include various ion channels [8,9], proton-pumps [10], mechanosensitive channels [11], tissue factor [12], and multidrug resistance transporters [13], just to name a few. Interestingly, many of the tasks that MPs are specialized in performing are highly pursued nowadays for engineered materials to accomplish. Little is known on how to use the sophisticated nature-evolved MP functions in practical devices.

 

We are studying the design principles to integrate two solar-energy harvesting MPs, proteorhodopsin (PR) and photosynthetic reaction center (RC), respectively, with engineered materials.

PR_lipid PR was discovered in 2000 as the light-driven proton pump in marine bacterioplankton [14], where it converts solar energy into a trans-membrane proton gradient to power ATP synthesis [14,15]. It has seven transmembrane α-helices that form a pocket in which retinal is covalently linked to lysine in the 7th helix (helix G) as a protonated Schiff base (Figure 1, the light-absorbing pigment shown in ball-and-stick model). Upon absorption of light, the retinal cofactor undergoes a conformational change (from all-trans to 13-cis) that initiates a cascade of chain movement within the protein scaffold to change the local chemical environment. As a result, PR will go through a photocycle with a number of intermediates characterized by protonation and de-protonation of a series of amino acids through the network of H-bonds in the water channel, which eventually leads to the proton pumping across the membrane [16-18]. The proton pump builds a ~90 mV potential across a single lipid bilayer only ~5 nm thick [14]. This energy density is remarkable compared to many man-made energy materials. The PR-exploiting bacteria are estimated to account for 13% of microorganisms in the photic zone [19], and function photophysiologically for global energy and matter transformations [14,20,21]. Interestingly, different PR variants are spectrally tuned to different habitats by Darwinian evolution to absorb different wavelength of solar light available [20], which offers great potential to harvest a wide spectrum of solar energy in practical devices. We have demonstrated recently that directed, cooperative assembly of PR and liposomes into 2D and 3D polarized arrays is possible [22,23]. Current research focuses on developing composite materials that convert solar energy into chemical energy.

 

The photosynthetic RC on the other hand, is a light-activated electrical charge transfer generator that uses light to power the separation of electrical charge across a lipid membrane. In its native state this process is coupled to a series of redox reactions to pump protons across the membrane as well. The RC from Rhodobacter (Rb.) sphaeroides consists of three protein subunits termed L, M, and H having 281, 307, and 260 amino acid residues, respectively [24] (Figure 2, left panel). The three

RC_La protein subunits serve as a protein scaffold to hold 10 cofactors involved with the light-activated electron charge transfer. Those cofactors include four molecules of BChl, two molecules of BPhe, two molecules of ubiquinone, a single photoprotective carotenoid and a non-heme iron atom that are held precisely in a 3D configuration by the protein scaffold (Figure 2, right panel). Synthetic effort to mimic this function in an artificial photosynthetic system has not been succeeded yet. The process begins when solar light is collected by the antenna pigment (the two BChls termed PA and PB) to generate a donor-acceptor pair. Before recombination occurs, a series of electron transfer reactions takes place that eventually leads to the formation of a transmembrane electron-hole dipole with a field density of ~106 V/cm and an extremely high quantum efficiency (approaching 100%) [25]. The light-activated charge separation sustains a large open circuit voltage of ~1.1V without significant electron-hole recombination [26], which offers great advantages to be exploited for photovoltaic applications. Since the determination of the 3D structure of RC (from Rhodopseudomonas-Viridis) in 1980s (which was the first successfully identified MP 3D structure and was honored with a Nobel Prize in Chemistry in 1988) [4], tremendous amount of research has been spent on understanding the structure, kinetics, and mechanism of photosynthesis in general and apply that on artificial systems [27-32]. We are interested in developing RC-based photovoltaic devices with strategies to intercept the photocycle of RC in the early stage of charge separation to harness its highly efficient solar-energy harvesting function.

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