Natural photosynthesis mechanisms

Extremely fine craftsmanship of nature is the ultimate goal for chemist.Natural photosystem plays a role of solar energy harvesting and energy conversion. The importance and complexity of photosynthesis has led many researchers to look for ways to duplicate the fundamental characteristics of these electron transfer reactions in simplified chemical systems. And it’s an integrated system including light harvesting, photo-induced charge separation, and synthesis of higher-energy compounds.

Photosynthetic apparatus consists of a number of photoactive proteins, which harvest solar energy and synchronize all of their function to make photosynthesis unabridged and efficient. During photosynthesis, plants and certain bacterial systems convert light energy into electrochemical energy and eventually into chemical potential energy stored in carbohydrates and other compounds.

Photosynthesis start by the absorption of a photon by light-harvesting (antenna) complexes that usually comprise a large number of pigments embedded in protein matrices. This process is followed by an efficient energy migration over many pigments within the antenna system until a reaction center is encountered.

Knowing how plants and bacteria harvest light for photosynthesis so efficiently may provide a new idea for energy requirements.

Photosynthesis, natural biosphere drives energy from sunlight energy. Photosynthetic organisms, i.e., plants, algae and photosynthetic bacteria, have developed efficient systems to harvest the light of the sun and to use the light energy to drive their metabolic reactions.

1.3.1 Bacterial photosynthesis -Purple bacteria

Purple bacteria are the paragon of harvesting light. Nearly all energy gained from the absorption

of a photon is transferred to the reaction center. In purple bacteria, the photosynthetic membranes contain two types of light-harvesting complexes: light harvesting complex I (LH-I) and light harvesting complex II (LH-II). The structure is showed in Fig.1-4.⁴

Figure 1-4 Photoexcited electron transfer in the bacterial photosynthetic unit. LH-II contains two types of BChls: B800 (dark blue) and B850 (green), which absorb at 800 nm and 850 nm.

BChls in LH-I absorb at 875 nm and labeled B875 (green). PA and PB refer to the RC special pair, and BA, BB refer to the accessory BChls in the RC. The figure demonstrates the coplanar arrangement of the B850 BChl ring in LH-II, the B875 BChl ring of LH-I, and the RC BChls PA, PB, BA, BB.⁴

X-ray structure of RC (from Rhodopseudomonas viridis, Rps. V) was first published in 1985.

Typically, RC (from Rb. S) is embedded in the bacterial cytoplasmic membrane and is composed of three subunits labeled L, M and H. The L and M subunits, each forming five transmembrane α-helices, are related by an axis of approximate two-fold symmetry. The more hydrophilic subunit H is located at the cytoplasmic surface and binds to both the L and M subunits. Except the one carotenoid molecule, cofactors including a bacteriochlorophyll (BChl) dimer (termed as P) known as the primary donor, two monomer bacteriochlorophylls (BChlA and BChlB), two bacteriopheophytins

(BPheA and BPheB), two quinones (QA and QB) and one non-heme iron are symmetrically arranged in the L and M subunits .⁵

All of the cofactors are binding to the proteins non-covalently. It is the well established photo-induced electron transfer (ET) system. Via the photoexcitation of P or received excitation energy transferring from the antenna, the excited state of P (P*) forms and then decays in about 3–4 ps to form the charge separation state P⁺BPheA−. Subsequently, BPheA− transfers an electron to QA

with a constant of about 200 ps at room temperature, yielding the state P⁺QA−. And ET from QA− to QB takes a few hundred μs. (Fig.1-5)⁵

.

Figure 1-5 (A) Three-dimensional structure of RC from Rhodobacter sphaeroides (Rb. S). The L, M and H subunits are shown as orange, green and blue ribbons. (B) Schematic of the cofactors arrangement and the photo-induced ET present in RC.⁵

1.3.2 Plant photosynthesis

Light-dependent reactions take place in the thylakoid membranes of the chloroplasts in green plant. Plants synthesize ATP and NADPH by utilizing light energy. One molecule of the pigment chlorophyll absorbs one photon and drives the electron to phenophytin. And pheophytin passes the electron to a quinine molecule, starting a flow of electrons down an electron transport chain that leads to reduction of NADP⁺ to NADPH at last. A proton gradient is created across the chloroplast

membrane. Its dissipation is used by ATP synthase for the concomitant synthesis of ATP. The chlorophyll molecule regains the lost electron from a H2O molecule and releases a dioxygen (O2) molecule. The overall reaction for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:

2H2O +2 NADP⁺ +2 ADP + 2 Pi + light→2 NADPH +2 H⁺ + 2 ATP +O2

Some specific wavelengths of light can support photosynthesis. It depends on the type of accessory pigments present. There are two forms of light-dependent reaction: non-cyclic and cyclic reaction. In the non-cyclic reaction, the photons are captured in the light-harvesting antenna complexes of photosystem II by chlorophyll and other accessory pigments. When a chlorophyll molecule of the photosystem II reaction center obtains sufficient excitation energy from the adjacent antenna pigments, an electron is transferred to the primary electron-acceptor molecule, pheophytin.

This process called photo-induced charge separation. These electrons are transported through an electron transport chain, called Z-scheme, that generates a chemiosmotic potential across the membrane. In Z-scheme, electrons are excited due to the light absorbed by the photosystem I. A second electron carrier accepts electrons, which again is passed down lowering energies of electron acceptors. The energy created by the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen. Electrons are used to reduce the co-enzyme NADP⁺, which has functions in the light-independent a reaction. An ATP synthase enzyme uses the chemiosmotic potential to make ATP during photophosphorylation. NADPH is the terminal product of redox reaction in the Z-scheme. The cyclic reaction is similar to that of the non-cyclic, but differs in the form that its generates only ATP, and no NADPH is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns back to photosystem I, from where it was emitted, hence the

name cyclic reaction.⁶ (Fig.1-6)

  Figure 1-6 Light-dependent reactions of photosynthesis at the thylakoid membrane.

1.3.3 The importance of protein scaffold in photosystem

Photosynthetic apparatus is a typical electronic device, including molecular optical and electronic circuitry organized by a protein scaffold. It occurs with an amazingly high efficiency.

Photo-induced electron transfer to yield long-lived charge-separated (CS) states is the most fundamental energy conversion process.Photosynthetic membrane proteins play a role of bio-device for solar energy harvesting and energy conversion. They can stabilize the charge-separated state and excited-state lifetimes. Protein structural deformation will induce lifetime shortening of excited state.^7-10