Fundamentals of Molecular Biology. Part 1: Molecular Cell Biology - A. N. Ogurtsov 2011

Photosynthetic Systems
Photosystems

Light absorption and The conversion of photon energy into chemical energy take place within multi-Structure/178.html">Protein Complexes known as photosystems. Photosystems are found in all photosynthetic organisms.

Photosystems consist of two linked components:

✵ a reaction center, where a high-energy electron is excited—the primary event in the photosynthetic process,

✵ Light-Harvesting Complexes (LHCs), which form a light-harvesting antenna that absorbs light of various wavelengths and transfers this absorbed energy to the reaction center.

Both Reaction Centers and antennas contain light-absorbing pigment molecules. The primary pigment in photosystems is chlorophyll a, which is a component of both antennas and reaction centers. In addition to chlorophyll a, antennas contain other pigments: chlorophyll b in plants, and carotenoids in both plants and photosynthetic organisms. These carotenoids feature hydrocarbon chains with alternating single and double (saturated and unsaturated) carbon-carbon bonds (such as polymethine or isoprene chains). Carotenoids are polyunsaturated Hydrocarbons of the terpene series. A carotene molecule (a derivative of the isoprenoid hydrocarbon—the tetraterpene C₄₀H₆₄) is built from eight isoprene units C₅H₈ (СН₂-С(СН ₃)=СН-СН₂) (Figure 176).

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Figure 176 - Diagram of the ß-carotene molecule

The presence of diverse pigments in antennas that absorb light of different wavelengths significantly broadens the spectrum range that can be absorbed and utilized in Photosynthesis.

Figure 177 shows that the curve relating photosynthetic rate to the wavelength of incident light correlates with the absorption spectra of the pigments present in the light-harvesting antenna complexes.

Figure 177 - Dependence of photosynthetic rate on the wavelength of incident light and the absorption spectra of chlorophyll a, chlorophyll b, and ß-carotene pigments

As an example, Figure 178 presents a molecular model of a photosynthetic center located within a cyanobacterial membrane. It utilizes a "special pair" (dimer) of chlorophyll molecules (shown in dark gray in the center of the molecule) to generate a high-energy electron. (The plant analogue of such a center is photosystem PSI; see section 10.2).

Under normal conditions, such an excited electron would rapidly dissipate its energy as thermal phonons or re-emit a light quantum of slightly lower energy than what was absorbed (fluorescence).

However, the photosynthetic reaction center is structured to bypass this natural pathway of energy relaxation. Instead, the excited electron is guided away from the chlorophyll in a Relay fashion along a chain of Cofactors and iron-sulfur clusters (the pathway is indicated by arrows).

Figure 178 - Cyanobacterial photosynthetic reaction center

Finally, the electron is transferred to a Water-soluble carrier protein (such as ferredoxin) for transport to its destination.

The copper-containing protein plastocyanin, shown at the bottom of the figure, replaces the missing electron in the reaction center with a low-energy electron, returning the chlorophyll to its ground state.

As a result, we achieve Electron transfer from a low-energy source to a high-energy carrier.

In most photosynthetic organisms, the electron source is a water molecule, which is oxidized to molecular oxygen. The electron is excited within the photosynthetic center and then transferred to a metalloprotein (such as ferredoxin) to transport it to the required site within the system.

Photosystems contain efficient complexes of pigment molecules (light-harvesting complexes, LHCs) that absorb light and deliver it to the reaction center. Figure 179 depicts the light-harvesting antenna photosystem whose reaction center is shown in Figure 178.

Figure 179 - Light-harvesting antenna complex of a cyanobacterium

The photosystem consists of three identical subunits, each featuring its own reaction center (shown in dark gray). Surrounding them, dozens of chlorophyll and carotenoid molecules act as an antenna, absorbing light of various wavelengths and transferring energy from molecule to molecule (via resonant energy transfer) to the chlorophylls in the subunit reaction centers.

Light energy is also used to perform mechanical work. For instance, the protein Bacteriorhodopsin transports protons across the membrane using the energy of absorbed light, while the light-sensitive protein opsin changes its globular conformation upon light absorption.

The MOLECULAR MECHANISMS OF photosystems will be discussed in more detail below.