Lecture-17 Electron Transfer in Proteins I The sun is main source of energy on the earth. The sun is consumed by the plant and cyanobacteria via photosynthesis process. In this process CO2 is fixed to carbohydrate. After that via oxidation process the carbohydrate are metabolized in presence of O2. These two processes are main reason for life on earth. Or in one sentence one can say these oxidation and reduction processes are the basic primary metabolic reaction step in life. And all these processes are electron transfer process in protein. Photosynthesis: The photosynthesis is the driven process by which CO2 is fixed to produce carbohydrates. CO 2 + H 2 O (CH 2 O) + O 2 In this process both CO2 and water are reduced to carbohydrate and oxygen. Photo synthetically produced carbohydrate is the main source of energy for the photosynthetic cell and normal cell. The final ingredients of overall photosynthetic recipe were demonstrated by the German physiologist Robert Mayer who concluded that plants convert (solar energy) to carbohydrate (chemical energy) from CO2. Chloroplasts: The site of the photosynthesis in the eukaryotes (algae and plant) is chloroplast. In chloroplast, harvesting and carbon assimilation reactions are take place in side the chloroplast. These chloroplasts are surrounded by two membranes, the outer membranes are permeable to small molecule and ions, and an inner membrane which is encloses the internal compartment. This compartment contains many flattened vesicles or sacs known as thylakoids and the aqueous phase enclosed by the inner membrane called as stroma. These thylakoids arranged in stacks called grana. The photosynthetic pigments and enzyme complex are present inside the thylakoid membrane. In the stroma, lots of enzymes are present. Photosynthesis occurs in two distinct phases: 1. The reactions, which use energy to generate NADPH and ATP in thylakoid membrane. 2. The dark reactions, actually -independent reactions, which use NADPH and ATP to synthesis carbohydrate from CO2 and H2O in stroma.
Light reactions: In the first decades of 20 th century it was assumed that was absorbed by the photosynthetic pigments which directly reduced CO2 to carbohydrate combined with water. In this view in 1931, Corneils van Neil performed photosynthesis process anaerobically using green photosynthetic bacteria in presence of water using H2S, generate sulfur. CO 2 + 2H 2 S (CH 2 O) + 2S + H 2 O Between the chemical similarity between H2S and H2O Neil proposed this general photosynthetic reaction. CO 2 + 2H 2 A (CH 2 O) + 2A + H 2 O Here H2A is H2O in green plants and H2S in photosynthetic sulfur bacteria. On the basis of this result Neil hypothesized that photosynthesis is the two step process in which energy is used to dissociate H2A ( reaction): 2H 2 A 2A + 4[H] And the resulting reducing agent [H] subsequently reduced CO2 to CH2O and H2O (the dark reactions): 4[H] + CO 2 (CH 2 O) + CO 2 In 1937 Robert Hill found that when leaf extracts containing chloroplasts in presence of non biological electron acceptors like dichlorophenolindophenol or ferricyanide are reduced and oxygen evolved in presence of. But in the dark neither these reagents are reduced nor oxygen evolved. This was the first evidence that absorbed energy causes electrons to flow from H2O to an electron acceptor. In 1941, when the oxygen isotope became available, Samuel Ruben and Martin Kamen directly demonstrated that the source of the O2 formed in photosynthesis is H2O: H 18 2 O + CO 2 (CH 2 O) + 18 O 2
Several years later Severo Ochao showed that NADP + is the biological electron acceptor in thylakoid membrane of chloroplasts in ( reaction) according to the equation: 2H 2 O + 2NADP + 2NADPH + 2H + + O 2 Light Absorption: Visible is the electromagnetic radiation of wavelengths from 400-700 nm ranging from violet to red, with the former at higher energy and red of lesser energy. The energy of the photon (a quantum of ) follows the Plank equation: E = hν = hc λ Where h is the Plank constant (6.626 10-34 J.s), ν is the frequency of the, c is the spped of the (3 10 8 m/s) λ is the wavelength. When a photon is absorbed, an electron in the absorbing molecule is lifted to a higher energy level. The energy of absorbed photon (a quantum) exactly matches with the energy of the electronic transition. A molecule that has absorbed a photon is in an excited state, which is generally unstable. An electron lifted to the higher energy orbital level usually returns to its lower energy level via various processes. The excited molecule decays to the stable ground state giving up the absorbed quantum as or heat or using it to do chemical work. The electron can jump from ground state (S0) to first (S1), second (S2) singlet excited state. Also this radiation process is called as fluorescence process. Electron moves from S2 to S1 via irradiative path way which is known as internal conversion. Also from S1 state to electron can move to triplet state (T1) state irradiative pathway and this process is called internal conversion. From this triplet state to electron can go to the ground state via radiative pathway which known as phosphorescence. Exciton transfer (resonance energy transfer) in which an excited molecule directly transfers its excitation energy to nearby unexcited molecules with similar electronic properties. This process occurs through interactions between the molecular orbitals of the participating molecules in a manner analogous to the interactions between the two pendulums of the similar frequencies. The amount of is absorbed by a substance at a given wavelength is decribed by Beer-Lambert law: I 0 A = log = εcl I Where A is the absorbance, I 0 and I are the intensities of the incident and transmitted, c is the molar concentration of the sample, l is the length of the path through the sample in cm and ε is the molar excitation coefficient. Consequently A versus λ plot for a given molecule is called its absorption spectrum.
Chlorophylls are the most important absorbing pigments in the thylakoid membranes. These green pigments are planar, polycylic containing porphyrin ring containing Mg 2+.the heterocyclic five ring system that surrounds the Mg 2+ has an extended polyene structure with an alternating single and double bonds. These moieties have characteristically high absorption in the visible region of the spectrum. The chlorophylls have high molar extinction coefficient and therefore are well suited for absorbing visible during photosynthesis. Chloroplasts contain both chlorophyll a and chlorophyll b. Although both are green in color they are absorbed at different wavelength complement each other. Normally chlorophylls a are twice with respect to chlorophylls b. The pigments in the algae and cyanobacteria are different sly from the plant pigment. These chlorophyll moieties are bound with the protein and formed -harvesting complexes. The pigments are fixed in relation to each other in other protein complexex and to the membrane. In cyanobacteria and red algae have phycoerythrobilin and phycocyanobiln as harvesting agent. In addition there are harvesting pigments are presents called carotenoids which are yellow, red and purple in color. The harvesting complexes in the thylakoid or bacterial membranes are arranged in a pattern called photo systems. In chloroplasts, each photosystem contains about 200 chlorophyll and 50 carotenoid molecules. All the pigments are able to absorb but only a few chlorophyll molecules attached with the reaction centre are engaged to transform energy to chemical energy. The other pigment molecules are called harvesting or antenna molecules. They absorb and transmit rapidly and efficiently to the reaction center. In this process a positive charge is formed in one center and in the other center a negative charge is created that forms a potential gap.
Electron transport in chloroplast The electron transport in the chloroplast is very complex process. Three alkaloid membrane bound proteins (1) PSII (2) cytochrome b6f complex and (3) PSI are engaged. in this process. The electrons are transferred via mobile electron carriers. The plastoquinone are reduced to plastoquinol in PSII and linked with cytochrome b6f complex. This cytochrome b6f complex is linked with PSI via mobile protein plastocyanin. The electron from PSI is used to reduce NADP + to NADPH in stroma.and the electron transfer is happened from water to electron whole of P680 and generate O2 and proton.