Lecture-17
Electron Transfer in Proteins I
The sun is main source of energy on the earth. The sun light 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 light driven process by which CO2 is fixed to produce
carbohydrates.
light
(CH2O) + O2
CO2 + H2O
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 light (solar energy) to carbohydrate (chemical energy) from CO2.
Chloroplasts:
The site of the photosynthesis in the eukaryotes (algae and plant) is chloroplast. In
chloroplast, light 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 light reactions, which use light energy to generate NADPH and ATP in
thylakoid membrane.
2. The dark reactions, actually light-independent reactions, which use NADPH and
ATP to synthesis carbohydrate from CO2 and H2O in stroma.
Light reactions:
In the first decades of 20th century it was assumed that light 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.
light
CO2 + 2H2S
(CH2O) + 2S + H2O
Between the chemical similarity between H2S and H2O Neil proposed this general
photosynthetic reaction.
light
CO2 + 2H2A
(CH2O) + 2A + H2O
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 light
energy is used to dissociate H2A (light reaction):
light
2A + 4[H]
2H2A
And the resulting reducing agent [H] subsequently reduced CO2 to CH2O and H2O (the
dark reactions):
4[H] + CO2
(CH2O) + CO2
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 light. But in the dark neither these reagents
are reduced nor oxygen evolved. This was the first evidence that absorbed light 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:
light
(CH2O) + 18O2
H218O + CO2
Several years later Severo Ochao showed that NADP+ is the biological electron acceptor
in thylakoid membrane of chloroplasts in (light reaction) according to the equation:
light
2NADPH + 2H+ + O2
2H2O + 2NADP+
Light Absorption:
Visible light 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 light) 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 light, c is the
spped of the light (3 × 108 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 light 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 light is absorbed by a substance at a given wavelength is decribed by
Beer-Lambert law:
I
A = log 0 = εcl
I
Where A is the absorbance, I 0 and I are the intensities of the incident and transmitted
light, c is the molar concentration of the sample, l is the length of the light 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 light absorbing pigments in the thylakoid
membranes. These green pigments are planar, polycylic containing porphyrin ring
containing Mg2+.the heterocyclic five ring system that surrounds the Mg2+ 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 light during photosynthesis.
Chloroplasts contain both chlorophyll a and chlorophyll b. Although both are green in
color they are absorbed light 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 slightly from the plant pigment.
These chlorophyll moieties are bound with the protein and formed light-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 light harvesting agent. In addition there are light harvesting pigments
are presents called carotenoids which are yellow, red and purple in color.
The light 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 light but
only a few chlorophyll molecules attached with the reaction centre are engaged to
transform light energy to chemical energy. The other pigment molecules are called light
harvesting or antenna molecules. They absorb light 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.