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BIOINORGANIC

CHEMISTRY

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Chetna

(Assistant prof. in chemistry)

Photosynthesis :

Photosynthesis is defined as the process of preparation of food (mainly carbohydrates) in green plants. In the presence of sunlight, carbon dioxide(from atmosphere) and water(from lithosphere) reacts together in green plants to produce carbohydrates and oxygen. The green pigment chlorophyll, present in plants, is essential for this process.

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^xCO2 ^{+ xH}2^{O→ (CH}2^O)x ^{+ xO}2

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All the oxygen come from the oxidation of water. It was established by labeling experiment. When labeled water is used, the product oxygen also contains labeling.

Photosynthesis is endothermic in nature. This process is initiated by visible solar radiation in the range of 600-700 nm. Plants absorb this solar radiation with the help of chlorophylls. That’s why chlorophyll is sometimes called- ‘light harvesting material’.

Chlorophyll :

Chlorophylls are the green pigments of plants. Chlorophylls are a mixture of four pigments- chlorophyll-a , chlorophyll-b , chlorophyll-c , chlorophyll-d . They are complexes of magnesium ion with macrocyclic ligand derived from porphine. The central Mg²⁺ ion is nearly _{30- 50pm above of the plane containing 4 nitrogen atoms of the ligand. One or two molecules of H2O} coordinate to the Mg²⁺ ion and connect the stacks of chlorophyll by H-bonding interaction. The presence of conjugated polyene structure in chlorophyll may be the reason for their light absorbing property.

Structure of chlorophyll

Structure of porphin

Mechanism :

The mechanism was proposed by Robert Hill and Fey Bendall in 1960. Chlorophylls absorb the solar light and pass the photon energy to the reaction site through excitation transfer within picoseconds. The overall photosynthesis process is the outcome of a series of complex reactions. These reactions involve light absorption at two different regions of wavelength. These two regions are as called PS-I and PS-II.

Photosystem – II (PS-II) :

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^{In this process, a cluster of Mn}4 ^{catalyzes the oxidation of water to dioxygen. The metalloenzyme} used is called – oxygen-evolving complex(OEC). Basic reaction is :

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^2H2^{O → O}2 ^{+ 4H+ + 4e}

The electron produced are excited photochemical by P-680 to the primary electron accepter

pheophytin. From there, it is transferred to PS-I involving the following intermediates-

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1. ^{Cytochrome b}3^{(cyt b}3⁾
2. Plastoquinone (PQ)
3. Rieske’s iron –sulphur protein (Fe-S protein)
4. Cytochrome f
5. Plastocyanin (PC)

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Photosystem-I (PS-I) :

The electrons received from PS-II are further excited by P-700 and is ultimately used up in the reduction of carbon dioxide involving the following intermediates-

1. An Fe-S centre.
2. Ferridoxins.
3. NADP reductase.
4. NADPH.

This flow of electrons also results in the formation of ATP from ADP.

Role of Mg²⁺ ion :

1. It maintains the rigidity of the macrocyclic structure of chlorophyll and minimizes molecular vibrations. As a result the loss of energy by thermal vibrations during transfer process is highly minimized.

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2.Magnesium ion provides a centre for the co-ordination of water molecules which maintains adjacent chlorophyll molecules in a stack by H-binding interaction. This H- bonding interaction enhances the rate of electron transfer in the redox chain reaction.

3.The initial photon absorption leads to formation of a short-lived singlet state. This singlet state is converted to a triplet state in the next step. The magnesium ion increases the rate of this conversion process.

Ovearall reaction :

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^{PS-II : 2H}2^{O → O}2 ^{+ 4H+ + 4e}

PS-I : 2NADP⁺ + 2H⁺ + 4e → 2 NADPH

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^{Dark reaction : 12 NADPH + 12 H+ + 6CO}2 ^{→ 12 NADP+ + 6 H}2^{O + (CH}2^O)6

^{Net reaction : 6CO}2 ^{+ 6H}2^{O → 6O}2 ^{+ C}6^H12^O6

• Iron :

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It is the major constituent of many proteins. Iron proteins are versatile in nature and occurance. They can be classified as-

1. Iron-porphyrin proteins : hemoglobin, myoglobin, cytochrome P-450
2. Non-heme iron proteins: transferritin, ferritin, hemosiderin
3. Non-heme di-iron di-oxo bridged species :hemerythrin, methane monoxygenase, ribolucleotide reductase
4. Non- heme protein containing iron-sulfur clusters : nitrogenase, ferridoxins, rubredoxins

1. Hemoglobin and Myoglobin :

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Both hemoglobin and myoglobin are metal porphyrins which contain the ‘heme’ group in their structure. The heme group consists of an iron atom which is coordinated to four nitrogen atoms of porphyrin- IX.

Structure of Myoglobin : Myoglobin is a monomeric protein. In myoglobin the heme group is embedded in a crevice formed by the coiling of its polypeptide chains containing nearly 153 amino acid residue. The nature of the pocket is hydrophobic due to the presence of large hydrocarbon chains. The polypeptide chain is coordinated to iron atom(satisfies 5^th coordination site) of heme group through the nitrogen atom of the histidine residue

Structure of Hemoglobin : Hemoglobin is a tetrameric protein. It consists of four myoglobin- like subunits. The entire heme unit is encapsulated in a water repelling pocket of protein chains. Hemoglobin has 4 protein chains each with a heme group, packed in the form of a roughly ^tetrahedral 2 ^β2 ^{cluster. The chains contain approximately 141 amino acids and the β chains} contain nearly 146 amino acids.

Special structural feature in hemoglobin and myoglobin :

The polypeptide chains of both hemoglobin and myoglobin coil in a similar manner to produce the cavity for the accommodation of the heme group. The 4 subunits of hemoglobin are linked with one another through salt-bridge interaction. This salt-bridge interaction is basically the ^{electrostatic attraction present in between the NH}3^{+ and COO- groups of adjacent amino acid} residues. Due to such salt-bridge interaction in between the polypeptide-chains, the hemoglobin molecule experience a state of strain.

The penta-coordinated high spin Fe(II) present in hemoglobin and myoglobin lies about 0.5 A^o above the plane of the porphyrin ring containing 4 nitrogen atoms. This is because the radius of high spin Fe(II) is larger than the size of the cavity of the porphyrin ring. The sixth coordination site is sterically overcrowded by the groups from the side chains attatched with the main-coiled polypeptide chain.

Biological role and characteristics of hemoglobin and myoglobin :

Hemoglobin picks up oxygen from lungs and carries it to the muscle tissues via the circulary system. The oxygeneated hemoglobin then transfers this oxygen to myoglobin present in the muscle tissue. Myoglobin then transport oxygen to the site of its use inside the cells ; when it is required. In addition, hemoglobin also carries carbon dioxide from the muscle to the lungs. It ^{can be noted that the transport of CO}2 ^{by hemoglobin does not involve the heme group, rather} the polypeptide chains get involved in this process.

Oxygenation of myoglobin can be represented as-

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^{Mb + O2 MbO}2

Oxygenation of myoglobin can be represented as-

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^{Mb + O}2 ^MbO2

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where, K is called the oxygen binding constant of myoglobin

K = [MbO2]/[𝑀𝑏][O2]

If f represents the fraction of myoglobin molecules bearing oxygen (oxy-myoglobin) and p is the equilibrium pressure then-

K = 𝑓/(1−𝑓)𝑝

f = Kp/(1+Kp)

due to tetrameric nature, the oxygenation of hemoglobin is little complex and it approximately follows the following equations

Hb + n O2 Hb(O2)n

K = [Hb(O2)n]/[𝐻𝑏][𝑂2]𝑛

where, K is called the oxygen binding constant of myoglobin

If f represents the fraction of myoglobin molecules bearing oxygen (oxy-myoglobin) and p is the equilibrium pressure then-

K = 𝑓/(1−𝑓)𝑝

f = 𝐾𝑃𝑛/1+ 𝐾𝑃𝑛

The above equation is called Hill equation and n is known as Hill constant. The value of n is pH dependent. The value of n ranges from n= 1 to 4.

It can be seen from the graph that, at high partial pressure of oxygen(nearly 100-120 mm Hg; generally found at lungs) both hemoglobin and myoglobin are almost completely saturated with oxygen. However, at low partial pressure (nearly 20-40 mm Hg; generally found at muscles), hemoglobin becomes a much poorer binder of oxygen (or, hemoglobin possesses lower affinity towards oxygen) compared to that of myoglobin. Hence in the muscle tissue, oxygenated hemoglobin transfer its oxygen to myoglobin.

The oxygen- binding power of hemoglobin depends upon pH of the medium. The affinity of hemoglobin towards oxygen decreases with decrease in the PH. This is called Bohr-effect. Due to this effect, the transfer of oxygen from oxygenated hemoglobin becomes more favorable in the working muscle, where concentration of CO2 is high(low pH).