Engineering Chemistry S Datta

ROLE OF METALS IN BIOLOGY 567
The iron-sulfur proteins play important role as electron carriers in all living organisms
and participate in the important functions like plant photosynthesis, nitrogen fixation, steroid
metabolism, oxidative phosphorylation etc. The simplest is rubredoxin found in anaerobic
bacteria. The ferredoxins (2Fe-2S) are found in plant chloroplasts and mammalian tissues.
The other types of ferredoxin (4Fe-4S) are found in many strains of bacteria. In these the Fe is
co-ordinated to the S of cysteins.
Lastly, the cytochromes are the most thoroughly characterized electron transferases.
Cytochrome contains heme cofactors and according to the type of heme i.e., heme a, heme b
and heme c, cytochromes are also classified. Class I cytochrome c have a distinctive role in the
mitochondrial electron transfer in mammalian system. Class II cytochrome c are found in
photosynthetic bacteria. Class III cytochrome c contain four hemes which are associated with
the cytoplasmic membrane of sulfate-reducing bacteria.
Electron transfer reactions play key roles in biological processes like collagen synthesis,
steroid metabolism, immune response, drug activation, neurotransmission, nitrogen fixation,
respiration and photosynthesis. From the point of global bioenergetics, aerobic respiration
and photosynthesis are complementary processes (Fig. 26.2).
CO 2
ATP
Catabolic cycles
Aerobic
respiration
Organic
compounds
Anabolic cycles
Photosynthesis
ADP+P i
P i+ADP
ATP
2H 2 O
2H 2 O
O 2
Fig. 26.2 Aerobic respiration and photosynthesis—Complementary processes.
The oxygen evolved by photosynthesis is consumed by aerobic bacteria and animals.
Similarly, the end products of aerobic respiration (CO 2
and H 2
O) are the major nutritional
requirements of photosynthetic organisms.
The energy extracted from the organic compounds by several catabolic pathways (e.g.
citric acid cycle) involves oxidation of these compounds to CO 2
and H 2
O with concomitant
production of water soluble reductants like NADH and succinate. These reductants donate
electrons to the components of mitochondrial electron transfer chain leading to reduction of O 2
to H 2
O.
In aerobic system the terminal oxidant is O 2
and in anaerobic system it is the inorganic
sulfates and nitrates. These reactions are coupled to ATP synthesis (called oxidative
phosphorylation).
Electron Transport and Oxidative Phosphorylation
Electron transport from NADH—Electrons are transported from NADH to oxygen
along the electron transport chain (respiratory chain). NADH passes electrons to NADH-