Questions&AnswersonBioinorganicChemistry(DRay) 12. Explain the iron binding by transferrin. Ans. Iron, when taken up with the food and processed in the mouth is mostly present in Fe 3+ form and thus gets into the gastrointestinal tract as Fe 3+ . In case of an intact milieu in the small intestines, ferric iron is reduced to its ferrous form (Fe 2+ ). The Fe 3+ ions are then taken up by apotransferrin (H 2 Tf); simultaneously, carbonate is coordinated to iron. Fe 3+ -Tf is the transport form for iron. The iron-loaded transferrin delivers iron to sites of potential use (e.g. incorporation into protoporphyrin IX and generation of haemoglobin), or stored in iron storage proteins (ferritins). The delivery of iron affords reduction from the ferric to the ferrous state; a reductant employed here is ascorbate (vitamin C). Transferrin is a glycoproteid of molecular weight 80 kDa (containing ca. 6% carbohydrate), having available two almost equivalent binding sites for iron(III), in the C- and N-teminal lobes, respectively. The pK (K = stability constant; see inset on p. 5) at pH 7.4 (the pH of blood) is -20.2. Transferrin is also an effective transporter for other tri- and divalent metal cations, and even for anions (e.g. vanadate). Since its loading capacity for iron commonly is only ca. 40%, other ions can be transported simultaneously. 13. Show and explain the iron storage protein ferritin. Ans. Ferritins (Fig. 4) are iron storage proteins, built up of a hollow protein sphere (apo-ferritin, M = 450 kDa, 24 subunits of 163 amino acids each) with an outer diameter of 130 and an inner diameter of 70 Å. The inner surface of this capsule is lined with carboxylate functions, which can coordinate Fe3+. Up to 4500 Fe3+ can be taken up. The various iron centres are connected by bridging oxido and hydroxido groups very much as in the colloidal form of ferric hydroxide. The overall composition of the iron nucleus is 8FeO(OH)·FeO(H2PO4). Channels of threefold symmetry and a width of 10 Å allow for an exchange of Fe3+ between the interior and exterior. For the primary uptake process, iron has to be in the oxidation state +II. Its transport along the channels and built-in into the core is accompanied by oxidation to the +III state.
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Questions & Answers on Bioinorganic Chemistry (D Ray)
12. Explain the iron binding by transferrin.
Ans. Iron, when taken up with the food and processed in the mouth is mostly present in Fe3+ form and thus gets into the gastrointestinal tract as Fe3+. In case of an intact milieu in the small intestines, ferric iron is reduced to its ferrous form (Fe2+). The Fe3+ ions are then taken up by apotransferrin (H2Tf); simultaneously, carbonate is coordinated to iron. Fe3+-Tf is the transport form for iron. The iron-loaded transferrin delivers iron to sites of potential use (e.g. incorporation into protoporphyrin IX and generation of haemoglobin), or stored in iron storage proteins (ferritins). The delivery of iron affords reduction from the ferric to the ferrous state; a reductant employed here is ascorbate (vitamin C). Transferrin is a glycoproteid of molecular weight 80 kDa (containing ca. 6% carbohydrate), having available two almost equivalent binding sites for iron(III), in the C- and N-teminal lobes, respectively. The pK (K = stability constant; see inset on p. 5) at pH 7.4 (the pH of blood) is -20.2. Transferrin is also an effective transporter for other tri- and divalent metal cations, and even for anions (e.g. vanadate). Since its loading capacity for iron commonly is only ca. 40%, other ions can be transported simultaneously.
13. Show and explain the iron storage protein ferritin.
Ans. Ferritins (Fig. 4) are iron storage proteins, built up of a hollow protein sphere (apo-ferritin, M = 450 kDa, 24 subunits of 163 amino acids each) with an outer diameter of 130 and an inner diameter of 70 Å. The inner surface of this capsule is lined with carboxylate functions, which can coordinate Fe3+. Up to 4500 Fe3+ can be taken up. The various iron centres are connected by bridging oxido and hydroxido groups very much as in the colloidal form of ferric hydroxide. The overall composition of the iron nucleus is 8FeO(OH)·FeO(H2PO4). Channels of threefold symmetry and a width of 10 Å allow for an exchange of Fe3+ between the interior and exterior. For the primary uptake process, iron has to be in the oxidation state +II. Its transport along the channels and built-in into the core is accompanied by oxidation to the +III state.
Apoferritin (the inside of the subunit structure and channels one of the subunits hollow sphere is lined with of C2, C3 (for iron exchange carboxylates) with the surroundings) and C4 symmetry 14. Explain with a schematic view the tetrameric structure of the hemoglobin.
Ans. Tetrameric adult human hemoglobin is a heterotetramer. Hb in the human blood carries O2
from the respiratory machines to the remaining part of the body where it releases it to burn
nutrients to provide energy to power the functions of the organism, and collects the formed CO2
to bring it back to the respiratory organs to be released from the organism.
When O2 binds to the iron complex, it causes the Fe atom to fit nicely move to the plane of the
porphyrin ring. The imidazole side-chain of the histidine residue interacting at the other pole of
the iron is pulled toward the porphyrin ring which forces the plane of the ring sideways toward
the outside of the tetramer, and also induces a strain in the protein helix containing the histidine
as it moves nearer to the Fe atom. This strain is transmitted to the remaining three monomers in
the tetramer, where it induces a similar conformational change in the other heme sites such that
binding of O2 to these sites becomes easier. This is known as the cooperative binding of O2. The
binding affinity of Hb for O2 is increased by the O2 saturation of the molecule. The positive
cooperative binding is achieved through conformational changes of the Hb. Thus the oxygen
binding curve of hemoglobin is sigmoidal as opposed to the hyperbolic one for non-cooperative
binding.
15. Graphically compare the O2 affinity of hemoglobin and myoglobin.
Ans. Affinity of Hb and Mb to O2 is shown below. The O2 partial pressure at saturation (100%)
is ca. 100 Torr (ca. 0.13 bar = 13 kPa). The graphs apply to the normal blood pH of 7.35 and
temperature of 37 °C. Decreasing the pH and increasing the temperature decreases the affinity
for O2.
16. List the common forms of O2 and oxygen bearing species in relation to ROS.
Ans. One commonly distinguishes three oxygen modifications: singlet-O2 (1O2; high energy
content, unstable, diamagnetic), triplet-O2 (3O2, stable, biradical and hence paramagnetic), and
ozone (O3; toxic; very reactive and strong oxidant).
Reduction of O2 produces superoxide (O2•-) or peroxide (O2
2-), both of which are strong
oxidants and physiologically harmful (reactive oxygen species, ROS). To cope with these
oxidants, the body holds ready catalases (H2O2 → H2O + O2) and superoxide dismutases
(2O2- + 2H+ → H2O2 + O2).
Another ROS species is the hydroxyl radical, formed, e.g. by the Fenton reaction:
Fe2+ + H2O2 + H+ → Fe3+ + H2O + HO•
17. Show with a picturesque presentation the role of distal imidazole heterocycle for trapping of
O2 by heoxy-Hb.
Ans. Deoxy and oxy forms of haemoglobin or myoglobin are shown below. The central ligand
system, protoporphyrin IX, is shown here without the appropriate ring substituents.
18. Explain transport, formation and degradation of hydrogencarbonate in our body.
Ans. Oxygen is ultimately reduced to water in the mitochondrial respiratory chain. The reduction
equivalents come from organic compounds (such as glucose), which are degraded to CO2. CO2 is
converted enzymatically to hydrogencarbonate according to CO2 + OH- → HCO3-, most of
which is extruded out of the erythrocytes (concomitantly, chloride is taken up) and transported,
via the blood plasma, to the pulmonary aveoli, where carbonic acid is formed through the
reaction with Hb·H+, coupled with binding of O2 to haemoglobin. Carbonic acid finally is
enzymatically split into CO2 und H2O. The enzyme catalysing the formation and degradation of
hydrogencarbonate/carbonic acid is called carbonic anhydrase (CA). CA has a molecular weight
of 29.7 kDa and contains Zn2+ in its active centre. Zn2+ is coordinated to three histidine residues
plus an aqua ligand (in its resting state) or a hydroxido ligand (in its active state). A histidine
close to the active centre participates in the proton shuttle. For the catalytically conducted
mechanism see figure for the mechanism of the formation of hydrogencarbonate catalysed by
carbonic anhydrase. The reverse reaction (formation of CO2 from carbonic acid) is also catalysed
by this enzyme.
19. Briefly present the aqueous iron chemistry in relation to the mineralization.
Ans. The redox potential for the pair Fe2+/Fe3+ at pH = 7 demonstrates that FeII is easily oxidized
to FeIII under aerobic conditions. Hexaaquairon(III) ions are cationic Brønstedt acids: