12.2 Bio-inorganic Chemistry: Haemoglobin, Cytochromes, Fe–S Proteins

12.2.1 Metals in biology and the porphyrin ring ★★

• Haem b = Fe(II) in protoporphyrin IX: four pyrrole N in a flat, fully conjugated 18 π-electron ring with methyl, vinyl and propionate side chains.SN
• Chlorophyll a = Mg(II) in a chlorin: one pyrrole ring partly reduced, an extra five-membered ring (with C=O), and a long phytol ester tail that anchors it in the membrane. Without Mg it is pheophytin IFoS 2023 Q5(b) 8m.SN
• Free-base porphyrin (two N–H) has lower symmetry, so four Q bands; a metal in the cavity raises the symmetry, leaving two. Mg²⁺ (d⁰, light, diamagnetic) keeps the ring fluorescent; open-shell metals such as Fe or Cu quench the excited state through their d levels, so haem does not fluoresce.

12.2.2 Haemoglobin and myoglobin ★★★

• Haemoglobin is an α₂β₂ tetramer with four haems; myoglobin is one chain and one haem (storage in muscle). Each haem sits in a non-polar pocket: proximal His F8 below, distal His E7 above, which hydrogen-bonds the bound O₂ CSE 2023 Q7(a) 10m IFoS 2024 Q6(b) 15m.SN
• ν(O–O) ≈ 1105 cm⁻¹ (free O₂ 1555; O₂⁻ 1145): bound O₂ is superoxide-like.
• Colour: both colours come from porphyrin π→π* bands (Soret ≈ 415–430 nm, Q bands 540–580 nm). Low-spin oxy Fe changes the d–π mixing: the Soret moves to shorter wavelength and the Q band splits into two sharp bands, so oxyHb absorbs more green-yellow and looks bright red; high-spin deoxy Fe gives one broad Q band and a purple colour Geo 2024 Q4(a) 10m.SN
• Cytochrome c vs haemoglobin: same haem and proximal His, but in cyt c the sixth site is methionine S, so no O₂ binds and Fe cycles Fe(II)/Fe(III) CSE 2023 Q7(a).

12.2.3 Cooperativity, T and R, Bohr and Haldane effects ★★★

• T (tense): deoxy form, low O₂ affinity; salt bridges between subunits, 2,3-BPG in the central cavity. R (relaxed): oxy form, high affinity, salt bridges broken CSE 2025 Q7(a) 15m.SN
• Cooperativity: O₂ binding to one haem moves Fe into the plane, pulls His F8 and helix F, and breaks the salt bridges; after one or two O₂ the whole tetramer switches to R, so the rest bind more easily. The curve is sigmoid, Hill coefficient n ≈ 2.8 (myoglobin n = 1, hyperbolic) IFoS 2022 Q8(a) 10m.SN
• Allosteric effect: a ligand at one site changes affinity at another. Homotropic: the ligand is the same as the substrate (O₂ for O₂ binding); heterotropic: a different one (H⁺, CO₂, 2,3-BPG). Other allosteric proteins: aspartate transcarbamoylase, phosphofructokinase CSE 2022 Q5(c) 10m.SN

• Bohr effect: lower pH or higher CO₂ lowers O₂ affinity (curve shifts right). H⁺ protonates groups such as β-His146 that form salt bridges only in T, so T is stabilised; working tissue (acidic, CO₂-rich) gets more O₂ CSE 2024 Q6(c)(ii) 10m.SN
• Haldane effect (the reverse view): deoxyHb is the better base and binds more CO₂ (as carbamate on terminal –NH₂) and H⁺; in the lungs O₂ binding releases them, so CO₂ is unloaded. CO₂ travels about 70 % as HCO₃⁻ (carbonic anhydrase, Zn), 20 % as carbamino-Hb, 10 % dissolved IFoS 2024 Q6(b).SN
• Blue baby syndrome: nitrate in well water is reduced to nitrite in an infant's gut; nitrite oxidises Hb Fe(II) to methaemoglobin Fe(III), which cannot carry O₂; the baby turns blue (cyanosis). Infants are at risk because fetal Hb oxidises more easily and their methaemoglobin reductase is low.

12.2.4 Why haem needs its protein: hematin and CO ★★

1. Bare haem in water: PFe(II) + O₂ ⇌ PFe(III)–O₂⁻. A second haem attacks the outer O: μ-peroxo PFe(III)–O–O–Fe(III)P.
2. The O–O bond breaks: two ferryl PFe(IV)=O; each oxidises another PFe(II): net 4 PFe(II) + O₂ → 2 PFe(III)–O–Fe(III)P (μ-oxo dimer, hematin), which cannot carry O₂ CSE 2021 Q5(c) 10m.SN
3. The globin prevents it by site isolation: each haem sits deep in its own hydrophobic cleft, so two haems never meet; water is kept out; methaemoglobin reductase repairs any Fe(III).SN

CO binds free haem about 20 000 times more strongly than O₂; in Hb only about 200 times, because the distal His forces CO to tilt away from its preferred linear Fe–C–O. That margin is what makes CO poisoning slow rather than instant.

12.2.5 Haemocyanin and haemerythrin ★★

Oxyhaemocyanin is blue from peroxide → Cu(II) charge transfer (≈ 345 and 570 nm) and diamagnetic because the two Cu(II) spins couple antiferromagnetically through the peroxide bridge; Cu···Cu shortens from ≈ 460 to 360 pm on O₂ binding CSE 2022 Q8(b) 10m.SN