Bioinorganic Chemistry Short And Very Important Notes And MCQ Pdf Of Metalloporphyrins, Metal complexes in Medicine, Metal Poisoning

 

🧬 Bioinorganic Chemistry – Complete Notes

JEE ADV NEET IIT-JAM GATE CSIR-NET TGT/PGT

📌 What is Bioinorganic Chemistry? A growing interdisciplinary field focusing on the roles of metal ions in living systems.

1. Metalloporphyrins – The Core Concept

• Metal ion coordinated to 4 nitrogen atoms inside porphyrin ring → square planar geometry
• Axial sites are free for other ligands
• Examples: Hemoglobin Myoglobin Cytochromes Chlorophylls
• Porphyrin = derivative of macrocyclic ligand Porphine (unsubstituted tetra-pyrole + CH bridges)
• Positions labeled α,β,γ,δ or 5,10,15,20 → tetraphenyl derivatives (tpd) commonly used
• Metal-free porphyrin ligand has –2 charge (two inner H atoms replaced by metal ion)
• More rigid than crown ethers; stronger preference for d⁸ Ni²⁺ ion

⚠️ Size Effect: If metal ion too small (Ni²⁺) → ring ruffles. If too large → domed (sits above ring).

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2. Role of Iron in Living Systems

Iron = most important transition metal in living systems (vital for plants + animals)

Three Iron Systems:

① Heme Iron (H)
Hemoglobin, Myoglobin, Cytochrome P₄₅₀
Contains porphyrin ring

② Non-heme Iron – Fe-S (NH)
Rubedoxin, Ferredoxins, Nitrogenase
Iron-sulphur compounds

③ Non-heme diiron oxo-bridged (NH)
Hemerythrin, Ribonucleotide reductase, Methane monooxygenase

📋 Table – Iron Proteins (MOST ASKED IN EXAMS!)

Protein	Mol. Mass	Fe atoms	Oxidation	Source	H/NH	Function
Hemoglobin	64500	4	Fe²⁺	Animals	H	O₂ transport
Myoglobin	17500	1	Fe²⁺	Animals	H	O₂ storage
Cytochromes	12500	4	Fe²⁺	Plants/Animals/Bacteria	H	Electron transfer
Ferredoxin	6000–12000	2–8	Fe²⁺/Fe³⁺	Bacteria/Plants/Animals	NH	Electron transfer
Ruberodoxin	6000	1	Fe³⁺	Bacteria	NH	Electron transfer
Ferritin	45000	20% Fe	Fe²⁺	Animals	NH	Iron storage
Transferritin	76000	2	Fe³⁺	Animals	NH	Scavenging iron
Hemerythrin	108000	2	Fe²⁺	Marine invertebrates	NH	O₂ transport
FeMo Protein	220000	24–36	—	Nitrogenase enzymes	NH	N₂ fixation in bacteria
Catalase	280000	—	Fe³⁺	Living organism	H	Decomp of H₂O₂
Peroxidase	44000	—	Fe³⁺	Living organism	H	Decomp of H₂O₂

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3. Hemoglobin (Hb) – O₂ Transport

• Contains heme groups + globin proteins
• Molar mass ≈ 64500; found in red blood cells (erythrocytes)
• 4 subunits: 2α (141 aa) + 2β (146 aa)
• Fe(II) attached via imidazole N of histidine (proximal); 6th site for O₂ (distal)
• Deoxy-Hb = high spin Fe(II), d²ₓ₋ᵧ² orbital occupied → Fe too large → sits 40 pm above ring → T state (tense)
• Oxy-Hb = low spin Fe(II) → smaller (75 pm) → slips into ring → R state (relaxed)

🔑 Key: Fe–N(imidazole) distance: Deoxy = 282 pm | Oxy = 197 pm

Cooperative Effect (Very Important!)

• Binding of 1st O₂ is hardest; each subsequent O₂ binds easier
• Equilibrium constants: K₁ < K₂ < K₃ < K₄
• K₄ is much larger than K₁ → last O₂ binds most readily
• This is the cooperative effect → sigmoidal O₂ dissociation curve

CO₂ Transport (Bohr's Effect)

• CO₂ (from muscles) → lowers pH → decreases Hb affinity for O₂ → releases O₂ to muscles
• CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (enzyme: carbonic anhydrase)
• ~10–15% CO₂ transported as carbaminohemoglobin (CO₂ binds to –NH₂)
• Oxygenated Hb is more acidic: HbHₓ⁺ + 4O₂ ⇌ Hb(O₂)₄ + xH⁺

Hill Equation for Hemoglobin:

f = Kpⁿ_O₂ / (1 + Kpⁿ_O₂)    where n = Hill constant
For Hb: n = 2.8 (physiological pH in muscle)
For Mb: n = 1 → hyperbolic curve

☠️ CO Poisoning: CO binds Fe²⁺ 200× stronger than O₂. CO + CN⁻, NO, PF₃ are stronger π-acceptors → prevent O₂ binding → Asphyxia

Genetic Defects:

1. Sickle Cell Anemia (SCA)
Hydrophilic Glutamic acid → replaced by Hydrophobic Valine in β-chain → reduces solubility → sickle-shaped RBCs

2. Absence of Methemoglobin Reductase
Fe²⁺ → Fe³⁺ (methemoglobin) → can't carry O₂
Normal blood has ~3% methemoglobin
Enzyme NADH-methemoglobin reductase converts back

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4. Myoglobin (Mb) – O₂ Storage

• Single heme group; molar mass ≈ 17000
• 5-coordinate high spin Fe(II) complex; t²₄g e²g config
• 153 amino acids; Fe(II) lies 40 pm above porphyrin plane
• Binds O₂ more strongly than Hb at low partial pressures (muscles)
• Hyperbolic oxygenation curve (n=1)
• In lungs: Hb + 4O₂ → Hb(O₂)₄ | In muscles: Hb(O₂)₄ + 4Mb → 4Mb(O₂) + Hb

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5. Hemerythrin – Marine O₂ Carrier

• Non-heme iron protein in marine invertebrates
• MW = 108000; 8 subunits; each with 113 aa + 2 Fe(II) active site
• Bridging groups: 2 carboxylates (glutamate + aspartate) + OH⁻
• 1 O₂ binds to 2 Fe(II) → both oxidized to Fe(III) + peroxide (O₂²⁻)
• No cooperativity between subunits (unlike Hb)
• Oxyhemerythrin: antiferromagnetically coupled Fe(III) → diamagnetic, EPR inactive
• Raman spectroscopy: μ(O–O) at 845 cm⁻¹ (bound peroxide)

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6. Hemocyanin – Invertebrate O₂ Carrier

• Copper-containing protein (NOT heme); blue blood protein
• Found in: Mollusca (whelks, snails, squid) and Arthropoda (crabs, lobsters, shrimps)
• Deoxy form: Cu(I) → colorless | Oxy form: Cu(II) → bright blue
• Active site: 2 Cu(I) ions (~460 pm apart); each bound by 3 histidine residues
• Blue color due to O²⁻→Cu²⁺ LMCT transition
• Like Hb: multiple subunits, binds O₂ cooperatively
• Oxy form: Cu–Cu = 360 pm | v(O–O) ≈ 750 cm⁻¹

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7. Cytochromes – Electron Transfer

• Found in plants + animals; contain heme prosthetic groups
• Prosthetic group = compound required by enzyme → also called coenzyme
• Apoenzyme + prosthetic group = Holoenzyme
• Three types: Cyt-a, Cyt-b, Cyt-c
• Fe(II) attached to N of histidine (one side) + S atom of methionine (sixth site) → inert to O₂ and CO
• Reduction potentials: cyt-b (0.26 V) → cyt-c (0.26 V) → cyt-a (0.4 V)
• Electron flow: b → c → a → O₂

Heme Types:

Heme	R groups	Found in
Heme A	R₁=CH=CH₂, R₂=C₁₈H₃₀OH	Cytochrome a
Heme B (Protoporphyrin IX)	R₁=R₂=CH=CH₂	Hb, Mb, cyt-b, peroxidase
Heme C	R₁=R₂=CH(CH₃)S–Protein	Cytochrome c

Cytochrome P-450

• Absorbs at 450 nm with CO complexes (π→π* Soret bond)
• Functions as monooxygenase: inserts one O atom into substrate
• Active site: Fe in Fe(III) low spin octahedral
• One S of cysteine (not histidine) + H₂O at 6th site
• Reactions: RH → ROH | Aldehyde → Carboxylic acid | Alkene → Epoxide
• Found in kidney; oxidizes insoluble hydrocarbons → water-soluble ROH (excreted)

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8. Iron-Sulfur Proteins (Non-heme)

Ruberodoxin

• Simplest NHIP; found in anaerobic bacteria; only 1 Fe atom
• Fe coordinated to 4 S atoms (cysteine) → distorted tetrahedral
• Oxidation state: +3; Fe–S distance: 224–233 pm; S–Fe–S angle: 104–114°
• No labile sulfur (inorganic S²⁻); one electron transfer agent

Ferredoxins (NHIP with multiple Fe)

Type	Fe atoms	Structure	Properties
[2Fe–2S] (Fe₂S₂)	2	Di-μ-sulfido bridged high spin tetrahedral Fe(III)	S=0 oxidized (diamagnetic); S=½ reduced (EPR active)
[3Fe–4S] (Fe₃S₄)	3	Cubane [4Fe–4S] with one corner removed	1e⁻ transfer; oxidized: ESR active (S=5/2 paramagnetic)
[4Fe–4S] (Fe₄S₄)	4	Cubane structure; most stable	S=0 oxidized (diamagnetic, EPR inactive); S=½ reduced (EPR active)

💡 [4Fe–4S] = most common and most stable ferredoxins. Found in anaerobic bacteria. Oxidation state: +2.5 (delocalized)

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9. Nitrogen Fixation

• N₂ → NH₃ (breaking N≡N, BDE = 945 kJ/mol)
• Haber-Bosch: 400–500°C, 200 atm, Fe/Mo catalyst → expensive
• Biological fixation: ambient T and P by bacteria
• Key bacteria: Clostridium pasteurianum, Azotobacter vinerlandii, Rhizobium (root nodules of legumes)
• Three types of nitrogenases: Vanadium, Iron, Molybdenum (most important)
• Molybdenum nitrogenase = 2 proteins: Fe protein (MW 60000) + MoFe protein (MW 220000–240000)
• Overall: N₂ + 16MgATP + 8H⁺ + 8e⁻ → 2NH₃ + 16MgADP + 16Pᵢ + H₂

Mo–N≡N →(2e⁻,2H⁺)→ Mo–N=NH₂ →(2e⁻,2H⁺)→ Mo–NH–NH₃ →(2e⁻,2H⁺)→ Mo + 2NH₃

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10. Ferritin & Transferrin – Iron Storage/Transport

Ferritin (Iron Storage)

• 24 protein chains → hollow sphere 100 Å diameter (apoferritin)
• Iron core: ~45000 Fe(III) ions + hydroxo + oxo + phosphate ligands
• Core similar to ferrihydrite: (FeO·OH)₈(FeO·H₂PO₄)
• Found in liver, spleen, bone marrow
• Fe(II) enters via hydrophilic channels; oxidized by ferroxidase enzyme

Transferrin (Iron Transport)

• Binds Fe(III) very strongly (not Fe(II))
• MW ≈ 80000; 2 Fe(III) per molecule
• Fe(III) in distorted octahedral: 1N + 3O + chelating CO₃²⁻ or HCO₃⁻
• Binding constant ≈ 10²⁶ → extremely efficient scavenger
• Fe absorbed as Fe(II) → oxidized to Fe(III) by ceruloplasmin → binds transferrin

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11. Electronic Spectra of Porphyrins

• Soret band (~400 nm) = strong; due to S₀→S₂ transition
• Q bands (450–750 nm) = weaker; due to S₀→S₁ transition
• Metal insertion → Soret becomes broad, Q-bands disappear
• Colors of Hb, Mb, catalase, cytochromes = intraligand π→π* transition
• d⁰, d², d³, d¹⁰ metals → little effect on spectrum
• dⁿ (n=4–9) metals → form metal-to-ligand π-bonds → hypsochromic (blue) shift

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12. Photosynthesis

6CO₂ + 6H₂O →(hν)→ C₆H₁₂O₆ + 6O₂
(i) 2H₂O → 4H⁺ + O₂    (ii) 6CO₂ + 12H⁺ → C₆H₁₂O₆ + 3O₂

• Occurs in chloroplasts
• Chlorophylls = Mg²⁺ complexes with porphyrin ring (one pyrrole reduced = chlorin)
• Chlorophyll-a: R = CH₃ | Chlorophyll-b: R = CHO
• Absorption: red region 680–700 nm
• Mg²⁺ preferred → binds strongly + keeps macrocycle rigid → energy not dissipated by thermal vibrations

Photosystems:

PS-II (P₆₈₀)
Absorbs at 680 nm; initiating photosystem
Water oxidation: 2H₂O → 4H⁺ + O₂
Mn₄ cluster (3Mn(II) + 1Mn(III)) catalyzes O₂ evolution

PS-I (P₇₀₀)
Absorbs at 700 nm
Reduces NADP⁺ → NADPH via ferredoxin
Net: 2H₂O + 2NADP⁺ + 8 photons → 2NADPH + 2H⁺ + O₂

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13. Zinc Metalloenzymes

Carboxypeptidase-A

• Pancreatic enzyme; catalyzes hydrolysis of peptide bonds
• 307 amino acids + 1 Zn²⁺; MW ≈ 34800
• Zn²⁺ coordinated to: His-69, His-196, Glu-72 (bidentate) + H₂O
• Hydrophobic pocket accommodates organic group of peptide

Carbonic Anhydrase

• Catalyzes: CO₂ + H₂O ⇌ HCO₃⁻ + H⁺ (rate increased ~10⁶×)
• MW ≈ 30000; 260 aa; Zn²⁺ coordinated tetrahedrally to His-94, His-96, His-119 + H₂O
• Zn²⁺ more acidic than in carboxypeptidase → neutral histidines (not glutamate)
• Mechanism: Zn–OH⁻ attacks CO₂ → transient Zn²⁺ with carbonato oxygen from HCO₃⁻

💡 Nature prefers Zn(II) at hydrolytic enzyme active sites because: no accessible redox states + forms 4 and higher coordination + strong Lewis acid

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14. Superoxide Dismutases (SOD)

• Catalyzes: 2O₂⁻ + 2H⁺ → H₂O₂ + O₂
• Three types: CuZnSOD (eukaryotes) | MnSOD (bacteria) | FeSOD (bacteria)
• CuZnSOD: MW ≈ 16000; Cu²⁺ = functional (square pyramidal, 4 His-N + H₂O); Zn²⁺ = structural support
• Zn²⁺ can be replaced by Co or Cd → retains activity; Cu²⁺ cannot be replaced

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15. Blue Copper Proteins

Type	Features	Examples
Type 1	Intense blue (λ=600nm), S→Cu²⁺ LMCT, EPR active (narrow hyperfine), trigonal coordination	Plastocyanin, Azurin, Ceruloplasmin
Type 2	Normal EPR, tetragonal coordination, no blue colour	Galactase oxidase
Type 3	Two Cu(I), ~360 pm apart, EPR inactive (antiferromagnetic), O₂ binding	Hemocyanin, Laccase

Plastocyanin

• Found in chloroplasts; MW ≈ 10500; 1 Cu atom; 97–104 aa
• Involved in electron transfer in photosynthesis (PS-I and PS-II bridge)
• Distorted tetrahedral: 2 His-N + 1 S(Met) + 1 S(Cys)

Ceruloplasmin

• Intensely blue copper protein; MW ≈ 135000; 6–7 Cu atoms
• Oxidizes Fe(II) → Fe(III) for transferrin binding
• Deficiency → Wilson's disease

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16. Vitamin B₁₂ / Coenzyme B₁₂ (Cyanocobalamin)

• Nature's only organometallic compounds
• Co(III) coordinated to 4 N of corrin ring (modified porphyrin; one less CH bridge)
• 5th: imidazole N; 6th: CN⁻ (in vitro) / water (in vivo)
• Deficiency → pernicious anaemia
• Vitamin B₁₂ = cyanocobalamin; Coenzyme B₁₂ = 5′-deoxyadenosylcobalamin
• Co(III) → d⁶ low spin octahedral → diamagnetic + EPR inactive
• Coenzyme B₁₂ catalyzes 1,2-rearrangements; methyl group transfer
• Bacteria can methylate heavy metals (Hg, Pb, Sn, Pd, Pt) → toxic species like Hg(CH₃) and Pb(CH₃)₄

⚠️ Corrin ring vs Porphyrin: Corrin is less symmetric + less unsaturated than porphyrin → can stabilize Co(I) state (porphyrin can't).

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17. Cisplatin – Anticancer Drug

• Discovered 1969 by B. Rosenberg: cis-[Pt(NH₃)₂Cl₂]
• Inhibits tumor cell division; binds N-7 of adjacent guanine bases on DNA
• trans-isomer is inactive → chelation at cis-positions is essential
• Side effects: kidney damage + neurotoxicity → replaced by Carboplatin
• Most effective against testicular cancer

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18. Metal Complexes in Medicine – Quick Table

Disease	Treatment / Complex	Details
Lead poisoning	EDTA (CaNa₂), BAL, Penicillamine	Chelation therapy
Wilson's disease	D-penicillamine	Cu overload; ceruloplasmin deficiency
Arthritis	Na₃[Au(S₂O₃)₂] (Sanochrysin), Auranofin	Au(I) complexes
Hypercalcemia	Ga(NO₃)₃ (gallium nitrate)	Rapid Ca loss from bones
Cancer	Cisplatin [Pt(NH₃)₂Cl₂]	cis-isomer only active
Siderosis	Desferrioxamine-B	Excess iron; high affinity for Fe(III)
MRI contrast	Gd(III), Fe(III), Mn(II)	Paramagnetic metals alter relaxation

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19. Siderophores

• Iron-containing complexes in microorganisms
• Also called siderochromes (many intensely coloured)
• Low MW (500–1000); classified as: ferrichromes, ferrioxamines, entrabactins
• Chelating ligands → high spin octahedral complexes with Fe(III)
• Stable but labile → iron transported + transferred within bacteria

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20. Xanthine Oxidase

• Contains: 2 identical subunits; each with 1 Mo + 2 Fe₂S₂ + 1 FAD
• MW ≈ 275000; catalyzes: Xanthine → Uric acid
• Mo(VI) site: 2e⁻ oxidation of xanthine → reduces to Mo(IV) → regenerated
• Electron flow: Xanthine → Mo(VI) → 2Fe₂S₂ → FAD → O₂
• Excess uric acid → gout; treated with xanthine oxidase inhibitors

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21. Na⁺/K⁺-ATPase Pump

• Active transport; hydrolyzes ATP → ADP + Pᵢ
• Pumps: 3 Na⁺ out + 2 K⁺ in per ATP hydrolyzed
• Enzyme = α + β subunits (tetramer αβ₂); 2 ATP binding sites on α-subunit
• Ionophores (ion-bearers): Nonactin, Valinomycin → selectivity for K⁺ over Na⁺
• 3Na⁺(in) + 2K⁺(out) + ATP + H₂O → 3Na⁺(out) + 2K⁺(in) + ADP + Pi

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22. Biological Roles of Metals – Master Table

Metal	Related Compound	Function
Fe	Hemoglobin, Myoglobin, Hemerythrin, Cytochrome P-450, Catalase, Ferredoxin, Ferritin, Transferrin, Siderophores	O₂ transport/storage, Oxygenases, Electron transfer, Iron storage/transport
Co	Coenzyme B₁₂	Methylation of organic compounds
Cu	Hemocyanin, Ceruloplasmin, Amine oxidase	O₂ carrier, Fe transport, Amine oxidation
Zn	Carboxypeptidase, Carbonic anhydrase	Hydrolysis of peptide bonds / CO₂ hydration
Mg	Chlorophyll, Phosphotransferase	Photosynthesis, Phosphate hydrolysis
Mn	Arginase, PS-II (Mn₄ cluster)	Electron transfer, Water oxidation
Fe,Mo	Nitrogenase	Nitrogen fixation
Cu,Zn,Mo	Superoxide dismutase	Dismutation of O₂⁻
Ni	Urease	Hydrolysis of urea → CO₂ + NH₃

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🎯 MCQ Practice

Q1. Which is a heme iron protein?
(a) Ruberodoxin (b) Transferrin (c) Hemerythrin (d) Cytochrome-c
✅ Answer: (d) Cytochrome-c

Q2. Metal ions involved in electron transport in biological systems:
(a) Na⁺ and K⁺ (b) Zn²⁺ and Mg²⁺ (c) Ca²⁺ and Mg²⁺ (d) Cu²⁺ and Fe²⁺
✅ Answer: (d) Cu²⁺ and Fe²⁺

Q3. In transformation of oxyhemoglobin to deoxyhemoglobin:
(a) Fe²⁺ low spin → Fe²⁺ high spin (b) Fe²⁺ low spin → Fe³⁺ low spin
(c) Fe²⁺ high spin → Fe²⁺ low spin (d) Fe²⁺ high spin → Fe²⁺ high spin
✅ Answer: (a) Fe²⁺ in low spin changes to Fe²⁺ in high spin

Q4. Oxygen-free form of myoglobin is:
(a) 5-coordinate high spin Fe(II) (b) 6-coordinate low spin Fe(II)
(c) 5-coordinate high spin Fe(III) (d) 5-coordinate low spin Fe(III)
✅ Answer: (a) 5-coordinate high spin Fe(II) complex

Q5. Iron-sulphur clusters in biological systems are involved in:
(a) proton transfer (b) atom transfer (c) group transfer (d) electron transfer
✅ Answer: (d) electron transfer

Q6. Metal involved in dioxygen transport besides Fe:
(a) Co (b) Zn (c) Mg (d) Cu
✅ Answer: (d) Cu (in Hemocyanin)

Q7. Myoglobin is a:
(d) di-oxygen binding metalloprotein
✅ Answer: (d)

Q8. Oxidation state of iron in met-hemoglobin:
(a) three (b) two (c) four (d) zero
✅ Answer: (a) three (Fe³⁺)

Q9. Metal ions in active site of nitrogenase cofactor:
(a) Fe, Mo (b) Fe, W (c) Fe, Cu (d) Fe, Ni
✅ Answer: (a) Fe, Mo

Q10. Metalloprotein that converts oxygen to water:
(a) catalase (b) cytochrome-c oxidase (c) haloperoxidase (d) hemoglobin
✅ Answer: (b) cytochrome-c oxidase

Q11. In oxyhemoglobin, iron centre is best described as:
(a) high spin Fe(II) (b) high spin Fe(III) (c) low spin Fe(III) (d) low spin Fe(II)
✅ Answer: (d) low spin Fe(II)

Q12. Metal atom in active site of carboxypeptidase enzyme:
(a) cobalt (b) zinc (c) iron (d) magnesium
✅ Answer: (b) zinc

Q13. Coordination geometry of Cu(II) in type-1 copper protein plastocyanin:
(a) square planar (b) tetrahedral (c) octahedral (d) distorted tetrahedral
✅ Answer: (d) distorted tetrahedral

Q14. Ligand system in vitamin B₁₂:
(a) porphyrin (b) corrin (c) phthalocyanin (d) crown ether
✅ Answer: (b) corrin

Q15. Carboxypeptidase contains:
(a) Zn(II) + hydrolysis CO₂ (b) Mg(II) + hydrolysis CO₂ (c) Zn(II) + hydrolysis peptide bonds (d) Mg(II) + hydrolysis peptide bonds
✅ Answer: (c) Zn(II) and hydrolysis peptide bonds

Q16. Bacterial ruberodoxin — Fe atoms / Sulfur bridges / Cysteine ligands:
(a) 4,4,4 (b) 2,2,4 (c) 2,2,2 (d) 1,0,4
✅ Answer: (d) 1, 0, 4

Q17. Well-known naturally occurring organometallic compound:
(a) Vitamin B₁₂ coenzyme (b) chlorophyll (c) cytochrome P-450 (d) myoglobin
✅ Answer: (a) Vitamin B₁₂ coenzyme

Q22. Changes when O₂ binds to hemerythrin:
(a) B and C (b) B and D (c) A and D (d) A and C
✅ Answer: (b) B and D — both Fe atoms oxidized + O₂ binds to both + H-bonded

Q25. Mg²⁺ preferred in photosynthesis by chlorophyll because:
(a) strong spin-orbit coupling (b) weak spin-orbit coupling (c) heavy metal (d) binds strongly with chlorophyll
✅ Answer: (d) it binds strongly with chlorophyll and keeps macrocycle rigid

Q26. Transition metal in vitamin B-12:
(a) Fe (b) Cu (c) Co (d) Mg
✅ Answer: (c) Co

Q27. Metal in carbonic anhydrase:
(a) Mn (b) Fe (c) Zn (d) Cu
✅ Answer: (c) Zn

Q28. Nature chose Zn(II) at hydrolytic enzyme active sites because:
(a) poor Lewis acid (b) no chemically accessible redox states ✅ (c) forms 4+ coord (d) weak with O-donors
✅ Answer: (b) Zn(II) does not have chemically accessible redox states

Q31. Predominant metal in PS-II reaction centre:
(a) Zn (b) Cu (c) Mn (d) Fe
✅ Answer: (c) Mn

Q34. Oxymyoglobin Mb(O₂) and oxyhemoglobin Hb(O₂)₄ are respectively:
(a) paramagnetic and paramagnetic (b) diamagnetic and diamagnetic
(c) paramagnetic and diamagnetic (d) diamagnetic and paramagnetic
✅ Answer: (b) diamagnetic and diamagnetic

Q35. Number of O₂ molecules one hemerythrin molecule can transport:
(a) 4 (b) 8 (c) 3 (d) 4
✅ Answer: (b) 8 (8 subunits × 1 O₂ per subunit)

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🔥 Exam Tips & High-Priority Topics

TOP 10 Most Asked Concepts:

• ✅ Hemoglobin structure, T-state vs R-state, cooperative effect
• ✅ Fe(II) spin state change: deoxy ↔ oxy hemoglobin
• ✅ Table of iron proteins (functions, oxidation states)
• ✅ Cisplatin mechanism + why cis is active, trans inactive
• ✅ Vitamin B₁₂ – corrin ring, Co(III), organometallic
• ✅ Nitrogen fixation – Haber-Bosch vs biological; FeMo protein
• ✅ Ferredoxin types [2Fe-2S], [3Fe-4S], [4Fe-4S] – spin states
• ✅ Zn enzymes: carboxypeptidase + carbonic anhydrase (coordination)
• ✅ Hemocyanin vs Hemerythrin vs Hemoglobin (differences)
• ✅ Photosystem I and II – metals involved (Mn, Mg, Fe, Cu)

❌ Common Mistakes:

• Hemerythrin is NON-heme (not heme iron protein)
• Myoglobin stores O₂ (not transports); Hemoglobin transports
• Vitamin B₁₂ = corrin ring (NOT porphyrin)
• Ruberodoxin has ZERO labile sulfur (inorganic S²⁻)
• Trans-cisplatin = INACTIVE; only cis-isomer is anticancer
• Hemocyanin uses Cu (NOT Fe or Heme)
• Ceruloplasmin deficiency = Wilson's disease (copper overload)

🧠 Memory Tricks:

• "HMC Transport O₂" = Hemoglobin, Myoglobin (vertebrates), Hemerythrin (marine), Hemocyanin (arthropods/molluscs)
• Fe size: High spin (big, 92 pm) → can't fit in ring | Low spin (small, 75 pm) → fits in ring
• Ferredoxin types: 2-2, 3-4, 4-4 (Fe–S numbers)
• Cytochrome electron flow: b → c → a → O₂ (alphabetical!)
• Bohr's Effect: More CO₂ + exercise → lower pH → Hb releases O₂ → muscles get O₂

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