Fe-Metalloenzymes

Question 1: Explain the electronic configuration, spin state, and coordination geometry of Fe in deoxyhemoglobin and oxyhemoglobin. How does oxygen binding alter the crystal field splitting and spin state of Fe? Discuss the structural basis of cooperativity in hemoglobin.

Question 2: Iron-containing enzymes often exist in multiple oxidation and spin states. Explain how Mössbauer spectroscopy can distinguish between Fe(II) and Fe(III) in metalloenzymes. What information can EPR spectroscopy provide for high-spin and low-spin iron centers? Why is Fe(II) often EPR silent?

Question 3: Describe the catalytic cycle of cytochrome P450 enzymes. Include the oxidation states of iron involved and explain the role of Compound I. Why is the Fe(IV)=O species considered a powerful oxidizing intermediate?

Question 4: Using crystal field theory and ligand field theory, explain the electronic structure of Fe in [2Fe–2S] and [4Fe–4S] clusters. How do bridging sulfide ligands influence redox potentials and electron transfer efficiency?

Question 5: Non-heme iron enzymes catalyze difficult oxidation reactions. Explain the role of Fe(II) in activating molecular oxygen. Describe the formation and role of Fe(IV)=O intermediate in these reactions.

Question 6: Explain how protein environment modulates the redox potential of iron centers in metalloenzymes. Discuss the role of ligand type, coordination number, and geometry in controlling Fe(III)/Fe(II) redox potential.

Question 7: Consider Fe(III) coordinated by the following ligands in metalloenzymes:

Histidine
Cysteine
Water
Cyanide
Arrange these ligands in order of increasing ligand field strength and explain how this affects the spin state of iron.

Question 8: Compare hemoglobin and myoglobin in terms of:

Coordination geometry
Spin state
Oxygen affinity
Biological function
Explain why myoglobin has higher oxygen affinity than hemoglobin.

Question 9: Catalase decomposes hydrogen peroxide efficiently. Explain the mechanism of catalase involving Fe(III)/Fe(IV) redox cycle. Why is iron particularly suited for this redox process?

Question 10: Ferritin stores iron in biological systems. Describe the structure of ferritin and explain how iron is stored as Fe(III). Why is iron stored in ferritin rather than as free Fe(II)?

Question 11: Discuss the role of π-backbonding between iron and ligands such as CO and O₂ in metalloenzymes. How does π-backbonding stabilize oxygen binding in hemoglobin?

MCQ

Q1. The oxidation state and spin state of iron in deoxyhemoglobin is

A. Fe(II), low spin
B. Fe(III), low spin
C. Fe(II), high spin
D. Fe(III), high spin

Correct Answer: C. Fe(II), high spin

Explanation:

In deoxyhemoglobin:

Iron oxidation state = Fe²⁺
Ligand field is weak (water or vacant site)
Configuration = d⁶ high spin

Electronic configuration:

t2g4eg2

High spin results in larger ionic radius and Fe lies slightly out of porphyrin plane.

Q2. Upon oxygen binding in hemoglobin, iron undergoes:

A. Oxidation from Fe(II) to Fe(III)
B. Reduction from Fe(III) to Fe(II)
C. Spin state change from high spin to low spin
D. No change

Correct Answer: C. Spin state change from high spin to low spin

Explanation:

O₂ is a strong-field ligand.

Effect:

increases crystal field splitting (Δ₀)
causes pairing of electrons

High spin→Low spin

Iron moves into porphyrin plane → triggers cooperativity.

Q3. Why is Fe(II) often EPR silent in metalloenzymes?

A. It has no unpaired electrons
B. It has integer spin state
C. It has half-filled orbitals
D. It has fully filled orbitals

Correct Answer: B. It has integer spin state

Explanation:

EPR detects species with half-integer spin.

Fe²⁺ high spin:

d6, S=2

Integer spin → fast relaxation → EPR silent.

Q4. The active oxidizing species in cytochrome P450 is:

A. Fe(III)
B. Fe(II)
C. Fe(IV)=O
D. Fe(I)

Correct Answer: C. Fe(IV)=O

Explanation:

Compound I structure:

FeIV=O + porphyrin radical

This species:

has very high oxidation potential
performs C–H activation

Q5. Iron in ferritin is stored as:

A. Fe(II)
B. Fe(III) oxide-hydroxide
C. Fe(0)
D. Fe(I)

Correct Answer: B. Fe(III) oxide-hydroxide

Explanation:

Ferritin stores iron as:

FeO(OH)FeO(OH)FeO(OH)

This prevents:

toxicity
Fenton reaction
Q6. The primary function of iron–sulfur proteins is:

A. Oxygen transport
B. Electron transfer
C. DNA synthesis
D. Protein synthesis

Correct Answer: B. Electron transfer

Explanation:

Fe–S clusters undergo:

Fe2+↔Fe3+; without structural change → ideal electron carriers.

Q7. Which ligand provides strongest field to iron?

A. H₂O
B. Histidine
C. Cysteine
D. CN⁻

Correct Answer: D. CN⁻

Explanation:

Spectrochemical series:

CN−>Cys−>His>H2O: CN⁻ is strong π-acceptor → strong field ligand.

Q8. Catalase converts hydrogen peroxide into

A. O₂ and H₂O
B. OH radicals
C. H₂ and O₂
D. FeO and H₂O

Correct Answer: A. O₂ and H₂O

Reaction:

2H2O2→2H2O+O2: Iron cycles between Fe(III) and Fe(IV).

Q9. The geometry around iron in hemoglobin is:

A. Tetrahedral
B. Square planar
C. Octahedral
D. Trigonal planar

Correct Answer: C. Octahedral

Explanation:

Coordination:

4 N from porphyrin
1 His
1 O₂ or H₂O
Total coordination number = 6

Q10. Iron is preferred in metalloenzymes because:

A. It is rare
B. It has only one oxidation state
C. It can exist in multiple oxidation states
D. It is inert

Correct Answer: C. It can exist in multiple oxidation states

Explanation:

Iron common states:

Fe2+,Fe3+,Fe4+

This enables redox catalysis.

Q11 The electronic configuration of Fe(III) in high spin octahedral field is:

A. t₂g⁵ eg⁰
B. t₂g³ eg²
C. t₂g⁴ eg¹
D. t₂g⁶ eg⁰

Correct Answer: B. t₂g³ eg²

Explanation:

Fe³⁺ = d⁵

High spin:

t2g3eg2

5 unpaired electrons.

Q12 Which interaction stabilizes Fe–O₂ bonding in hemoglobin?

A. σ donation only
B. π backbonding only
C. Both σ donation and π backbonding
D. No bonding interaction

Correct Answer: C. Both σ donation and π backbonding

Explanation:

O₂ bonding involves:

σ donation: O₂ → Fe
π backbonding: Fe → O₂

This stabilizes oxygen binding.

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