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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2019 · Paper I

Q3(b). Active site of oxyhaemerythrin

  • 10 marks
  • Answered
Question

Describe the active site ligand environment around the coordination sphere of each iron in hemerythrin in the oxygenated form.

Answer in brief

Two high-spin Fe(III) ions are bridged by one oxo ion and two carboxylates (Glu, Asp). Fe1 also has three histidines (N₃O₃, octahedral). Fe2 has two histidines and the bound hydroperoxide HO₂⁻ (N₂O₄, octahedral).

Key points

  • haemerythrin carries O₂ in sipunculid and brachiopod marine worms. It is a non-haem iron protein; each subunit (about 14 000 Da) has two Fe atoms and binds one O₂.
  • one μ-oxo (O²-) and two carboxylates (from a glutamate and an aspartate) each bridging through both O atoms.
  • three terminal histidine imidazole N atoms + μ-O + two carboxylate O: six-coordinate, octahedral N₃O₃. It does not bind O₂.
  • two histidine N + μ-O + two carboxylate O + the terminal hydroperoxide (HO₂⁻): octahedral N₂O₄. In deoxyhaemerythrin this Fe is five-coordinate; its vacant site takes up O₂.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. Protein: haemerythrin carries O₂ in sipunculid and brachiopod marine worms. It is a non-haem iron protein; each subunit (about 14 000 Da) has two Fe atoms and binds one O₂.
  2. Bridges shared by both Fe: one μ-oxo (O²-) and two carboxylates (from a glutamate and an aspartate) each bridging through both O atoms.
  3. Fe1: three terminal histidine imidazole N atoms + μ-O + two carboxylate O: six-coordinate, octahedral N₃O₃. It does not bind O₂.
  4. Fe2: two histidine N + μ-O + two carboxylate O + the terminal hydroperoxide (HO₂⁻): octahedral N₂O₄. In deoxyhaemerythrin this Fe is five-coordinate; its vacant site takes up O₂.
  5. Binding step: Fe^(II)Fe^(II)(μ-OH) + O₂ → Fe^(III)Fe^(III)(μ-O)(OOH). Each Fe(II) gives one electron (O₂ becomes peroxide), and the proton of the bridging hydroxide moves to the peroxide. The OOH group stays hydrogen-bonded to the μ-oxo bridge, which holds O₂ at one Fe only.
  6. Electronic state: both Fe(III) are high-spin d⁵, antiferromagnetically coupled through the oxo bridge. Deoxy is colourless; oxy is violet-pink from peroxide → Fe(III) charge transfer.

Application. the same Fe₂(μ-O)(μ-carboxylate) core appears in methane monooxygenase, ribonucleotide reductase and purple acid phosphatase, where it activates O₂ instead of only carrying it.

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Model answer to Q3(b), 2019 Paper I: solution page 13
Solution page 13 of 40
Read the whole 2019 solution2019 question paper

All parts of Question 3