Metals in Biology: Active Sites, Electron Transfer and Chelation · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

How proteins put metals to work

Science · Chemistry · ages 20-21
Name ______________________   Date ____________

  1. Where does oxygen bind in hemoglobin?

    • Directly to the iron held in heme
    • Directly to a nitrogen atom of the porphyrin ring
    • Directly to the histidine donor instead of the iron

  2. What makes a molecule or ion a ligand in a coordination complex?

    • It is the central metal that accepts electron pairs from surrounding binding partners
    • It binds the metal through a donor atom that supplies an electron pair
    • It only balances the complex charge while remaining outside the coordination sphere

  3. In the carbonic anhydrase site described in the lesson, what does zinc-bound hydroxide attack?

    • A peptide carbonyl, cleaving the enzyme's backbone
    • A histidine side chain, detaching zinc from the protein
    • CO2, leading to bicarbonate formation

  4. Why does a large lead-chelator formation constant not establish a safe treatment by itself?

    • It describes binding equilibrium, leaving distribution and excretion unresolved
    • It describes ligand-exchange speed, leaving the equilibrium binding preference unresolved
    • It describes membrane passage, leaving the formation of metal complexes unresolved

  5. For a mononuclear metal complex with no other magnetic centers, which electronic feature gives paramagnetism?

    • All occupied orbitals contain paired electrons with opposite spins
    • Coordinate bonds form while every electron remains paired
    • The complex has one or more unpaired electrons

  6. Two proteins bind the same metal element. How can their surroundings change its reduction potential?

    • By changing exchange rates alone while leaving the relative energies of oxidation states unchanged
    • By stabilizing oxidized and reduced forms differently
    • By changing reaction speed alone while leaving the equilibrium preference for electron gain unchanged

  7. In one equilibrated mixture, Pb2+ and Fe2+ bind the same ligand L in 1:1 complexes. At the stated pH and medium, K'(Pb) = 10^8 and K'(Fe) = 10^5. Free Pb2+ and Fe2+ concentrations are equal. What is [PbL]/[FeL]?

    • 0.001, so the Pb complex is less abundant than the Fe complex
    • 1, because equal free metal concentrations give equal complex concentrations
    • 1000, so the Pb complex is more abundant than the Fe complex

  8. Two coordination complexes with the same formula must bind and react identically, even when their ligand arrangements differ.

    Circle one:   True   False

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Answer key

For grown-ups. Fold this page away before handing over the rest.

How proteins put metals to work W1-mt_gKhjhRXfMU-s1

  1. Directly to the iron held in heme · The porphyrin ring and histidine coordinate iron. Oxygen binds directly to that iron at the available site.
  2. It binds the metal through a donor atom that supplies an electron pair · A ligand coordinates through a donor atom. A counterion that only balances charge outside the coordination sphere is not a ligand of that metal.
  3. CO2, leading to bicarbonate formation · Zinc favors formation of bound hydroxide from water. That hydroxide attacks CO2 in the bicarbonate-forming reaction.
  4. It describes binding equilibrium, leaving distribution and excretion unresolved · A formation constant describes equilibrium binding under stated conditions. Distribution, excretion and effects on useful ions require separate evidence.
  5. The complex has one or more unpaired electrons · Unpaired electrons give this single-center complex a paramagnetic response. Coordinate bonding or orbital splitting alone does not establish that response.
  6. By stabilizing oxidized and reduced forms differently · Donor atoms, geometry and local charges can change the relative stability of oxidized and reduced forms. This can shift the reduction potential without replacing the metal element.
  7. 1000, so the Pb complex is more abundant than the Fe complex · Both complexes share the same free ligand pool in one equilibrium mixture. Equal free metal concentrations cancel, giving [PbL]/[FeL] = 10^8/10^5 = 1000. This does not prove safe treatment.
  8. False · Different arrangements can produce isomers with different binding or reactivity. Sharing a formula does not establish identical behavior.
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