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

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

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

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

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

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

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

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