Hemoglobin binds O₂ reversibly while free Fe²⁺ in solution is rapidly and irreversibly oxidized to Fe³⁺ by O₂. What feature of the protein environment enables reversible binding?
AThe protein prevents any interaction between Fe²⁺ and O₂
BThe proximal histidine, distal histidine pocket, and hydrophobic environment work together — the proximal His tunes the Fe redox potential, the distal His stabilizes bound O₂ through hydrogen bonding, and the hydrophobic pocket excludes water that would facilitate irreversible oxidation
CThe porphyrin ring makes iron completely inert to oxidation
DHemoglobin contains Fe³⁺, not Fe²⁺, which binds O₂ without risk of further oxidation
In free solution, Fe²⁺ + O₂ readily forms Fe³⁺ + O₂⁻ (superoxide) because water stabilizes the oxidized products. In hemoglobin, the protein architecture prevents this. The iron sits in a porphyrin (a macrocyclic ligand providing 4 N donors) with a proximal histidine as the fifth ligand. O₂ binds at the sixth position in a bent geometry, hydrogen-bonding to the distal histidine, which stabilizes the Fe-O₂ adduct in an Fe²⁺-O₂ (or Fe³⁺-O₂⁻) state without full electron transfer. The hydrophobic pocket excludes water molecules that would protonate coordinated superoxide and drive irreversible oxidation. This is nature's solution to a fundamental coordination chemistry problem.
Question 2 True / False
Zinc in carbonic anhydrase activates a water molecule for nucleophilic attack on CO₂ by lowering the pKa of the coordinated water from ~15.7 to ~7.
TTrue
FFalse
Answer: True
Zn²⁺ in carbonic anhydrase is coordinated by three histidine residues and one water molecule in a tetrahedral geometry. The Lewis acidity of Zn²⁺ polarizes the coordinated water, dramatically lowering its pKa from the normal value of ~15.7 (for free water) to ~7. At physiological pH, the coordinated water is deprotonated to form a zinc-hydroxide, Zn-OH⁻, which is a potent nucleophile that attacks CO₂ to form bicarbonate. This is one of the fastest enzyme reactions known (~10⁶ turnovers per second), and it depends entirely on the Lewis acid properties of the zinc center.
Question 3 True / False
Iron-sulfur clusters in electron transfer proteins use the variable oxidation states of iron (Fe²⁺/Fe³⁺) to shuttle electrons one at a time through metabolic pathways.
TTrue
FFalse
Answer: True
Iron-sulfur clusters ([2Fe-2S], [3Fe-4S], [4Fe-4S]) are among the most ancient and ubiquitous metallocofactors in biology. Each cluster contains iron atoms bridged by sulfide ions, coordinated by cysteine (or sometimes histidine) residues from the protein. The irons can individually cycle between Fe²⁺ and Fe³⁺, and the cluster's redox potential is tuned by the protein environment (hydrogen bonds, dielectric constant, nearby charges). This tunability allows evolution to place iron-sulfur clusters at precise positions in the electron transfer chain, each with the correct redox potential to pass electrons downhill to the next carrier.
Question 4 Short Answer
Explain why nature predominantly uses first-row transition metals (Fe, Cu, Zn, Mn, Co) in metalloenzymes rather than second- and third-row metals (Ru, Pd, Pt), despite the latter often being better catalysts in synthetic chemistry.
Think about your answer, then reveal below.
Model answer: Several factors favor first-row metals in biology: (1) Bioavailability — first-row transition metals are far more abundant in Earth's crust and oceans than second- and third-row metals, so organisms evolved to use what was available. (2) Lability — first-row metals generally form more labile complexes, allowing the rapid ligand exchange needed for catalytic turnover. Second- and third-row metals form kinetically inert complexes that would be too slow for biological catalysis. (3) Redox accessibility — the biologically relevant redox potentials (roughly −0.5 to +0.8 V) are well-matched to first-row metal couples (Fe²⁺/³⁺, Cu⁺/²⁺, Mn²⁺/³⁺). (4) Kinetic selectivity — the faster ligand exchange of first-row metals allows the protein to control reactivity through selective binding and release.
The exception that proves the rule is molybdenum, a second-row metal used in nitrogenase and other enzymes. Mo is unusually bioavailable (relatively soluble as molybdate MoO₄²⁻) and has unique chemical properties (multiple accessible oxidation states from +2 to +6, ability to form multiple bonds to O and S) that first-row metals cannot replicate for nitrogen fixation.