The Hill coefficient (n) derived from the Hill plot of human hemoglobin oxygen binding is approximately 2.8. The most appropriate interpretation of this value is:

• A All four subunits bind oxygen simultaneously in a single concerted step
• B Binding of one oxygen molecule decreases the affinity of remaining sites, indicating negative cooperativity
• C Hemoglobin behaves as a non-cooperative monomeric oxygen-binding protein like myoglobin
• D Oxygen binding shows positive cooperativity, and at least three subunits behave as interacting binding units ✓

Explanation

The Hill coefficient quantifies cooperativity: n greater than 1 indicates positive cooperativity, n equal to 1 indicates none, and n less than 1 indicates negative cooperativity. A value of 2.8 implies that binding of oxygen at one site increases affinity at others, and that at least three sites act concertedly. If all four bound simultaneously in a fully concerted model, n would equal 4, the theoretical maximum.

Reference: Lehninger Principles of Biochemistry, 7th ed.

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