Shown below is a Hill plot for binding of oxygen (pO_2 in mmHg) to squid hemocya

Question

Shown below is a Hill plot for binding of oxygen (pO_2 in mmHg) to squid hemocyanin (a copper-containing protein that functions as the oxygen carrier for most mollusks and some arthropods). Does binding of oxygen to squid hemocyanin show positive cooperativity, no cooperativity, or negative cooperativity Explain. Use the Hill plot to determine the following - describe how each of these values was obtained from the Hill plot. the p_so value for the most tightly bound oxygen molecule the p_so value for the most loosely bound oxygen molecule the Hill coefficient for oxygen binding Assuming hemocyanin contains multiple subunits, and that each subunit can bind a single molecule of oxygen, what is the minimum number of subunits in squid hemocyanin? Hill Plot: Binding of Oxygen to Hemocyanin (pO_2 in mmHg): Slopes and intercepts of lines given below.

Explanation / Answer

A)Most hemocyanins bind with oxygen non-cooperatively and are roughly one-fourth as efficient as hemoglobin at transporting oxygen per amount of blood. Hemoglobin binds oxygen cooperatively due to steric conformation changes in the protein complex, which increases hemoglobin's affinity for oxygen when partially oxygenated. In some hemocyanins of horseshoe crabs and some other species of arthropods, cooperative binding is observed, with Hill coefficients of 1.6 - 3.0. Hill coefficients vary depending on species

c)Hemocyanin is made of many individual subunit proteins, each of which contains two copper atoms and can bind one oxygen molecule (O2). Each subunit weighs about 75 kilodaltons (kDa). Subunits may be arranged in dimers or hexamers depending on species; the dimer or hexamer complex is likewise arranged in chains or clusters with weights exceeding 1500 kDa. The subunits are usually homogeneous, or heterogeneous with two variant subunit types. Because of the large size of hemocyanin, it is usually found free-floating in the blood, unlike hemoglobin.[8

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