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But how did proteins make physiological reactions possible? Hemoglobin, the oxygen carrier in blood, for instance, performs one of the simplest and yet most vital reactions in physiology. When exposed to high levels of oxygen, hemoglobin binds oxygen. Relocated to a site with low oxygen levels, it willingly releases the bound oxygen. This property allows hemoglobin to shuttle oxygen from the lung to the heart and the brain. But what feature of hemoglobin allows it to act as such an effective molecular shuttle? The answer lies in the structure of the molecule. Hemoglobin A, the most intensively studied version of the molecule, is shaped like a four-leaf clover. Two of its “leaves” are formed by a protein called alpha-globin; the other two are created by a related protein, beta-globin.II Each of these leaves clasps, at its center, an iron-containing chemical named heme that can bind oxygen—a reaction distantly akin to a controlled form of rusting. Once all the oxygen molecules have been loaded onto heme, the four leaves of hemoglobin tighten around the oxygen like a saddle clasp. When unloading oxygen, the same saddle-clasp mechanism loosens. The unbinding of one molecule of oxygen coordinately relaxes all the other clasps, like the crucial pin-piece pulled out from a child’s puzzle. The four leaves of the clover now twist open, and hemoglobin yields its cargo of oxygen. The controlled binding and unbinding of iron and oxygen—the cyclical rusting and unrusting of blood—allows effective oxygen delivery into tissues. Hemoglobin allows blood to carry seventyfold more oxygen than what could be dissolved in liquid blood alone. The body plans of vertebrates depend on this property: if hemoglobin’s capacity to deliver oxygen to distant sites was disrupted, our bodies would be forced to be small and cold. We might wake up and find ourselves transformed into insects.
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