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II. GLOBULAR HEMEPROTEINS Hemeproteins are a group of specialized proteins that contain heme as a tightly bound prosthetic group. (See p. ) The role of the heme group is dictated by the environment created by the three-dimensional structure of the protein. For example, the heme group of a cytochrome functions as an electron carrier that is alternately oxidized and reduced (see p. 76). In contrast, the heme group of the enzyme catalase is part of the active site of the enzyme that catalyzes the breakdown of hydrogen peroxide (see p.

4 . Schematic diagram showing structural changes resulting from oxygenation and deoxygenation of hemoglobin. ] Ionic and hydrogen bonds also occur between the members of the dimer. In contrast, the two dimers are able to move with respect to each other, being held together primarily by polar bonds. 4). [Note: The binding of O2 to the heme iron pulls the iron into the plane of the heme. ] a. T form: The deoxy form of hemoglobin is called the “T,” or taut (tense) form. In the T form, the two αβ dimers interact through a network of ionic bonds and hydrogen bonds that constrain the movement of the polypeptide chains.

For example, hemoglobin can transport H+ and CO2 from the tissues to the lungs, and can carry four molecules of O 2 from the lungs to the cells of the body. Furthermore, the oxygen-binding properties of hemoglobin are regulated by interaction with allosteric effectors (see p. 29). Obtaining O2 from the atmosphere solely by diffusion greatly limits the size of organisms. Circulatory systems overcome this, but transport molecules such as hemoglobin are also required because O2 is only slightly soluble in aqueous solutions such as blood.