468 Chapter 15 Enzyme RegulationThe myoglobin polypeptide chain and the - and -chains of hemoglobin are com-posed of 8 -helical segments denoted by the letters A through H. The short, un-ordered regions that connect the helices are named for the segments they connect,as in the AB region or the EF region. In an amino acid numbering system unique toglobin chains, successive residues in the helices are numbered, such as the histidineat position 8 in the F helix, known as His F8.The tetrameric nature of Hb is crucial to its biological function: When a moleculeof O2 binds to a heme in Hb, the heme Fe ion is drawn into the plane of the porphyrin ring.This slight movement sets off a chain of conformational events that are transmitted to adja-cent subunits, dramatically enhancing the affinity of their heme groups for O2. That is, thebinding of O2 to one heme of Hb makes it easier for the Hb molecule to bind ad-ditional equivalents of O2. Hemoglobin is a marvelously constructed molecular ma-chine. Let us dissect its mechanism, beginning with its monomeric counterpart, themyoglobin molecule.Myoglobin Is an Oxygen-Storage ProteinMyoglobin is the oxygen-storage protein of muscle. The muscles of diving mammalssuch as seals and whales are especially rich in this protein, which serves as a storefor O2 during the animal’s prolonged periods underwater. Myoglobin is abundantin skeletal and cardiac muscle of nondiving animals as well. Myoglobin is the causeof the characteristic red color of muscle.VenouspO2Myoglobin0 20 40 60 80 100 120Partial pressure of oxygen (pO2, torr)10080604020Percent O2 saturationWorkingmuscleRestingmuscleHemoglobinArterial pO20FIGURE 15.20 O2-binding curves for hemoglobin andmyoglobin.Myoglobin (Mb)Hemoglobin (Hb)22 11FIGURE 15.21 The myoglobin (pdb id 2MM1) and hemoglobin (pdb id 2HHB) molecules.Fe2+–OOCCH2CCH3CHCC CCCOO–H2CCH2CCH3C NHHCC NCC H2C CCCHN CCCCNCHHCCH3 CHCH2CH3CCH3CHCC CCCCH3C NHCC NCCCCN CCCCNCHHCCH3CH3CH2H–OOCCH2CH2COO–H2CCH2CHCH2H2C CHProtoporphyrin IX Heme(Fe-protoporphyrin IX)FIGURE 15.22 Heme is formed when protoporphyrin IXbinds Fe2.
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