8885ac05157-1898/12/038:55 AM Page159mac78mac78:385 Protein Function CH CHI Fe- ch CH CH CH FIGURE 5-1 Heme. The heme group is present in myoglobin, hemo- role rings linked by methene bridges, with substitutions at one or more globin, and many other proteins, designated heme proteins. Heme of the positions denoted X. (b, c) Two representations of heme(De- onsists of a complex organic ring structure, protoporphyrin IX, to rived from PDB ID 1CCR The iron atom of heme has six coordina- hich is bound an iron atom in its ferrous(Fe2+)state. (a) Porphyrins tion bonds: four in the plane of, and bonded to, the flat porphyrin ring of which protoporphyrin IX is only one example, consist of four pyr- stem,and(d) two perpendicular to it. other is the binding site for molecular oxygen(O2)(Fig Myoglobin Has a Single Binding Site for Oxygen 5-2). When oxygen binds, the electronic properties of heme iron change; this accounts for the change in color Myoglobin(Mr 16, 700; abbreviated Mb) is a relatively from the dark purple of oxygen-depleted venous blood simple oxygen-binding protein found in almost all mam- to the bright red of oxygen-rich arterial blood. Some mals, primarily in muscle tissue. As a transport protein, it facilitates oxygen diffusion in muscle. Myoglobin is tric oxide (NO), coordinate to heme iron with greater particularly abundant in the muscles of diving mammals affinity than does O, When a molecule of co is bound such as seals and whales, where it also has an oxygen- to heme, O2 is excluded, which is why Co is highly toxic storage function for prolonged excursions undersea to aerobic organisms(a topic explored later, in Box Proteins very similar to myoglobin are widely distrib- 1). By surrounding and sequestering heme, oxygen uted, occurring even in some single-celled organisms binding proteins regulate the access of CO and other Myoglobin is a single polypeptide of 153 amino acid small molecules to the heme iron residues with one molecule of heme. It is typical of the family of proteins called globins, all of which have sim- ilar primary and tertiary structures. The polypeptide is made up of eight a-helical segments connected by bends Edge view (Fig. 5-3). About 78% of the amino acid residues in the protein are found in these a helices. Any detailed discussion of protein function in- evitably involves protein structure. To facilitate our treatment of myoglobin, we first introduce some struc tural conventions peculiar to globins. As seen in Figure 5-3, the helical segments are named A through H. An individual amino acid residue is designated either by its Histidine Plane of position in the amino acid sequence or by its location residue porphyrin within the sequence of a particular a-helical segment ring system For example, the His residue coordinated to the heme FIGURE 5-2 The heme group viewed from the side. This view shows in myoglobin, His(the 93rd amino acid residue from the two coordination bonds to Fe2+ perpendicular to the porphyri the amino-terminal end of the myoglobin polypeptide ring system. One of these two bonds is occupied by a His residu sequence), is also called His F8(the Sth residue in a sometimes called the proximal His. The other bond is the binding site helix F). The bends in the structure are designated AB for oxygen. The remaining four coordination bonds are in the plane CD, EF, FG, and so forth, reflecting the a-helical seg of, and bonded to, the flat porphyrin ring system ments they connect.other is the binding site for molecular oxygen (O2) (Fig. 5–2). When oxygen binds, the electronic properties of heme iron change; this accounts for the change in color from the dark purple of oxygen-depleted venous blood to the bright red of oxygen-rich arterial blood. Some small molecules, such as carbon monoxide (CO) and ni￾tric oxide (NO), coordinate to heme iron with greater affinity than does O2. When a molecule of CO is bound to heme, O2 is excluded, which is why CO is highly toxic to aerobic organisms (a topic explored later, in Box 5–1). By surrounding and sequestering heme, oxygen￾binding proteins regulate the access of CO and other small molecules to the heme iron. Myoglobin Has a Single Binding Site for Oxygen Myoglobin (Mr 16,700; abbreviated Mb) is a relatively simple oxygen-binding protein found in almost all mam￾mals, primarily in muscle tissue. As a transport protein, it facilitates oxygen diffusion in muscle. Myoglobin is particularly abundant in the muscles of diving mammals such as seals and whales, where it also has an oxygen￾storage function for prolonged excursions undersea. Proteins very similar to myoglobin are widely distrib￾uted, occurring even in some single-celled organisms. Myoglobin is a single polypeptide of 153 amino acid residues with one molecule of heme. It is typical of the family of proteins called globins, all of which have sim￾ilar primary and tertiary structures. The polypeptide is made up of eight
-helical segments connected by bends (Fig. 5–3). About 78% of the amino acid residues in the protein are found in these
helices. Any detailed discussion of protein function in￾evitably involves protein structure. To facilitate our treatment of myoglobin, we first introduce some struc￾tural conventions peculiar to globins. As seen in Figure 5–3, the helical segments are named A through H. An individual amino acid residue is designated either by its position in the amino acid sequence or by its location within the sequence of a particular
-helical segment. For example, the His residue coordinated to the heme in myoglobin, His93 (the 93rd amino acid residue from the amino-terminal end of the myoglobin polypeptide sequence), is also called His F8 (the 8th residue in
helix F). The bends in the structure are designated AB, CD, EF, FG, and so forth, reflecting the
-helical seg￾ments they connect. Chapter 5 Protein Function 159 O C O O Fe
CH3 CH N
CH2 CH2 CH2 CH2 CH2 C H3 C H3 CH3 CH CH CH CH CH O C C C C C C C C C C C C C C C N N N CH2 C (b) C (a) NH X N HN N X X X X X X X (c) (d) Fe FIGURE 5–1 Heme. The heme group is present in myoglobin, hemo￾globin, and many other proteins, designated heme proteins. Heme consists of a complex organic ring structure, protoporphyrin IX, to which is bound an iron atom in its ferrous (Fe2
) state. (a) Porphyrins, of which protoporphyrin IX is only one example, consist of four pyr￾role rings linked by methene bridges, with substitutions at one or more of the positions denoted X. (b, c) Two representations of heme. (De￾rived from PDB ID 1CCR.) The iron atom of heme has six coordina￾tion bonds: four in the plane of, and bonded to, the flat porphyrin ring system, and (d) two perpendicular to it. FIGURE 5–2 The heme group viewed from the side. This view shows the two coordination bonds to Fe2
perpendicular to the porphyrin ring system. One of these two bonds is occupied by a His residue, sometimes called the proximal His. The other bond is the binding site for oxygen. The remaining four coordination bonds are in the plane of, and bonded to, the flat porphyrin ring system. HN CH2 C CH Edge view ring system residue C N Fe O2 Histidine Plane of porphyrin H 8885d_c05_157-189 8/12/03 8:55 AM Page 159 mac78 mac78:385_REB: