Protein Tertiary and Quaternary Structures: -Protein Quaternary Structures Range from Simple Dimers to Large Complexes

Many proteins have multiple polypeptide subunits. The association of polypeptide chains can serve a variety of functions. Many multisubunit proteins have regulatory roles; the binding of small molecules may affect the interaction between subunits, causing large changes in the protein’s activity in response to small changes in the concentration of substrate or regulatory molecules (Chapter 6). In other cases, separate subunits can take on separate but related functions, such as catalysis and regulation. Some associations, such as the fibrous proteins considered earlier in this chapter and the coat proteins of viruses, serve primarily structural roles. Some very large protein assemblies are the site of complex, multistep re actions. One example is the ribosome, site of protein synthesis, which incorporates dozens of protein sub units along with a number of RNA molecules. A multisubunit protein is also referred to as a multimer. Multimeric proteins can have from two to hundreds of subunits. A multimer with just a few subunits is often called an oligomer. If a multimer is composed of a number of nonidentical subunits, the overall structure of the protein can be asymmetric and quite com plicated. However, most multimers have identical sub units or repeating groups of nonidentical subunits, usually in symmetric arrangements. As noted in Chapter 3, the repeating structural unit in such a multimeric protein, whether it is a single subunit or a group of sub units, is called a protomer. The first oligomeric protein for which the three-dimensional structure was determined was hemoglobin (Mr 64,500), which contains four polypeptide chains and four heme prosthetic groups, in which the iron atoms are in the ferrous (Fe²⁺) state (Fig. 4–17). The protein portion, called globin, consists of two α chains (141 residues each) and two β chains (146 residues each).

Note that in this case α and β do not refer to secondary structures. Because hemoglobin is four times as large as myoglobin, much more time and effort were re quired to solve its three-dimensional structure by x-ray analysis, finally achieved by Max Perutz, John Kendrew, and their colleagues in 1959. The subunits of hemoglobin are arranged in symmetric pairs (Fig. 4–23), each pair having one α and one β subunit. Hemoglobin can therefore be described either as a tetramer or as a dimer of αβ protomers.

Identical subunits of multimeric proteins are generally arranged in one or a limited set of symmetric pat terns. A description of the structure of these proteins requires an understanding of conventions used to de fine symmetries. Oligomers can have either rotational symmetry or helical symmetry; that is, individual subunits can be superimposed on others (brought to co incidence) by rotation about one or more rotational axes, or by a helical rotation. In proteins with rotational symmetry, the subunits pack about the rotational axes to form closed structures. Proteins with helical symmetry tend to form structures that are more open-ended, with subunits added in a spiraling array. There are several forms of rotational symmetry. The simplest is cyclic symmetry, involving rotation about a single axis (Fig. 4–24a). If subunits can be superimposed by rotation about a single axis, the protein has a symmetry defined by convention as Cn (C for cyclic, n for the number of subunits related by the axis). The axis itself is described as an n-fold rotational axis. The αβ protomers of hemoglobin (Fig. 4–23) are related by C2 symmetry. A somewhat more complicated rotational symmetry is dihedral symmetry, in which a twofold rotational axis intersects an n-fold axis at right angles. The symmetry is defined as Dn (Fig. 4–24b). A protein with dihedral symmetry has 2n protomers. Proteins with cyclic or dihedral symmetry are particularly common. More complex rotational symmetries are possible, but only a few are regularly encountered. One example is icosahedral symmetry. An icosahedron is a regular 12-cornered polyhedron having 20 equilateral triangular faces (Fig. 4–24c). Each face can be brought to coincidence with another by rotation about one or more of three rotational axes. This is a common structure in virus coats, or capsids. The human poliovirus has an icosahedral capsid (Fig. 4–25a). Each triangular face is made up of three protomers, each protomer containing single copies of four different polypep tide chains, three of which are accessible at the outer surface. Sixty protomers form the 20 faces of the icosahedral shell enclosing the genetic material (RNA). The other major type of symmetry found in oligomers, helical symmetry, also occurs in capsids. To bacco mosaic virus is a right-handed helical filament made up of 2,130 identical subunits (Fig. 4–25b). This cylindrical structure encloses the viral RNA. Proteins with subunits arranged in helical filaments can also form long, fibrous structures such as the actin filaments of muscle (see Fig. 5–30).

FIGURE 4–23 Quaternary structure of deoxyhemoglobin. (PDB ID 2HHB) X-ray diffraction analysis of deoxyhemoglobin (hemoglobin without oxygen molecules bound to the heme groups) shows how the four polypeptide subunits are packed together. (a) A ribbon representation. (b) A space-filling model. The subunits are shown in gray and light blue; the β subunits in pink and dark blue. Note that the heme groups (red) are relatively far apart.

FIGURE 4–24 Rotational symmetry in proteins. (a) In cyclic symmetry, subunits are related by rotation about a single n-fold axis, where n is the number of subunits so related. The axes are shown as black lines; the numbers are values of n. Only two of many possible Cn arrangements are shown. (b) In dihedral symmetry, all subunits can be related by rotation about one or both of two axes, one of which is twofold. D2 symmetry is most common. (c) Icosahedral symmetry. Relating all 20 triangular faces of an icosahedron requires rotation about one or more of three separate rotational axes: twofold, threefold, and fivefold. An end-on view of each of these axes is shown at the right.

FIGURE 4–25 Viral capsids. (a) Poliovirus (derived from PDB ID 2PLV). The coat proteins of poliovirus assemble into an icosahedron 300 Å in diameter. Icosahedral symmetry is a type of rotational symmetry (see Fig. 4–24c). On the left is a surface contour image of the poliovirus capsid. In the image on the right, lines have been super imposed to show the axes of symmetry. (b) Tobacco mosaic virus (de rived from PDB ID 1VTM). This rod-shaped virus (as shown in the electron micrograph) is 3,000 Å long and 180 Å in diameter; it has helical symmetry.

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مؤسّسة الوافي تُهدي مكتبةَ مسجد الكوفة نسخةً من موسوعة فتوى الدفاع الكفائي

المجمع العلمي يقيم برنامج (سراج المنتظرين) في محافظة المثنى

قسم الشؤون الفكرية ينظم ندوة فكرية عن أبعاد نهضة الإمام الصادق (عليه السلام)

العتبة العباسية المقدسة تناقش تكثيف جهودها الإغاثية لدعم الشعبين الإيراني واللبناني

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دواء جديد يحمي القلب ويعزز علاج السرطان في آن واحد

سر غذائي بسيط لحماية القلب مع التقدم في العمر

علامات تشير إلى اختلال عمل الغدة الدرقية

السمنة واللقاحات.. دراسة تكشف تأثيراً سلبياً

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الآخبار التكنلوجية

البعوض في آيسلندا إشارة مبكرة على تحولات بيئية في القطب الشمالي

صاروخ يطلق قمرا اصطناعيا في مدار خاطئ

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مواضيع ذات صلة

Complementary Interactions between Proteins and Ligands: The Immune System and Immunoglobulins: -The Immune Response Features a Specialized Array of Cells and Proteins

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Sickle-Cell Anemia Is a Molecular Disease of Hemoglobin

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Oxygen Binding to Hemoglobin Is Regulated by 2,3-Bisphosphoglycerate

Protein Denaturation and Folding: - Loss of Protein Structure Results in Loss of Function

Protein Tertiary and Quaternary Structures: -There Are Limits to the Size of Proteins

Protein Tertiary and Quaternary Structures: -Protein Quaternary Structures Range from Simple Dimers to Large Complexes

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Protein-Ligand Interactions Can Be Described Quantitatively

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Myoglobin Has a Single Binding Site for Oxygen

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Oxygen Can Be Bound to a Heme Prosthetic Group

Protein Denaturation and Folding:- Some Proteins Undergo Assisted Folding

Protein Denaturation and Folding:- Polypeptides Fold Rapidly by a Stepwise Process

Protein Denaturation and Folding: - Amino Acid Sequence Determines Tertiary Structure