How a tertiary structure is different from the quaternary structure of a protein? Apart from the peptide bonds, what other types of bonds can you find in α-helix?
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(a) Bond-line structure. (b) Molecular model of heme oxygen complex.
The two identical α chains and two identical β chains are arranged tetrahedrally in a three-dimensional structure (Figure 14.13). These units are held together by hydrophobic interactions, hydrogen bonding, and salt bridges. The four protein subunits of hemoglobin do not behave independently. When one heme molecule binds O2, the conformation of the surrounding polypeptide chain is slightly altered. Changes in conformation at one site caused by a change at a spatially separated site of a protein molecule are called allosteric effects. As a result of allosteric effects, each heme in the other subunits then can bind more easily to additional oxygen molecules. As each oxygen binds, there are further conformational changes in the other protein chains that enhance their binding capability. As a result, once oxygenation occurs at one heme, there is cooperation at all other sites in hemoglobin, which then can carry four oxygen molecules.
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Figure 14.13. Structure of Deoxyhemoglobin
The α and β subunits of hemoglobin interact cooperatively, and when one heme binds O2, each of the others rapidly binds O2.
Amino Acids, Peptides, and Proteins
Robert J. Ouellette, J. David Rawn, in Organic Chemistry (Second Edition), 2018
Hemoglobin
Hemoglobin has a quaternary structure. It consists of two pairs of different proteins, designated the α and β chains. There are 141 and 146 amino acids in the α and β chains of hemoglobin, respectively. As in myoglobin, each subunit is linked covalently to a molecule of heme. Thus, hemoglobin binds four O2 molecules. The two identical α chains and the two identical β chains are arranged tetrahedrally (Figure 29.20). These units are held together by hydrophobic interactions, hydrogen bonding, and ion pairs (salt bridges) between oppositely charged amino acid side chains.
Figure 29.20
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Figure 29.20. Structure of Deoxyhemoglobin.
The α and β subunits of hemoglobin interact cooperatively, and when one heme binds O2, then each of the others rapidly binds O2.
The subunits of hemoglobin do not act independently. When one subunit binds O2, its conformation changes. When a change in conformation at one site of an oligomeric protein is caused by a change in a spatially separated site of the oligomer, the change is called an allosteric effect, and the protein is called an allosteric protein. Hemoglobin is an allosteric protein. When one heme group in hemoglobin binds oxygen, it is easier for successive oxygen molecules to bind at the remaining three sites. Thus, once oxygenation occurs at one heme, there is cooperation at all other sites in hemoglobin.
Handbook of Modern Pharmaceutical Analysis
Yin Luo, ... John Steckert, in Separation Science and Technology, 2011
d Quaternary Structure
By definition, only multimeric proteins have quaternary structure; it refers to the arrangement of two or more folded polypeptide chains. The interactions stabilizing protein quaternary structure may be covalent or noncovalent. Although other forms of covalent side chain interactions have been observed, the most common is the cystinyl disulfide bond.
There are many ways to assess the structure of a folded protein. Many of them are spectroscopic, such as circular dichroism (CD), fluorescence, infrared, and Raman. These methods can be combined with the use of stress to measure the stability of the folded structure by perturbing it with heat or chemicals (Figure 8). High resolution structural information (determined in atomic detail) is often obtained by X-ray crystallography or nuclear magnetic resonance (NMR). These data are routinely deposited in the RSCB (Research Collaboratory for Structural Bioinformatics) protein database for publi
In addition to biologically relevant self-association processes (quaternary structure), proteins are prone to undergo the aberrant process of aggregation which is often linked to folding and misfolding. Aggregation can have profound effects on protein production and the final product. Many methods have been developed to assess and understand these processes and they are discussed in more detail below