Quaternary structure hemoglobin

Figure 4.3 Myoglobin and hemoglobin quaternary structures. (Reprinted from Figure 4.2 of Cowan, J. A. Inorganic Biochemistry, An Introduction, 2nd ed., Wiley-VCH, New York, 1997. Copyright 1997, Wiley-VCH.)...

Figure 7.12 The change in hemoglobin quaternary structure during oxygenation. 

Proteins may also Have a quaternary structure, in which neighboring polypeptide units stack together in a specific arrangement. The hemoglobin molecule, for example, has a quaternary structure of four polypeptide units, one of which is shown in Fig. 19.20. 

THE ALLOSTERIC PROPERTIES OF HEMOGLOBINS RESULT FROM THEIR QUATERNARY STRUCTURES... 

The properties of individual hemoglobins are consequences of their quaternary as well as of their secondary and tertiary structures. The quaternary structure of hemoglobin confers striking additional properties, absent from monomeric myoglobin, which adapts it to its unique biologic roles. The allosteric (Gk alios other, steros space ) properties of hemoglobin provide, in addition, a model for understanding other allosteric proteins (see Chapter 11). 

Figure 6-5. Developmental pattern of the quaternary structure of fetal and newborn hemoglobins. (Reproduced, with permission, from Ganong WF Review of Medical Physiology, 20th ed. McGraw-Flill, 2001.)...

Most proteins contain more than one polypeptide chain. The manner in which these chains associate determines quaternary structure. Binding involves the same types of noncovalent forces mentioned for tertiary structure van der Waals forces, hydrophobic and hydrophilic attractions, and hydrogen bonding. However, the interactions are now interchain rather than infrachain (tertiary structure determination). The quaternary structure of hemoglobin (four almost identical subunits) will be discussed in Chapter 4, that of superoxide dismutase (two identical subunits) will be discussed in Chapter 5, and that of nitrogenase (multiple dissimilar subunits) will be discussed in Chapter 6. 

Quaternary structure the four separate chains Of hemoglobin assembled S3 into an oligomeric protein... 

All proteins have at least three levels of structure primary, secondary, and tertiary. Proteins with more than one polypeptide chain— hemoglobin and nitrogenase are examples—also possess quaternary structure. The primary. 

Hemoglobin provides an example of a protein possessing quaternary structure... 

Quaternary structure the tour separate chains of hemoglobin assembled into an oligomeric proteir ... 

Using the 241-cm value for the Ni-histidine frequency in Mb, the T- R shift from the Hb- Mb comparison is 5 cm. This value is consistent with the increase observed in a comparison of the corresponding Fe proteins and with the T R shifts based on other Fe hemoglobins (3-8 cm ) (31). The similarity of the increases observed in the metal-histi3 e frequencies for the nickel and iron hemoglobins indicates that the effect of quaternary structure on the Ni-histidine bond is similar to the Fe case. Also, the effect of the protein conformation change is virturally independent of the particular metal in the porphyrin core. 

The term quaternary structure is employed to describe the overall shape of groups of chains of proteins, or other molecular arrangements. For instance, hemoglobin is composed of four distinct but different myoglobin units, each with its own tertiary structure that comes together giving the hemoglobin structure. Silk, spiderwebs, and wool, already described briefly, possess their special properties because of the quaternary structure of their particular structural proteins. 

The quaternary structure of hemoglobin features strong interactions between unlike subunits. The i/3i interface (and its a2 2 counterpart) involves more than 30 residues, and its interaction is sufficiently strong that although mild treatment of hemoglobin with urea tends to cause the tetramer to disassemble into a/3 dimers, these dimers remain intact. The a (and a2/3i) interface involves 19 residues (Fig. 5-8). Hydrophobic interactions predominate at the interfaces, but there are also many hydrogen bonds and a few ion pairs (sometimes referred to as salt bridges), whose importance is discussed below. 

Many proteins consist of a single polypeptide chain, and are defined as monomeric proteins. However, others may consist of two or more polypeptide chains that may be structurally identical or totally unrelated. The arrangement of these polypeptide subunits is called the quaternary structure of the protein. [Note If there are two subunits, the protein is called dimeric , if three subunits trimeric , and, if several subunits, multimeric. ] Subunits are held together by noncovalent interactions (for example, hydrogen bonds, ionic bonds, and hydrophobic interactions). Subunits may either function independently of each other, or may work cooperatively, as in hemoglobin, in which the binding of oxygen to... 

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