Protein Quaternary Structure Hemoglobin

In some cases proteins are divided into two or more domains (Fig. 1), each of which is like a globular protein but connected covalently to other domain(s) by the continuous polypeptide chain. Other proteins are oligomeric in that they are composed of several unconnected polypeptide chains (subunits) that usually, but not always, fold up independently and assemble to form the complete protein. The arrangement of the subunits relative to each other is referred to as the quaternary structure. Hemoglobin (a202) (Fig. 6) and aspartate transcarbamoylase (a6/36), where a and j3 refer to different types of subunits, are well-studied cases where different quaternary structures occur with significantly altered properties. 

Hemoglobin is a classic example of protein quaternary structure. The protein has 4 subunits, two a-chains and two p-chains, and it exhibits positive cooperativity. Binding of oxygen to one subunit makes it easier for oxygen to bind to other subunits. 

The globular proteins sometimes consist of an assembly of a small number of identical or very similar sub-units, not linked together by covalent bonds. This is usually designated as the quaternary structure. Hemoglobin consists of 4 units of the type indicated in Figure 10.18b. Insulin readily forms a dimer from the two units in Figure 10.19. 

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 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). 

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... 

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. 

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