Protein and tertiary and Quart.St.

Quaternary Structure: Hemoglobin

Learning Objective :
? Understand the general structure of Ouaternary protein and it’s function

While many proteins are biologically active as tertiary structures, some proteins require two or more
tertiary structures to be biologically active. When a biologically active protein consists of two or more
polypeptide chains or subunits, the structural level is referred to as a quaternary structure. Hemoglobin,
a globular protein that transports oxygen in blood, consists of four polypeptide chains: two ?-chains with
141 amino acids, and two ?-chains with 146 amino acids. Although the ?-chains and ?-chains have
different sequences of amino acids, they both form similar tertiary structures with similar shapes

In the quaternary structure, the subunits are held together by the same interactions that stabilize tertiary
structures, such as hydrogen bonds, salt bridges, disulfide links, and
hydrophobic interactions between R groups. Each subunit of the
hemoglobin contains a heme group that binds oxygen. In the adult
hemoglobin molecule, all four subunits (?2?2) must be combined for
hemoglobin to properly function as an oxygen carrier. Therefore, the
complete quaternary structure of hemoglobin can bind and transport four
molecules of oxygen.
Hemoglobin and myoglobin have similar biological functions. Hemoglobin
carries oxygen in the blood, whereas myoglobin carries oxygen in muscle.
Myoglobin, a single polypeptide chain with a molar mass of 17 000, has
about one-fourth the molar mass of hemoglobin 167 0002. The tertiary
structure of the single polypeptide myoglobin is almost identical to the
tertiary structure of each of the subunits of hemoglobin. Myoglobin
carries one molecule of oxygen, whereas hemoglobin carries four oxygen molecules. Table 2.1 and Figure 2.11
summarize the structural levels of proteins.