Question

Identify the protein with more than one polypeptide chain.
A. Myoglobin
B. Trypsin
C. Immunoglobulin
D. Lysozyme

Answer 1

Immunoglobulin (or antibody) molecules are glycoproteins that are composed of one or more units, each containing four polypeptide chains: two identical heavy chains (H) and two identical light chains (L).

Complete answer:
Immunoglobulins are heterodimeric proteins composed of four polypeptide chains, two small called light (L) chains and two longer called heavy (H) chains. Hence an antibody is represented as H2L2. The heavy chain has a large number of amino acids while the light chain has a smaller number of amino acids. In each chain of the antibody, there are two different regions namely the constant region and variable region. A disulfide bond joins a light chain with a heavy chain. Two disulfide bonds also link the two heavy chains. This part of the antibody has very flexibility in it and is therefore known as the hinge region. Due to this antibody “arms” can show some movement as the hinge bends, an antibody can be seen as a Y shaped molecule.

Among the given answers:
Myoglobin is known as a small oxygen-binding protein of muscle cells with a single polypeptide chain of 153 amino acid residues and a single iron protoporphyrin, or heme, group.
Trypsin is a globular protein with one polypeptide chain of 220 amino acid residues and serves as a proteolytic enzyme.
Lysozyme is abundantly found in egg white and human tears and catalyzes the hydrolytic cleavage of polysaccharides in the protective cell walls of some groups of bacteria. It is formed by a single peptide chain of 129 amino acid residues arranged mostly in α helical segments.

Hence, the correct answer is option (C).

Note: Immunoglobulin has four polypeptide chains: two large ones are called heavy chains and two light chains which are linked by non-covalent and disulfide bonds. The heavy chains of antibody interact at one end, then branches to interact separately with the light chains thereby forming a Y-shaped molecule.