Question

In immunoglobulins all of the following are true except A. there are four polypeptide chains. B. there are two copies of each type of chain. C. all chains are linked by disulfide bonds. D. carbohydrate is covalently bound to the protein. E. immunoglobulin class is determined by the \(\mathrm{C}_{\mathrm{L}}\) regions.

Short answer

A: There are four polypeptide chains. B: There are two copies of each type of chain. C: All chains are linked by disulfide bonds. D: Carbohydrate is covalently bound to the protein. E: Immunoglobulin class is determined by the CL regions. Answer: E Explanation: Immunoglobulin class is determined by the constant region of the heavy chain (CH) and not the constant region of the light chain (CL). All other options are true statements about immunoglobulins.

Step-by-step solution

Option A: There are four polypeptide chains.

This statement is true. Immunoglobulins, also known as antibodies, are composed of four polypeptide chains: two identical heavy chains and two identical light chains.

Option B: There are two copies of each type of chain.

This statement is also true. As mentioned above, immunoglobulins have two identical heavy chains and two identical light chains, making up a total of four polypeptide chains.

Option C: All chains are linked by disulfide bonds.

This statement is true. The heavy and light chains in an immunoglobulin are linked by disulfide bonds (S-S bonds). Additionally, the heavy chains are also connected to each other through disulfide bonds.

Option D: Carbohydrate is covalently bound to the protein.

This statement is true. Immunoglobulins contain carbohydrate groups that are covalently bound to the protein component of the molecule. These carbohydrate groups play a role in determining the properties and function of the immunoglobulin.

Option E: Immunoglobulin class is determined by the CL regions.

This statement is false. Immunoglobulin class (IgA, IgD, IgE, IgG, and IgM) is determined by the constant region of the heavy chain (CH) and not the constant region of the light chain (CL). The CL region contributes to the overall structure and function of the immunoglobulin, but it does not determine its class. Therefore, the correct answer is option E, as it is the only statement that is not true about immunoglobulins.

Key concepts

Polypeptide Chains in Antibodies

Understanding the structure of antibodies, also known as immunoglobulins, is critical for grasping how they function in the immune response. Antibodies are composed of four polypeptide chains -- two light chains and two heavy chains. Each chain is a sequence of amino acids folded into a specific three-dimensional structure. These chains pair up, with one light and one heavy chain joining to form a y-shaped molecule. It's essential to recognize that although there are different types of light and heavy chains, an individual antibody will have two identical light chains and two identical heavy chains, ensuring symmetry and functionality.

The diversity of antibodies is not due to variety in the light and heavy chains within a single antibody but results from the different possible combinations of chains across different antibodies. This intricate design allows for a vast array of antibodies, each specific to a unique antigen.

Disulfide Bonds in Immunoglobulins

The stability and structure of antibodies are vital to their role in the immune system, and disulfide bonds play a key part in maintaining these properties. Disulfide bonds are strong covalent bonds formed between sulfur atoms of cysteine residues from different parts of the polypeptide chains. In immunoglobulins, these bonds link the light and heavy chains to each other, and also connect the two identical heavy chains together.

Those linkages are critical for the protein's structural integrity, ensuring that the antibody maintains its shape under physiological conditions. They also facilitate the flexible yet sturdy architecture that allows antibodies to bind to antigens effectively. The presence and precise location of disulfide bonds within an antibody contribute to its classification and can influence its antigen-binding and effector functions.

Immunoglobulin Class Determination

Immunoglobulins are categorized into different classes based on the structure of their heavy chains, specifically in the constant (C) region. This region is the same for all antibodies within a class and is responsible for the effector functions of the antibody, such as complement activation or binding to cell receptors. The five primary classes are IgA, IgD, IgE, IgG, and IgM.

Contrary to the solution exercise's incorrect statement E, the light chain's constant (CL) region does not determine the class of the antibody. Instead, different heavy chain constant regions (CH) define the class, offering a variety of immune functions and responses. For example, IgG is known for its role in providing long-term immunity, while IgM is usually the first responder during an initial immune response. Understanding these classes helps dissect the complexity of the immune response and the strategic functions of various types of antibodies.