Questions and Answers
What is the primary structural difference between a B-cell receptor and an antibody?
The primary structural difference is that the B-cell receptor has a hydrophobic membrane-anchoring sequence at the C-terminus, while the antibody has a hydrophilic sequence that allows for secretion.
What are the two main functional tasks of antibodies?
Antibodies bind to a wide variety of antigens and interact with a limited number of effector molecules and cells.
Describe the general shape of antibody molecules.
Antibody molecules are roughly Y-shaped, consisting of three equal-sized portions connected by a flexible tether.
What are the variable regions of antibodies called and what is their function?
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What region of the antibody interacts with effector cells and molecules?
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How are antibodies and immunoglobulins related?
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What allows antibodies to be studied easily compared to B-cell receptors?
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What method was used to determine the structure of antibodies?
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What are the two types of polypeptide chains that make up IgG antibodies?
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How many identical antigen-binding sites does an IgG molecule typically have?
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What is the purpose of disulfide bonds in IgG antibodies?
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Name the two types of light chains present in antibodies.
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What determines the class of an antibody?
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What is the molecular weight of IgG antibodies?
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How many domains does the heavy chain of an IgG antibody contain?
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What is the significance of the variable regions in immunoglobulins?
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What happens to the ratio of λ to κ chains in different species?
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What functional modification can be inferred from an excess of λ light chains in a sample?
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Explain the role of the hinge region in the structure of antibodies.
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What are the five major classes of immunoglobulin?
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What feature of immunoglobulin chains suggests their evolutionary origin?
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What is the relationship between the V and C regions of immunoglobulins?
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What are the main components that construct a β barrel structure in immunoglobulin domains?
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How does the size and structural complexity of the variable (V) domain compare to the constant (C) domain?
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What role do flexible loops in the V domains of immunoglobulins play?
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What is the significance of the amino acids common to both V and C domains in immunoglobulins?
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In what types of proteins outside of immunoglobulins might you find immunoglobulin-like domains?
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What is meant by the term 'immunoglobulin superfamily'?
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How does crystallography contribute to our understanding of immunoglobulin-like domains?
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Describe the folded structure of the immunoglobulin protein domain.
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What are the two types of fragments produced when an antibody is cleaved by papain?
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How does papain's cleavage site differ from that of pepsin in relation to disulfide bonds in antibodies?
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What is the significance of the flexible hinge region in the immunoglobulin molecule?
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What is a single-chain Fv and how is it constructed?
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Describe the difference between Fab and F(ab′)2 fragments.
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Why are Fab fragments valued in therapeutic applications?
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What role do disulfide bonds play in the structure of antibody molecules?
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What structural similarities exist between the V and C domains of immunoglobulin molecules?
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How does the flexibility of antibodies influence their interaction with haptens?
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In what way does the Fc fragment differ in function compared to Fab fragments?
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What is the effect of partial proteolysis on immunoglobulin structure?
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Explain the term 'molecular ball-and-socket joint' in relation to antibodies.
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What are the capturing capabilities of immunotoxins made from Fv molecules?
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How does antigen density affect antibody binding, considering hinge flexibility?
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Study Notes
Antibody Structure and Function
- Antibodies are secreted forms of B-cell receptors, differing only at the C-terminus of the heavy-chain constant region: antibodies have a hydrophilic sequence for secretion, while B-cell receptors have a hydrophobic sequence for membrane anchoring.
- Antibody molecules are Y-shaped, composed of three equal-sized regions connected by a flexible tether, facilitating dual tasks of antigen binding and interaction with effector molecules.
- The two arms (V regions) of the Y are variable and responsible for binding to a broad range of antigens, while the stem (C region) is less variable, interacting with effector cells.
- All antibodies, or immunoglobulins, are constructed from paired heavy and light polypeptide chains, categorized into five classes: IgM, IgD, IgG, IgA, and IgE, which are differentiated by their heavy-chain constant regions.
IgG Antibody Features
- IgG antibodies have a molecular weight of around 150 kDa and consist of two heavy (H) chains (~50 kDa each) and two light (L) chains (~25 kDa each).
- The heavy chains are linked by disulfide bonds; each heavy chain is also linked to a light chain via disulfide bonds, resulting in two identical antigen-binding sites.
- Two types of light chains—lambda (λ) and kappa (κ)—are present in antibodies, with immunoglobulins exclusively containing one type. Variations in the κ to λ ratio exist among species, and distortions of this ratio can indicate B-cell tumors.
Heavy-Chain Classes and Functions
- Antibody classes are defined by their heavy chains, which contribute to functional activities. Five major immunoglobulin classes exist, with IgG being the most abundant and having several subclasses (IgG1, IgG2, IgG3, IgG4).
- IgG's distinctive functional properties arise from the carboxy-terminal part of the heavy chain.
Domains in Immunoglobulin Chains
- Immunoglobulin heavy and light chains comprise repeated structural units known as protein domains, with the light chain having two domains and the heavy chain having four.
- The variable (V) regions of heavy (VH) and light (VL) chains allow specific antigen binding, while constant (C) regions (CH and CL) provide structural stability.
Functional Fragmentation
- Antibodies can be cleaved into functionally distinct fragments using proteolytic enzymes. Papain digests antibodies into two identical Fab fragments (containing antigen-binding sites) and one Fc fragment (non-binding, crystallizable portion).
- Pepsin cleavage generates F(ab′)2 fragments, which retain antigen-bind characteristics but cannot interact with effector molecules.
Flexibility and Structural Dynamics
- The hinge region allows flexibility between Fab arms, enabling binding to antigens located at varying distances apart. This flexibility is crucial for effective immune responses.
- The V-C domain junction allows further movement, termed a 'molecular ball-and-socket joint,' enhancing spatial adaptation during antigen interactions.
Immunoglobulin Domain Structure
- Immunoglobulin domains share similar structural features characterized by β sheets and are known as the immunoglobulin fold, with V domains being larger due to an additional loop.
- Stability of the immunoglobulin fold is maintained by shared amino acids located in the core structure, and the immunoglobulin superfamily includes proteins involved in various immune and cellular recognition processes.
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Description
Test your knowledge on the structure and function of antibodies, especially focusing on their unique Y-shaped architecture and the specific characteristics of different classes like IgG. This quiz covers key concepts including B-cell receptors, antigen binding, and the roles of heavy and light chains. Dive into the fascinating world of immunology and enhance your understanding of these crucial immune proteins.