(02) Antibody Structure and Function Flashcards
Immunoglobulin
- Cells that produce these?
- expression of gene(s)
- Relationship to antibodies.
Produced by:
- Plasma Cells (B cell)
Gene Expression:
- Two genes expressed that code for Immunoglobulins
Antibodies:
- 2 immunoglobulins combine to form antibodies
What is the relationship of immunoglobulins produced by a B cell and B cell surface receptors?
- they are identical with the same antigen specificity as the antibodies that are secreted
What happens to B cells during an infection?
- B cells with specificity for any specific determinant or the pathogen are activated
- These are induced to clonally expand
- Cells differentiate into plasma cells
T or F: during a specific infection only one group of B cells become active and differentiate into plasma cells
False, each pathogen has many different determinants that can be recognized by distinct antibodies so many different B cells with different specificities can be activated to target a single pathogen
Antibody structure
- Heavy Chain and Light Chain linked by disulfide bonds is linked via disulfide bonds to another IDENTICLE heavy chain, light chain combo
What is the Hinge region of antibodies?
- is this found in all types?
- Stretch of Amino Acids in the HEAVY CHAIN they has no secondary structure and is flexible
- NO not all isotypes have a hinge region, instead they have an additional IMMUNOGLOBULIN domain
How many distinct regions are found in the heavy chain of antibodies?
- light chain?
- Purpose?
Heavy Chain:
4 distinct regions and hinge region (3 constant domains and 1 variable domain)
Light Chain:
2 distinct regions (1 constant and 1 variable domain)
**These are formed by highly organized secondary structure that makes antibodies vary stable in many different environments
T or F: both heavy and light chains have variable domains that align with each other to form a unique structure.
True
Antibody Structure (4 parts) **pay attn. to 3 and 4
- Each Immunoglobulin domain is stabilized by INTRACHAIN disulfide bonds
- Each monomeric antibody molecule has two antigen-binding regions (where heavy and light chains come together)
- Tertiary Structure permits binding of only one antigen (3D structure = idiotype) - C-terminal Immunoglobulin domain includes region of molecules that binds to antibody receptors or Fc receptors (on macrophages, neutrophils, mast cells etc.)
- CH2 immunoglobulin domain containis the determinant that complement component C1 binds to when intitiating the classical complement cascade
Fc region
- what does it consist of?
- what is it bound by?
- CONSTANT domains of heavy chains of antibody (vertical part of the Y)
- Bound by COMPLEMENT COMPONENT C1 and Fc receptors
What are the Fab fragments?
- Fragments of Antigen Binding (slanting part of the Y)
What region would you need to cleave to isolate Fc region from Fab fragments?
- Cleaving in the hinge domain produces Fc region piece and Fab fragments
What happens if a pathogen produces a protease that cleaves the hinge region?
- Fab Fragments still by their antigenic determinant
- BUT no Fc regions means they couldn’t
1. perform their effector functions
2. activate complement cascade
3. serve as opsonins for phagocytic uptake
4. in some cases they couldn’t even neutralize toxins
T or F: the constant domain of the light chain is almost identical to the constant domain of the heavy chain?
True
What is the secondary and tertiary structure of constant domains?
- what maintains tertiary structure?
- Beta-Sheet secondary structure separated by HIGHLY CONSERVED loops of amino acids that have no secondary structure
- Tertiary structure = Beta Barrel, maintained by SINGLE DISULFIDE bond
T or F: the light chain variable region is very different from the light chain constant region
False, these two regions are nearly identical
What is the main difference between the structure of the light chain variable and constant regions?
- Loops are NOT conserved and are EXTREMELY variable, aka HYPERVARIABLE region
What would result from a basepair substitution in the light chain loop region in the:
- Constant Region
- Variable Region
Constant Region:
- Loss of Functionality
Variable Region:
- Protein would still be functional
Note all concepts of the light chain variable and constant region apply to the heavy chain except the heavy chain has 3 variable regions
Note all concepts of the light chain variable and constant region apply to the heavy chain except the heavy chain has 3 variable regions
What does HV1, HV2, and HV3 stand for?
HyperVariable region 1-3 (located around 30, 50, and 90 residues)
***3 located on both the heavy and light chains
What do HV regions come together to form?
- The Antigen Binding Domain
**This defines the antibody’s IDIOTYPE
Define Antigen
Any molecule that is bound by a B cell or T cell immunoglobulin molecule
Define Antigenic Determinant or Epitope
The exact portion of the antigen that is recognized by the immunoglobulin
Define Idiotype
Antigen Binding Pocket
Define Affinity
Strength of the binding interaction between a single antigen-binding site of the immunoglobulin
Definen Avidity
the COMBINED strength of all antigen-binding domains of an antibody molecule when bound to its cognate antigen
How many binding domains can be found on each monomeric antibody?
- How do they bind to an antigen with two identical epitopes?
- How do they let go?
2 binding sites
- Each must bind simultaneously
- Must also let go simultaneously
T or F: antibody binding is a reversible reaction
True, if affinity is high the bound state is strongly favored
Why are multivalent antigens bound more tightly by antibodies than if they had only a single epitope?
Because both epitopes must be released simultaneously, so while the AFFINITY of each domain could be relatively low for a single epitope could be low the AVIDITY will be higher because the binding interactions onto the antigen are additive
What antibody has the highest POTENTIAL avidity for any antigen molecule and why?
IgM because it is a pentamer (has 10 binding domains)
T or F: weak interactions are what permits antibodies to interact with antigens
True, only non-covalent forces used
T or F: there are many restrictions to they type of antigens that can be bound by antibodies
False, antibodies can bind: - Native Proteins - Denatured Proteins - Carbohydrates - Lipids - Small molecules and chemicals -
T or F: antibodies can bind SOLUBLE OR PARTICULATE antigens
True
Why are T cells so much more limited in the type of antigens that they can recognize?
- They can only recognize PEPTIDES that are presented in the binding groove of an MHC molecule
Many different idiotypes are possible, while binding pockets are a common motif the variable region can actually slide into the binding pocket of an antigen*
Many different idiotypes are possible, while binding pockets are a common motif the variable region can actually slide into the binding pocket of an antigen*
What is the difference between a linear epitope and conformational epitope?
Linear Epitope:
- several amino acid stretch that is recognized by antibodies
Conformational (discontinuous epitope):
- Amino acids recognized by the antibody may be several residues away from each other but tertiary structure brings them together
What difference in binding ability of antibodies for linear and conformational epitopes (determinants) in different conditions?
Linear Epitopes can be bound even if the protein is denatured because it is not dependent on secondary or tertiary structure
Conformational (discontinuous epitopes) depend on tertiary structure and will likely no longer be bound by the antibody under denaturing conditions
T or F: T cell receptors can bind both linear and conformational determinants
False, they bind peptides so they only recognize linear determinants
What are multivalent antigens?
Antigens with multiple binding spots (determinants) these can be different types or epitope or the same type
Are pathogens typically univalent or multivalent?
Multivalent, gives the bodies lots of targets (viruses usually have fewer than bacteria)
What are the two versions of antibody found on each B cell?
- what are their functions?
- Antigen Specific Receptor is membrane bound antibody
- an identical antibody is also secreted, it serves as the EFFECTOR that RECRUITS phagocytes, NK cells, mast cells and eosinophils and proteins in the complement pathway
What is neutralization?
Secreted antibodies interact with toxins and prevent them from binding host cell receptors
What are the 5 distinct isotypes of antibody that can be produced by a B cell?
- IgG
- IgM
- IgD
- IgA
- IgE
What two isotypes have subtypes?
IgG (4 subtypes)
IgG1-4
IgA (2subtypes)
IgA1-2
What two isotypes lack a hinge domain
No Hinge:
- IgM
- IgE
- *They fill the void with another Constant Immunoglobulin domain
Hinge:
- IgG
- IgD
- IgA
How do the four IgG subtypes differ structurally?
- Length of Hinge Region
- Number of Disulfide Bridges that connect the two heavy chains
- Different Fc constant domains - *This gives them distinct functionality
The SPECIFICITY of antibodies secreted by B cells are identical yet FUNCTIONALITY can be changed. How is FUNCTIONALITY altered?
Class Switching
- Changes isotype by changing the constant region
- Different constant regions have different determinants for binding the Fc receptors produced by the innate immune cell types (neutrophils, macrophages, mast cells, NK cells, etc.)
T or F: Changing the isotype of the antibody changes the effector mechanism directed by that antibody
True, this allows great flexibility of the antibody response to pathogens
IgM
- how is it secreted
- when is it secreted
- what B cells secrete it
- Surface receptor form
- Fc receptor specificity
- Secreted in Pentameric form
- FIRST isotype produced by EVERY B Cell
- Surface receptor of IgM = MONOMERIC
- NO Fc RECEPTORS are specific to IgM
T or F: IgM can be transported into mucosal secretions
True, this process is very inefficient due to the size of IgM molecules and active transport is needed
What stabilizes both the pentamer of IgM and the dimer or IgA?
J chain
IgA
- different forms secreted
- most common secreted form
- mucosal secretion involvement
3 forms
- Monomer, dimer, Trimer
- Dimer = most common
Most common form found in mucosal secretions (much more efficient active transport)
Where is IgA found?
Lumens of:
- Respiratory Tract
- Urinary/ G.I. Tract
- Saliva
- Tears
- Breast Milk/colostrum
What is the primary function of IgA?
-neutralize pathogens, preventing them from colonizing mucosal surfaces
What is the largest antibody?
IgM
What is the antibody found in the lowest concentration in blood serum? which is next lowest? why?
Lowest:
IgE - produced in high concentrations but is stolen quickly by Mast Cells
Next to Lowest:
IgD - lacks a role in antibody response to infection
What is the most predominant antibody in serum?
IgG
What isotype has the longest half life?
IgG
