Lecture 5 - Ig Structure Flashcards
Describe clonal selection theory
- Any given antigen has a B cell specific for it
* Once a B cell sees the antigen, it proliferates
What are the two forms of Ig?
- surface Ig (detector form)
* secreted Ig (effector form)
What happens after clonal selection?
Selection of that B cell
• undergoes proliferation
• starts producing a lot of Ig
Describe the structure of Ig
Draw it
Two version of two different chains: • 2x light chains • 2x heavy chains Disulfide bonds holding the chains together Chains are made up of Ig domains Variable region (antigen binding) Constant region (effector function) Antigen binding arms
What is an Ig domain?
How many does each chain have?
A section of the heavy and light chains on the Ig
Light chain: 2 domains
Heavy chains: 4 domains
Describe the structure of an Ig domain Describe some key features: • stability • hydrophobicity What are the two types of Ig domains? How many strands in each?
Sandwich:
• two layers of antiparallel Beta Strands (thus, two beta sheets)
• Sheets held together with disulphide bond
- Very stable
- Antiparallel
- Hydrophobic residues pointing inwards
Two types:
• V like: 9 strands
• C like: 7 strands
How many strands in the V-like Ig domains?
9 strands
How many strands in the C-like Ig domains?
7 strands
What is the Ig superfamily?
Give some examples
A family of proteins that aren’t antibodies, but contain domains that are found in antibodies
- TCRs
- MHC class II
- CD4
- CD8
Where are Ig superfamily proteins generally found?
- Membrane bound
- Secreted
- ER, Golgi
i.e., not in the cytosol
Why is it important that Ig-like domains are encoded by a single exon?
It can be moved by transposons around the genome
It is for this reason that these domains are so conserved
Give an example of an Ig-like domain in various membrane proteins
- in TCR
- in CD4
- in MHC II
Which proteases were commonly used to digest Ig?
- Papain
* Pepsin
Where on Ig does papain cut?
What about pepsin?
Papain: Above hinge
Pepsin: below the hinge
Using papain, which two fragments were gotten?
- Above hinge: Fab
* Below hinge: Fc
Which fragments were gotten when Ig was digested with pepsin?
• Fab’2
Cut below the hinge
• Below the hinge degrades
What is the antigen binding site made up of?
What is the name for these loops?
3 loops from each chain
(Thus 6 loops)
The loops are called:
• CDR: complementarity determining regions
• Hypervariable loops
Where are the hyper variable regions located?
Throughout the variable domains of the Ig, but the loops are often at the ends, exposed.
This makes sense, because they are the ones that need to bind
What are the different Ig classes?
What are the corresponding greek letters?
IgA - α IgD - δ IgE - ε IgG - γ IgM - μ
What is the actual difference between the classes?
Different heavy chain
What is the further diversity in heavy chains?
There are subclasses within each heavy chain class.
For example:
IgG(1-4)
What is the diversity in light chains?
Two types:
• kappa
• lambda
** Any given B cell will only produce one of these types
Which is the most abundant Ig class in the serum? Which is the least abundant?
IgG
IgE is the least abundant; because it is bound to Mast cells
Describe IgE binding to Mast cells
IgE Fc region binds to FcR on the mast cells
Where is IgA mainly found?
Serum and secretions
Where is IgM mainly found?
Serum
Where is IgG mainly found?
Serum and lymph
Where is IgE mainly found?
On mast cells under epithelium
Which isotopes are monomers?
What about the others?
Monomers: • IgG • IgE • IgD Multimers: IgM: • pentamer IgA: • dimer
What are some features of IgG?
- Large hinge region
- High affinity
- 3 C Ig domains
What are some features of IgM?
- 4 C Ig domains
- Pentamer
- Low affinity, however, high avidity
- Stiff hinge region
- Tail piece (where J chain attaches)
What joins pentamers and dimers?
J-chain
What is a mu chain?
The heavy chain of the IgM class
What difference does avidity make?
When there are more binding sites, there are more chances for binding.
With low avidity molecules, the Ig is either bound or unbound
Differentiate between affinity and avidity
Affinity: closeness of binding between two molecules
Avidity: sum total of strength of binding at all the sites
Why is the tail piece important?
This is where the J-chain joins up the 5 IgM molecules to make the pentamer
What are the features of IgA?
- Dimer
- Secreted into mucosa
- Tail piece + J-chain
- Secretory component
- 3 C Ig domains
What are some features of IgD?
- Low abundance
* Found on mature B cells
What are some features of IgE?
- Involved in allergic reactions & parasitic immunity
- Binds to mast cells
- Has an extra C domain (Cε4)
Which isotopes have a stiff hinge region?
IgM
Who described clonal selection theory?
McFarlane Burnet
Where are disulphide bonds seen in a Ig molecule?
- Between light and heavy chain
- Within domains
- Between heavy chains below the hinge region
Give the size of light chains and heavy chains in kDa
Light chain: 25 kDa
Heavy chain: 50 kDa
How long is a single Ig domain?
70-110 aa
Describe the connection between the two Beta pleated sheets in Ig domains
Sandwich of two beta sheets is held together by a di-sulfide bond
Conserved cysteine residues form the bridge
Compare the valency of Fab and F(ab’)2
How do you get these two molecules?
Fab: monovalent
• digestion with papain
F(ab’)2: divalent
• digestion with pepsin
What determines the idiotype of an Ig molecule?
The CDRs in the variable domain
How many sulfur atoms in a di-sulfide bond?
Two
Which Ig isotypes have 4 constant Ig domains?
- IgM
* IgE
