Antibody structure Flashcards
What is electrophoresis?
Where was antobody activity mostly found?
Electrophoresis , which in its simplest form separates proteins on the basis of their net electrical charge. At pH 8.2 or so, most – COOH groups are – COO - (ionized,) and most NH 3 + are NH 2 (not ionized) , so many proteins have a net negative charge (are anionic ) and will migrate towards an anode (in this application the anode is positive ) .
► If serum is separated in an electrical field, the proteins segregate into an albumin and several globulin bands:
Antibody activity was found mostly in the gamma (γ) globulin zone ; antibodies were called, and still sometimes are, gamma globulin or immune globulin. Since some activity was also located in the beta region, it was decided to come up with a better generic term: immunoglobulin.
It had also become clear that “antibody” was not a single substance. Why? The antibodies to one toxin did not neutralize a different toxin, so that antibodies differed in specificity. In addition, there were classes of antibody that could be separated physically, for example by size in the ultracentrifuge, which can actually spin molecules out of solution, the heaviest spinning down soonest :
Describe the typical immunoglobulin structure:
How many antigens can Fab bind?
F (ab2) can bing to how many antigens?
If you adjust conditions during antibody digestion with proteolytic enzymes, you can get 2 Fab fragments (S — S bonds between the H chains fully reduced) or you can leave the 2 Fabs still joined; that’s called F(ab 2 ).
Fab is univalent (binds only 1 antigen) ; like IgG, F(ab 2 ) is divalent .
What do carbohydrates do to the Fc part of the IgG?
This is the structure of an IgG molecule as deduced from X - ray crystallography; each ball is an a tom . There is a patch of carbohydrate (purple & blue in this picture) that pushes the second constant domains (CH2) of the heavy chains out, allowing them to interact more easily with components of the complement system, which is how antibodies initiate inflammation.
► Each chain is composed of domains , which are compact areas held together by intra chain S — S bonds.
Light chains have one variable domain, V L , and one constant domain, C L .
Heavy chains have one variable domain, V H , and 3 - 4 constant domains, C H1 , C H2 , C H3 , (C H4 ).
There is considerable structural homology betwee n different domains, which suggests that there was once an ancestral gene for one domain, which has duplicated many times.
IgG
IgG
–> 2 light chains, and 2 gamma (γ) chains.
Molecular weight 150,000.
IgD
what is special about its hinge region?
IgD. 2 light and 2 delta (δ) chains.
–Extra-long hinge region; molecular weight 180,000.
IgE
What is important about its constant domain?
IgE. 2 light and 2 epsilon (ε) chains.
It has an extra constant domain,CHε4 and a lot of sugars.
Molecular weight 190,000.
IgM
What is the structure of IgM in blood?
Does it have extra CH domain?
What kind of chain does it have linked by disulfide bonds?
In blood as a pentamer of a basic unit which is 2 light chains and 2 mu (μ) chains
μ has an extra CHμ4 domain
Linked by S - S bonds and closed by a J chain
Molecular weight 900,000
IgA
The dimer is joined by what?
What wrapps its and prevents it degradation from proteases in mucosal secretions?
IgA.
-Secreted for m is a dimer; basic unit is 2 light chains and 2 alpha (α) chains.
Joined by a J chain, and wrapped by Secretory Component.
Molecular weight 400,000.
What does rotational symmetry means in antobodies?
► In any one antibody, the H chains are identical, and so are the L chains , so the molecule has rotational symmetry.
-The 5 kinds of H chains (gamma, alpha, mu, epsilon, delta) define the class of antibody to which the molecule belongs, and therefore its biological properties.
What changes and what doesn’t change when antobody switches isotype?
The heavy chain changes (mu replaced by alpha) but the L chain, either kappa or lambda, stays the same during the switch.
-The Fc region changes
How do you also call the hypervariable regions?
Amino acid sequence variability is not distributed uniformly along the V domain; most of the variability is in 3 areas called, therefore, hypervariable regions.
It is more functionally significant to call them ► complementarity - determining regions , CDR , because the amino acids in the hypervariable regions comprise the actual antigen-binding site.
What is allotype?
ALLOTYPES:
There are minor allelic differences in the sequence of immunoglobulins between individuals, just as blood types or eye color differ. These differences are called allotypes, and the allotypes you express are determined by the allotypes your parents had, in the usual Mendelian fashion. Allotyp es are useful in genetics, for example in determining relatedness, and sometimes in forensic medicine. Occasionally, an immunodeficient patient getting immunoglobulin treatments will make antibodies to someone else’s allotype; this could be awkward. If cer tain allotypes function more efficiently than others, it could explain why some people are more susceptible to some infections than other people; we don’t know much about that yet , though there is some evidence for the idea .
What are idiotypes?
Each antibody will have its unique combining region, made up of the CDR amino acids of its L and H chains; we can call this unique structure an idiotype (idio means self).
It might not surprise you that, under rather special circumstances, antibodies can be made (most easily in another species) that recognize the unique sequence of that combining site, and no other. Such an antibody is an anti-idiotype. ► In other words, it is almost completely correct to say that an idiotype is an antibody’s unique combining site considered as an antigen .
1- What is the most abundant antibody in serum?
What is the second antibody most abundant in serum?
3- Least abundant antibody?
1- IgG
2- IgA
3- IgE
