Antibody Structure

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Carbohydrate forces CH2 domains to bulge; this impacts the Fc regions ability perform effector functions.

Carbohydrate addition is important for structure & function.

Antibody Antigen Interactions

Epitopes may be composed of AA that are linear or conformational:

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Vp1 protein:- white= epitopes exposed to the immune system.

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AA in Ag are located adjacent to one another in the primary seq.

Protein Folding:May Not be essential

Ab will bind todenatured Ag.synthetic peptide.

Sequential / Linear epitope:

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Conformational / Nonsequential epitope / Discontinuous:

Primary AA seq of Ag:

Ab will not bind to:Denatured AgShort synthetic peptide

Protein Folding Required:

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Protein antigens can contain both sequential & conformational epitopes:

HEL- 8 epitopes with most being nonsequential.

Sperm whale myoglobin: - 5 Sequential epitopes (shown in

blue) - several nonsequential (not

shown)

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Experiment:Prepared synthetic peptides of the HEL loop:

Open or closed.Competition Studies:

Immunology (kuby), Fig. 3-8

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Sequential epitopes may require tertiary structure & disulfide bonding in the Ag:

Antibody Antigen Interactions

A closer look at the antigen-antibody molecular interaction

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Comparison of Ab interactions with small protein Ag to that of large globular proteins:

Kuby, fig 3-4

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The size & conformation/shape of Ag impacts the Ab surface contacts:

Small protein Ag HIV peptide HEL

Ab CDR

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Fig. 3-5, kuby: Influenza virus antigen

VL

VH

11/5/2009 16Antibody binding is Reversible.

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Larger globular antigens often attract by

electrostatic interactions & H bonding:

HEL: Glutamine on HEL is the critical interaction:

Salt bridge

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Affinity versus AvidityAffinity: Strength of one Ab binding site bound to a single antigenic epitope.

Strength of noncovalent interactions.

Low affinity Ab-binds Ag weaklyDissociates more quickly.

High affinity Ab-binds Ag tightlyRemains bound longer.

http://www.bio.davidson.edu/misc/movies/antibody.mov

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Avidity- Strength of multiple interactions between a multivalent Ab & multivalent Ag.

The affinity of 1 Ab binding site to 1 epitope does not reflect the true strength of an Ab-Ag interaction.

High avidity can compensate for a low affinity.

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Cross reactivitySpecificity- Ab recognizes one epitope.

Example: anti-Hemagglutinin (Measles virus) Ab.

S. PneumoniaeCapsule Ag-A-1

S. PyogenesCapsule Ag-A-2

Cross reactivity-Anti-S. pneumoniae capsular Ab

- Binds S. pneumoniae epitope A-1 - Binds also to S. pyogenes A-2 with lower aff

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