AQA · A-Level · Biology

    Course version: 7402 · Version from Sept 2016

    Cell recognition and the immune system: Antibody structure.

    An antibody (such as IgG) is a Y-shaped protein with a quaternary structure comprising four polypeptide chains: two long heavy chains and two short light chains, held together by disulfide bridges.

    Checked against the AQA 7402 specification (opens in a new tab) on .

    • Point 8 of 18
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    An antibody (such as IgG) is a Y-shaped protein with a quaternary structure comprising four polypeptide chains: two long heavy chains and two short light chains, held together by disulfide bridges. The tip of each arm is a variable region whose amino acid sequence and tertiary structure differ between antibodies. These form two identical binding sites complementary to one specific antigen. The rest of the molecule is the constant region, which is the same in all antibodies of a given class. This region binds to receptors on phagocytes, marking the antigen-antibody complex for engulfment. A flexible hinge region allows the arms to move apart and bind antigens on separate pathogens, causing agglutination.

    What earns the marks

    • The antibody has four polypeptide chains, two heavy and two light, giving it a quaternary structure.
    • Disulfide bridges hold the polypeptide chains together.
    • Two variable regions form two binding sites with a tertiary structure complementary to one specific antigen.
    • The constant region is the same in all antibodies of a specific class and binds to receptors on phagocytes.
    • The hinge region provides flexibility so that two separate antigens or pathogens can be bound simultaneously.

    Common mistakes and how to put them right

    • Labelling the binding site as an active site.

      Put it right: Antibodies are not enzymes.

    • Claiming the antibody changes shape to fit the antigen.

      Put it right: Antibodies do not show induced fit.

    • Saying the variable region is the same in all antibodies.

      Put it right: It is the constant region that is conserved within a class.

    • Drawing a single binding site, which makes explaining agglutination impossible.

    Exam tips

    • Label a sketch with variable region, binding site, constant region, hinge region, and disulfide bridge to clearly demonstrate structural knowledge.
    • Pair every structural feature with its function, such as linking the presence of two binding sites to the ability to cause agglutination.

    Try the lesson

    The start of the Cell recognition and the immune system: Antibody structure. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which part of an antibody molecule binds to an antigen?

    Listen

    An antibody (such as IgG) is a Y-shaped protein with a quaternary structure comprising four polypeptide chains: two long heavy chains and two short light chains, held together by disulfide bridges. The tip of each arm is a variable region whose amino acid sequence and tertiary structure differ between antibodies. These form two identical binding sites complementary to one specific antigen. The rest of the molecule is the constant region, which is the same in all antibodies of a given class. This region binds to receptors on phagocytes, marking the antigen-antibody complex for engulfment. A flexible hinge region allows the arms to move apart and bind antigens on separate pathogens, causing agglutination. A strong answer earns marks for these points. The antibody has four polypeptide chains, two heavy and two light, giving it a quaternary structure. Disulfide bridges hold the polypeptide chains together. Two variable regions form two binding sites with a tertiary structure complementary to one specific antigen. The constant region is the same in all antibodies of a specific class and binds to receptors on phagocytes. Exam tip: Label a sketch with variable region, binding site, constant region, hinge region, and disulfide bridge to clearly demonstrate structural knowledge.

    See it

    A flowchart starting from a node labelled 'Antibody: Y-shaped protein'. From it, branches lead to four features. The first branch goes to 'Four polypeptide chains', which splits into 'Two heavy chains' and 'Two light chains', both leading to 'Held together by disulfide bridges'. The second branch goes to 'Variable regions at tips of arms', then to 'Two identical binding sites complementary to one specific antigen'. The third branch goes to 'Constant region', which splits into 'Same in all antibodies of a given class' and 'Binds to receptors on phagocytes'. The fourth branch goes to 'Hinge region', then to 'Flexibility allows binding antigens on separate pathogens', then to 'Agglutination'.
    It shows the parts of an antibody and how each structural feature links to its function.

    Key words

    Say what each word means before you turn the card.

    variable region

    Try answering aloud or in your head before turning the card.

    constant region

    Try answering aloud or in your head before turning the card.

    disulfide bridges

    Try answering aloud or in your head before turning the card.

    See every definition
    variable region
    The tip of each arm, whose amino acid sequence and tertiary structure differ between antibodies and which forms a binding site complementary to one specific antigen.
    constant region
    The rest of the molecule, the same in all antibodies of a given class, which binds to receptors on phagocytes, marking the antigen-antibody complex for engulfment.
    disulfide bridges
    Bonds that hold the polypeptide chains of the antibody together.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart starting from a node labelled 'Antibody: Y-shaped protein'. From it, branches lead to four features. The first branch goes to 'Four polypeptide chains', which splits into 'Two heavy chains' and 'Two light chains', both leading to 'Held together by disulfide bridges'. The second branch goes to 'Variable regions at tips of arms', then to 'Two identical binding sites complementary to one specific antigen'. The third branch goes to 'Constant region', which splits into 'Same in all antibodies of a given class' and 'Binds to receptors on phagocytes'. The fourth branch goes to 'Hinge region', then to 'Flexibility allows binding antigens on separate pathogens', then to 'Agglutination'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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