Types of Ligand

In theory, any molecule can be used as a ligand in affinity chromatography, from small ones like amino acids to large proteins and even supramolecu-lar fragments such as membranes or whole cells. It must be possible to immobilize the ligand in such a way that it is still recognized by its biological partner. Thus, a random attachment of an antibody through lysines will allow only a small proportion of the antibody molecules to be oriented in an appropriate way to bind with their antigen, but a more directed attachment, for instance through the carbohydrate moiety on the Fc fragment, can increase this proportion significantly. Attachment of a ligand through an amine that is part of the biological recognition site will not produce a satisfactory affinity adsorbent, and some subtle chemistry may be needed to synthesize a ligand derivative that attaches elsewhere.

Highly specific adsorbents which are expected to interact with only one protein are the ideal, but may need to be synthesized individually (antibodies are a good example of this). On the other hand, group ligands which are expected to interact with a range of different proteins will obviously be less specific, but can be used for a wide range of different purifications. Not surprisingly, it is the latter which tend to be available commercially. An example is AMP-agarose, which interacts with enzymes that have ATP or NAD + as substrates. Despite the fact that AMP itself binds very weakly to most such enzymes, the additional nonspecific hydrophobic interactions with the hexyl spacer arm create sufficient binding energy. Lectins, such as concanavalin A which specifically binds to mannose residues in glycoproteins, have been extensively employed. Pseudo-affinity group adsorbents such as dye ligands have many advantages, including simplicity of synthesis.

Possibly the best known types of affinity adsorbents are antibodies, for purification of antigens; protein A and protein G for purification of antibodies; and an increasing range of affinity and pseudo-affinity materials for purification of recombinant fusion proteins. This last group will be discussed below, and antibodies (immunoafRnity chromatography) are described elsewhere.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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