Friday, December 06, 2013

Replacing Antibodies: Engineering New Binding Proteins

One aim of protein engineering is to generate mutant proteins with enhanced or novel functions. As the variation among ligand-binding receptors is governed by folding and the diverse structures of protein domains, this can be accomplished through manipulation of natural or synthetic DNA. Antibodies are currently the preferred proteins for clinical use as biomolecular recognition elements in disease pathology. However, antibodies have limitations and protein scaffolds, polypeptide frameworks amenable to mutations or insertions, allow new high-affinity binding domains to be evolved. For example, antibodies are structurally reliant on disulfide bridges which cannot form in the cytosol of microbial hosts, while protein scaffolds express and fold efficiently in bacterial cytoplasm.

X-ray crystallography and modeling software allow for identification of suitable sites for mutagenesis in proteins that do not naturally participate in biomolecular recognition. Mutant constructs and libraries are generated based on these identifications, and mutants with a desired trait are isolated by selection of translated protein binding domains.

One application of these mutant proteins is in selective recognition of membrane-bound proteins associated with cancerous tissue. Protein scaffolds, such as the affibody molecule, have more favorable structural attributes than antibodies. An affibody, the first non-Ig protein to reach clinical trials for in vivo cancer imaging, is derived from a domain of a staphylococcal protein and selects for potential binders to human epidermal growth-factor receptor 2 (HER2) which is linked to breast and ovarian cancers.

I recently had a conversation with my mother, who claimed that the concept of receptors is a relatively new phenomenon and that she did not learn about them when she attended medical school. Having been through a semester of physiology now, I have a hard time conceiving the subject without such a crucial concept. It seems that receptors govern almost everything in the body (even cancer), so protein engineering has the potential to be extremely effective in medical application. As I posted previously, I am increasingly interested in the field of biomaterials and I could easily see myself involved in a similar field.


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