Chemically based protein modification methods could provide useful strategies for the generation of antibody mimics. However, the highly complex structures of antibody domains make it exceptionally difficult to modify these proteins in a single or small number of locations. This complexity includes the presence of multiple polypeptide chains, extensive disulfide networks, and critically important glycosylation patterns, all of which must remain intact to obtain biological function. In this work, we have created novel antibody mimics by installing synthetic molecules at the N-termini of crystallizable fragment domains (Fc's) via a chemical modification approach. First, a pyridoxal 5′-phosphate (PLP) mediated N-terminal transamination reaction provided a compatible method for site-selectively installing ketones as reactive handles on Fc domains. High levels of conversion were achieved. For elaboration of the newly installed chemical handles, we used two strategies for the ligation of our desired compounds to the protein. In the first, we used alpha-effect amines to create oxime or hydrazone linkages. Alternatively, we used the ketone as a site to introduce a second reaction handle: an aniline group that can participate in a recently reported oxidative coupling reaction. The oxidative coupling provides a highly efficient ligation strategy requiring very short reaction times (two min or less) at room temperature. By combining the advantages of synthetic targeting agents (e.g. high stability, low cost, and facile and reproducible production and discovery) with the ability of Fc domains to mediate targeted cell death and extend plasma half-life, these new hybrid agents may possess the best qualities of both. As an initial proof of concept, Fc domains were functionalized with DNA aptamers. The specificity of the aptamers for binding their cellular targets was demonstrated, as was the ability of the modified Fc domains to bind to complement proteins. The full assessment of the immunological properties of these hybrid constructs is currently underway.
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