Modularity of serpins: A bifunctional chimera possessing α₁- proteinase inhibitor and thyroxine-binding globulin properties

An exciting application of protein engineering is the creation of proteins with novel functions by the retrofitting of native proteins. Such attempts might be facilitated by the idea of a mosaic architecture of proteins out of structural units. Even though numerous theoretical concepts deal with the delineation of structural 'modules,' their potential in the design of proteins has not yet been sufficiently exploited. To address this question we used a gain of function approach by designing modular chimeric molecules out of two structurally homologous but functionally diverse members of the superfamily of serine-proteinase inhibitors, α₁-proteinase inhibitor and thyroxine-binding globulin. Substitution of two of four α₁-proteinase inhibitor modules (Lys²²² to Leu²⁸⁸ and Pro³⁶62 to Lys³⁹⁴, respectively), identified by α-backbone distance analysis, with their thyroxine-binding globulin homologues resulted in a bifunctional chimera with inhibition of human leukocyte elastase and high affinity thyroxine binding. To our knowledge, this is the first report on a bifunctional chimera engineered from modules of homologous globular proteins. Our results demonstrate how a modular concept can facilitate the design of new functional proteins by swapping structural units chosen from members of a protein superfamily.