CAMEL SINGLE-DOMAIN ANTIBODY INHIBITS ENZYME BY MIMICKING CARBOHYDRATE SUBSTRATE

Whereas antibodies have demonstrated the ability to mimic various comp ounds, classic heavy/light-chain antibodies may be limited in their ap plications. First, they tend not to bind enzyme active site clefts. Se cond, their size and complexity present problems in identifying key el ements for binding and in using these elements to produce clinically v aluable compounds. me have previously shown how cAb-Lys3, a single var iable domain fragment derived from a lysozyme-specific camel antibody naturally lacking light chains, overcomes the first limitation to beco me the first antibody structure observed penetrating an enzyme active site. We now demonstrate how cAb-Lys3 mimics the oligosaccharide subst rate functionally (inhibition constant for lysozyme, 50 nM) and struct urally (lysozyme buried surface areas, hydrogen bond partners, and hyd rophobic contacts are similar to those seen in sugar-complexed structu res). Most striking is the mimicry by the antibody complementary deter mining region 3 (CDR3) loop, especially Ala104, which mimics the subsi te C sugar 2-acetamido group; this group has previously been identifie d as a key feature in binding lysozyme, Comparative simplicity, high a ffinity and specificity potential to reach and interact with active si tes, and ability to mimic substrate suggest that camel heavy-chain ant ibodies present advantages over classic antibodies in the design, prod uction, and application of clinically valuable compounds. (C) 1998 Wil ey-Liss, Inc.