**4.2 Functional epitope exchange**

Grafting epitopes with known function is a straightforward stratage for a protein to gain new function. Functional epitopes could be exchanged between proteins have a high sequence identity or share a high structural similarity. Based on the structural homologous, the β2-β3 hairpin of scyllatoxin, a scorpion toxin containing the CSαβ motif, is replaced by the CDR2-like loop of human CD4. The chimeric protein can bind to the HIV-1 envelope glycoprotein and the affinity is increased 100 fold as compared with the native scyllatoxin (Vita et al., 1999). In another case, VrD1 and VrD2, two defensins of *Vigna radiate*, share 80% sequence identities. The major difference of sequences concentrats on the loop 3. When the loop 3 of VrD1 is grafted to VrD2, the chimeric peptide exhibits the enzyme inhibitory function as VrD1 does (Lin et al., 2007).

## **4.3 Combinatorial chemistry approach**

Combinatorial chemistry approach are powerful in screening and selecting binders with high affinity and high specificity (Hosse et al., 2006). It has been widely applied in antibody scaffold and thousands of antibodies are generated through the technology. Combinatorial chemistry approach also has been recruited to develop and isolate artificial proteins with new functions (Zhao et al., 2004). The approach could accelerate protein engineering based on a CSαβ motif to develop novel peptides with biomedical interesting (Thevissen et al., 2007;Van Gaal et al., 2004). Based on scaffold of insect defensin A, an expression library of peptides with 29 residues is constructed and used in screeening novel binders to targets. The expression library is artificially synthesized and amino acid of seven positions on the loops are randomized (Zhao et al., 2004). Tumor necrosis factor α (TNF-α), TNF receptor 1, TNF receptor 2 and antibody against BMP-2 are selected as targets and the screening results show significant enrichment in all cases.
