Conformation nearby Trp residues of APIA and APIB modulates the inhibitory specificity of the protease.

The relationship between the micro-environment of the two tryptophan residues and the inhibitory specificity of arrowhead protease inhibitors A and B (APIA and APIB) was studied by mutagenesis and fluorescence spectroscopy. The environment of the two Trp residues at positions 93 and 122 in APIB is more hydrophobic than in APIA. Study after substitution of Trp with Ala revealed that the environment of Trp(122) is more hydrophobic than that of Trp(93). Substitution of Leu(82) and Arg(87) in APIB with Ser and Leu respectively made the tryptophan fluorescence of APIB to be like that of APIA and the inhibitory specificity to be closer to APIA, indicating that the inhibitory specificity of the enzyme may be modulated by the conformation around the tryptophan residues.