Specific inhibition of barley α‐amylase 2 by barley α‐amylase/subtilisin inhibitor depends on charge interactions and can be conferred to isozyme 1 by mutation

α‐Amylase 2 (AMY2) and α‐amylase/subtilisin inhibitor (BASI) from barley bind with Ki = 0.22 nm. AMY2 is a (β/α)8‐barrel enzyme and the segment Leu116–Phe143 in domain B (Val89–Ile152), protruding at β‐strand 3 of the (β/α)8‐barrel, was shown using isozyme hybrids to be crucial for the specificity of the inhibitor for AMY2. In the AMY2–BASI crystal structure [F. Vallée, A. Kadziola, Y. Bourne, M. Juy, K. W. Rodenburg, B. Svensson & R. Haser (1998) Structure6, 649–659] Arg128AMY2 forms a hydrogen bond with Ser77BASI, while Asp142AMY2 makes a salt‐bridge with Lys140BASI. These two enzyme residues are substituted by glutamine and asparagine, respectively, to assess their contribution in binding of the inhibitor. These mutations were performed in the well‐expressed, inhibitor‐sensitive hybrid barley α‐amylase 1 (AMY1)‐(1–90)/AMY2‐(90–403) with Ki = 0.33 nm, because of poor production of AMY2 in yeast. In addition Arg128, only found in AMY2, was introduced into an AMY1 context by the mutation T129R/K130P in the inhibitor‐insensitive hybrid AMY1‐(1–161)/AMY2‐(161–403). The binding energy was reduced by 2.7–3.0 kcal·mol−1 as determined from Ki after the mutations R128Q and D142N. This corresponds to loss of a charged interaction between the protein molecules. In contrast, sensitivity to the inhibitor was gained (Ki = 7 µm) by the mutation T129R/K130P in the insensitive isozyme hybrid. Charge screening raised Ki 14–20‐fold for this latter mutant, AMY2, and the sensitive isozyme hybrid, but only twofold for the R128Q and D142N mutants. Thus electrostatic stabilization was effectively introduced and lost in the different mutant enzyme–inhibitor complexes and rational engineering using an inhibitor recognition motif to confer binding to the inhibitor mimicking the natural AMY2–BASI complex.