Oligosaccharide binding to barley alpha-amylase 1

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Previous enzymatic subsite mapping suggested 10 binding subsites in barley alpha-amylase active sites.
• Understanding substrate binding is crucial for enzyme function and inhibitor design.

Purpose of the Study:

• To elucidate the three-dimensional structures of barley alpha-amylase isozyme 1 (AMY1) in complex with oligosaccharides.
• To gain detailed insight into substrate binding, identify specific residues, and compare binding modes with isozyme 2 (AMY2).
• To investigate the roles of surface binding sites and the catalytic nucleophile in enzyme activity.

Main Methods:

• X-ray crystallography of barley alpha-amylase isozyme 1 (AMY1) complexed with oligosaccharides (acarbose, maltoheptaose, thio-DP4).
• Structural analysis of wild-type and catalytic nucleophile mutant (AMY1D180A) complexes.
• Comparison of sugar binding modes between AMY1 and AMY2.

Main Results:

• Detailed structural description of 9 subsites (-7 through +2) in AMY1, confirming acarbose hydrolysis.
• Observation of sugar binding at the starch granule-binding site in AMY1, with comparative analysis to AMY2.
• Confirmation of the 'sugar tongs' surface binding site and proposal of a substrate entrance site.
• Evidence for an additional role of the catalytic nucleophile (Asp180) in stabilizing the Michaelis complex.
• Data support a model where AMY1 surface sites interact with folded amylose chains, facilitating active site access and potentially unraveling via Tyr-380.

Conclusions:

• The crystal structures provide unprecedented insight into the detailed substrate binding mechanism of barley alpha-amylase isozyme 1.
• Surface binding sites play critical roles in substrate recognition, orientation, and processing of amylose.
• The catalytic nucleophile contributes to enzyme-substrate complex stabilization, enhancing catalytic efficiency.