Subsite mapping of the human pancreatic alpha-amylase active site through structural, kinetic, and mutagenesis techniques
| Autor: | Yili Wang, Stephen G. Withers, Gary D. Brayer, Christopher M. Overall, Curtis Braun, Robert Maurus, Gary Sidhu, Edwin H. Rydberg, Nham T. Nguyen |
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| Rok vydání: | 2000 |
| Předmět: |
Models
Molecular Glycosylation Stereochemistry Protein Conformation Swine Molecular Sequence Data Oligosaccharides Cleavage (embryo) Crystallography X-Ray Biochemistry Catalysis chemistry.chemical_compound Fluorides Structure-Activity Relationship Nucleophile Hydrolase Animals Humans Maltose Pancreas chemistry.chemical_classification Binding Sites biology Chemistry Hydrolysis Leaving group Active site Kinetics Enzyme Aglycone Amino Acid Substitution Carbohydrate Sequence Models Chemical Mutagenesis biology.protein Thermodynamics Acarbose alpha-Amylases Alpha-amylase |
| Zdroj: | Biochemistry. 39(16) |
| ISSN: | 0006-2960 |
| Popis: | We report a multifaceted study of the active site region of human pancreatic alpha-amylase. Through a series of novel kinetic analyses using malto-oligosaccharides and malto-oligosaccharyl fluorides, an overall cleavage action pattern for this enzyme has been developed. The preferred binding/cleavage mode occurs when a maltose residue serves as the leaving group (aglycone sites +1 and +2) and there are three sugars in the glycon (-1, -2, -3) sites. Overall it appears that five binding subsites span the active site, although an additional glycon subsite appears to be a significant factor in the binding of longer substrates. Kinetic parameters for the cleavage of substrates modified at the 2 and 4' ' positions also highlight the importance of these hydroxyl groups for catalysis and identify the rate-determining step. Further kinetic and structural studies pinpoint Asp197 as being the likely nucleophile in catalysis, with substitution of this residue leading to an approximately 10(6)-fold drop in catalytic activity. Structural studies show that the original pseudo-tetrasaccharide structure of acarbose is modified upon binding, presumably through a series of hydrolysis and transglycosylation reactions. The end result is a pseudo-pentasaccharide moiety that spans the active site region with its N-linked "glycosidic" bond positioned at the normal site of cleavage. Interestingly, the side chains of Glu233 and Asp300, along with a water molecule, are aligned about the inhibitor N-linked glycosidic bond in a manner suggesting that these might act individually or collectively in the role of acid/base catalyst in the reaction mechanism. Indeed, kinetic analyses show that substitution of the side chains of either Glu233 or Asp300 leads to as much as a approximately 10(3)-fold decrease in catalytic activity. Structural analyses of the Asp300Asn variant of human pancreatic alpha-amylase and its complex with acarbose clearly demonstrate the importance of Asp300 to the mode of inhibitor binding. |
| Databáze: | OpenAIRE |
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