Amylases mammalian

To understand the inhibition of a-amylase by peptide inhibitors it is crucial to first understand the native substrate-enzyme interaction. The active site and the reaction mechanism of a-amylases have been identified from several X-ray structures of human and pig pancreatic amylases in complex with carbohydrate-based inhibitors. The structural aspects of proteinaceous a-amylase inhibition have been reviewed by Payan. The sequence, architecture, and structure of a-amylases from mammals and insects are fairly homologous and mechanistic insights from mammalian enzymes can be used to elucidate inhibitor function with respect to insect enzymes. The architecture of a-amylases comprises three domains. Domain A contains the residues responsible for catalytic activity. It complexes a calcium ion, which is essential to maintain the active structure of the enzyme and the presence of a chloride ion close to the active site is required for activation. 

The resulting protein is still active and has the same conformation as the native insulin. Similarly, mammalian a-amylase has been reported to react with Hg2+ to produce a protein which is still active and contains an S—Hg—S bridge.257 Thus the thiol groups are situated close together and are not required for activity. 

Other ions, or molecules, may be required for maximal activity of an enzyme, although the enzyme is active in their absence. Thus, mammalian alpha-amylases are activated by chloride ions, and rabbit-muscle phosphorylase a is activated by adenosine 5-phosphate. 

Mammalian aipha-amylases probably attack large substrates by a multiple-attack mechanism, but it seems unlikely that any of the other well studied types of alpha-amylase act in this way. 

Mammalian alpha-amylases are activated by monovalent anions, especially chloride. Equations, similar to those for enzyme inhibition (see p. 290), may be written for the formation of enzyme—activator complexes, and kinetic parameters may be derived. Dissociation constants of salivary alpha-amylase—anion complexes have been determined. ... 

The A domains of all mammalian and some other GH13 a-amylases have a chloridebinding site that is essential for activity together with a short, inserted downstream amino acid loop (residues 304 through 311 in salivary amylase). This loop is flexible and interacts with substrate after binding so that the catalytic residues are correctly positioned for hydrolysis. It is absent from enzymes that do not require chloride ions for activity. The C domain has no known function, but it may facilitate solubility, in part, because of glycan attached to asparagine-413 in this domain. 

Glucoamylases, also termed amyloglucosidase or y-amylase, are produced predominantly by fungi, especially by species of Aspergfllus, Rhizopus and Endomyces. They are rare in procaryots and absent in plants or in mammalian tissues. 

---