Enzyme active sites, peptide-carbohydrate

Peptide-Carbohydrate Mimicry in Enzyme Active Sites. 93... 

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. 

CE Family 1 is very large and contains members which do not act on carbohydrate-derived substrates. The crystal structure of a CE 1 domain of XynlOB modular enzyme from Clostridium thermocellum has been solved. " The CE 1 domain is a feruloyl esterase which hydrolyses the feruloyl groups attached to some arabinofuranosyl 05 groups in native xylan. (The Xyn lOB protein as a whole consists of two CBM 22 domains, a dockerin domain, and a GH 20 xylanase domain, and forms part of a cellulosome - see Section 5.10.) The enzyme has the common a/p hydrolase fold. Studies of ferulic acid complexes of the inactive alanine mutant of the active site serine revealed the classic catalytic triad, and two main-chain peptide NH bonds are in place to form an oxyanion hole . A remarkable feature is that the enzyme as repeatedly isolated was esterilied on the active site serine by phosphate or sulfate. 

The metal-dependent hydrolase class of enzymes uses catalytic metal ion(s) along with key active-site side chains to catalyze the hydrolysis of a wide variety of biologically important substrates, including carbohydrates, peptides, proteins, nucleotides, phosphodiesters, and xenobiotics. Interest in understanding the mechanisms of... 

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