Carbohydrates enzyme covalent bonding

The reaction mechanism of a-amylases is referred to as retaining, which means that the stereochemistry at the cleaved bond of the carbohydrate is retained. Hydrolysis of the glycosidic bond is mediated by an acid hydrolysis mechanism, which is in turn mediated by Aspl97 and Glu233 in pig pancreatic amylase. These interactions have been identified from X-ray crystallography. The aspartate residue has been shown to form a covalent bond with the Cl position of the substrate in X-ray structure of a complex formed by a structurally related glucosyltransferase. " The glutamate residue is located in vicinity to the chloride ion and acts as the acidic catalyst in the reaction. The catalytic site of a-amylases is located in a V-shaped depression on the surface of the enzyme. 

Glycopeptides substances of low molecular weight having many of the physical properties of a peptide, but containing covalently bonded carbohydrate components) mucopeptides, mucins muramyl peptides, products of enzymic degradation of glycoproteins... 

A peculiar sugar modification occurs in the biosynthesis of the aclacino-mycins. These anthracyclines contain a trisaccharide moiety attached to the aklavinone scaffold at the C-7 position (Scheme 1). The first two carbohydrates in the aclacinomycins are rhodosamine and 2-deoxyfucose, but they differ structurally in their third sugar component, which is rhodinose in AclN, cinerulose A in AclA, L-aculose in AclY and cinerulose B in AclB [157]. The conversion of rhodinose to L-aculose is catalysed in a two-step process by the FAD-dependent enzyme aclacinomycin oxidoreductase [71] (Scheme 5, step 31). The three-dimensional structure of this oxidase revealed that the cofactor FAD is bound via two covalent bonds to the enzyme. Crystal structure and functional data further established a mechanism where the two different reactions are catalysed in the same active site of the enzyme but by different active site residues [71]. 

H2CO is an abbreviation for a carbohydrate. The reaction is a part of the Calvin cycle. The enzyme necessary to carry out the binding of CO2 is called ribulose-1,5-biphosphate carboxylase oxygenase (RuBisCO). The mechanisms are not fully known. The cofactor ribulose-1,5-biphosphate, a rather small sugar phosphate molecule, binds to CO2 after abstraction of a proton. A covalent bond is formed between RuBis and CO2 (Figure 15.12). 

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