Oxidase Modification of Sugar Residues

Another method of forming aldehyde groups on carbohydrates and glycoproteins involves the use of specific sugar oxidases. These enzymes only affect the monosaccharide they are specific toward, 

6-D-Galactose Residue within Polysaccharide Chain with Terminal Sialic Acid Groups 

Selective Oxidation of Terminal B-D-Galactose (or N-acetyl-galactosamine) Groups 

The following protocol was used by Wilchek and Bayer (1987) to label cell-surface galactose residues. 

Prepare a 5 percent cell suspension in an appropriate buffer. Avoid amine-containing buffers, as these will interact with aldehydes. 

Hgure 93 Galactose oxidase may be used to transform spedfically the C-6 hydroxyl group of galactose residues into an aldehyde. 

---

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]. 

---