Many of the pectin polysaccharides contain sugars that have been modified by the addition of a methyl group. For example, HG has carboxymethyl esters on C-6, RG-I contains a GlcA — 4- O-methylether, and RG-II contains 2-O-methylfucose. Multiple methyltransferases are required to synthesize these polysaccharides, although it is not known ifthe methyl group is added after the sugar is inserted into the polymer, or rather on the nucleotide-sugar donor. The available evidence suggests that SAM is the methyl donor (227,280-285). Table 5.3 lists the non-glycosyltransferases “required” for the synthesis of pectin.

5.4.1 Acetyltransferases

Several of the pectic polysaccharides contain sugars modified by acetylation. These include acetylation of the O-3 position of GalA in HG and either O-2 or O-3 acetylation on the GalA in the RG-I backbone. In addition, RG-II contains acetylated Fuc and 3- O acetylated aceric acid. The acetyltransferases have not been identified.