The current view is that the bulk of pectin synthesis is catalyzed by glycosyltransferases (GTs) that transfer a glycosyl residue from an activated form of the sugar, most likely a nucleotide-sugar, to an acceptor. Like other polymer biosynthetic reactions, pectin syn­thesis is thought to proceed through three stages: initiation, elongation, and termination. There is no detailed understanding of the initiation phase for the synthesis of any of the pectic polysaccharides. It has traditionally been held that the pectic polysaccharides are not synthesized on a protein, thus distinguishing them from the synthesis of animal Golgi — localized proteoglycans (256). More recently, several investigators have presented evidence suggesting that some wall polysaccharide synthesis may occur on protein primers (257­259); however, definitive proof of this hypothesis has yet to be provided. To date all studies of pectin biosynthetic glycosyltransferases have been carried out by assaying the transfer of a glycosyl residue from a radiolabeled nucleotide-sugar substrate (with or without ad­ditional unlabeled substrate) onto endogenous acceptors in plant microsomal membranes or onto exogenous acceptors, or alternatively, by transfer of an unlabeled substrate onto fluorescently labeled exogenously added oligosaccharide or polysaccharide acceptors. Most nucleotide-sugars involved in pectin synthesis consist of a nucleoside-diphosphate (NDP) attached to the sugar, and thus the general reaction catalyzed by the GTs is NDP-sugar + acceptor^) ^ NDP + acceptor(„+i). The precise number and location of the glycosyl residues in the acceptor that are recognized by a GT will be unique for each GT. However, in this review, for the purposes of calculating a minimal number of GTs required for pectin synthesis, since all pectin biosynthetic GTs characterized to date have been shown to add onto the non-reducing end of the oligosaccharide/polysaccharide acceptor, the assumption has been made that each GT will recognize, as a minimum, the terminal two glycosyl residues at the non-reducing end of the acceptor (i. e., the sugar onto which the transfer takes place, and the adjacent sugar).

Table 5.2 lists the types of glycosyltransferases that are expected to be required for pectin synthesis, based on the premise that a unique glycosyltransferase will be required for the transfer of a unique sugar from a unique nucleotide-sugar onto a unique disaccharide acceptor region at the non-reducing end of the acceptor. In some cases, it is possible that the same enzyme may catalyze the synthesis of a similar region on diverse polysaccharides, (e. g., the same galacturonosyltransferase may catalyze the synthesis of the backbone of HG and the HG region of RG-II. Table 5.2 attempts to list all known or expected GTs that are required for pectin synthesis, and as such, is meant to serve as a reference table. However, in an effort to consider in more depth the synthesis of the different types of pectic polysaccharides, HG, RG-I, RG-II, XGA, and AG; a detailed summary of progress in understanding the synthesis of the specific pectic polysaccharides is described separately.

Table 5.2 List of glycosyltransferases expected to be required for pectin biosynthesis Enzymeb acceptor substrate Enzyme Type of glycosyl-transferase Parent polymer3 (side chain) activity (unless noted: enzyme adds to the glycosyl residue on the left*) Ref. c for structure Putative gene Identified (Ref.)d Gene identified (Ref.)e D-GalAT HG/RG-II *GalAa1,4-GalA а 1,4-GalAT (157) Put. QUA1- GAUT1 At3g25140 (135, At3g61130 260) (137) D-GalAT RG-I L-Rhaa1,4-GalA a 1,2-Gal AT (157, 261,262) — — D-GalAT RG-II (A) L-Rha|31,3′-Apif a1,2-GalAT (214, 157) — — D-GalAT RG-I I (A) L-Rha|31,3′-Apif fil,3GalAT (214, 157) — — D-GalAT RG-I/HG GalAal,2-L-Rha а і,4-Gal AT — — — D-GalAT HG attached to *GalAa1,4-GalA а 1,4-GalAT or (129, 263) Put. GAUT12 xylan Xyl-?__ а 1,4-GalAT (Xylpi,4-ХуЫ, IRX8 At5g54690 3-Rhaa1,2-GalAA ,4-Xyl) (GT8) (129, 132) L-RhaT RG-I GalAal,2-L-Rha a1,4c-RhaT (157, 261,262) — — L-RhaT RG-II (A) Apifpi,2-GalA pi, З’-ь-RhaT (214, 157) — — L-RhaT RG-II © Kdo2,3GalA a1,5-c-RhaT (214, 157) — — L-RhaT RG-II (B) L-Araa1,4-Gal a? ,2-c-RhaT (214, 157) — — L-RhaT RG-II (B)f L-Araa1,4-Gal |31,3-c-RhaT (158, 161) L-RhaT HG/RG-I GalAal,4-GalA a1,4-c-RhaT — — — D-GalT RG-I L-Rhaa1,4-GalA |3 1,4-GalT (157, 261) — D-GalT RG-I Gal|31,4-Rha p 1,4-GalT (157, 264) — — D-GalT RG-I Galpi,4-Gal $l,4-GalT (157, 190, 207, — — 264-267) D-GalT RG-I Galpi,4-Gal $1,6-GalT (157, 264) — — D-GalT RG-I/AGP^ Galpi,3-Gal $l,3-GalT (214) — — D-GalT RG-I/AGP Galpi,3-Gal $1,6-GalT (214) — — D-GalT RG-I/AGP Galpi,6-Gal pi,3-Gal p 1,6-GalT (214) — — D-GalT RG-I L-Araf-1,4-Gal 1-5-GalT (268) L-GalT RG-II (A) GlcApi,4-Fuc a? ,2-c-GalT (214, 269) — — D-GalT RG-II (B) c-AcefA a1,3-Rha p 1,2GalT (214, 269) — — L-AraT RG-I Galpi,4-Rha a1,3-c-ArafT (157, 264) — —