Biosynthesis of pectin

Pectin consists of a group of acidic polysaccharides that constitute a large part of the cell wall of plants. The pectic polysaccharides have a complex structure but can generally be divided into homogalacturonan, rhamnogalacturonan I, rhamnogalacturonan II (RGII) and xylogalacturonan (XGA). These polysaccharides appear to be present in all cells but their relative abundance and structural details differ between cell types and species. Pectin is synthesized in the Golgi vesicles and its complexity dictates that a large number of enzymes must be involved in the process. The biosynthetic enzymes required are glycosyltransferases and decorating enzymes including methyltransferases, acetyltransferases and feruloyltransferases. Biochemical methods successfully led to the recent identification of a pectin biosynthetic galacturonosyltransferase (GAUT1), and recent functional genomics and mutant studies have allowed the identification of several biosynthetic enzymes involved in making different parts of pectin. Strong evidence has been obtained for two xylosyltransferases (RGXT1 and RGXT2) with documented in vitro activity and apparently involved in making a side chain of RGII. Strong circumstantial evidence has been obtained for a putative glucuronosyltransferase (GUT1) involved in making RGII, a putative arabinosyltransferase (ARAD1) involved in making arabinan, and a putative xylosyltransferase (XGD1) involved in making XGA. In several other cases, enzymes have been identified as involved in making pectin but because of ambiguity in the cell wall compositions of mutants and lack of direct biochemical evidence their specific activities are more uncertain.