Biosynthesis pectic polysaccharides

Caffall KH, Mohnen D (2009) The structure, function, and biosynthesis of plant cell wall pectic polysaccharides. Carbohydr Res 344 1879—1900... 

A large number (>50) of glycosyltransferases and other types of transferase are required to synthesize pectic polysaccharides [20]. The activities of several glycosyltransferases have been identified in cell-free membrane preparations from plants. However, only a few of these enzymes have been partially characterized and shown to be involved in pectin biosynthesis. These include D-galacturonosyl transferase, galactosyl transferase, arabinosyl transferase, and apiosyl transferase [20]. An enzyme that catalyzes the methylesterification of homogalacturonan (homogalatur-onan methyltransferase) has been partially characterized [20]. There are also reports that plants contain methyltransferases that catalyze the methylesterification of RG-I and RG-II [25]. No 0-acetyl transferase involved in pectin biosynthesis has been characterized nor have the enzymes that catalyze the addition of phenolic acids to pectins [20]. 

Pectins are probably the most complex polysaccharides known, in terms of their chemistry and are certainly so in terms of their biosynthesis. Classically they were regarded as al,4-galacturonans, with various degrees of methyl esterification, and the terms pectic acid and pectinic acid referred to the non-esterified and partially esterified forms respectively. A third term protopectin , was used of insoluble pectin that could not be extracted from plant cell walls by hot solutions of chelating agents. It was considered that these three classes of pectin constituted a pectic triad . This view is now known to be erroneous, but it is still frequently put forward, especially in botanical texts. Consequently any discussion of the synthesis of pectins must be prefaced by a description of their chemistry, as it is now understood. 

---