Pectin Modifying Enzymes and Designer Pectins
Pectins are chemically heterogeneous plant cell wall polysaccharides localised in particular cell wall domains. They are found in the middle lamella, the region of interface between two walls from neighbouring cells, and in the primary plant cell wall as an independent but coextensive network associated with cellulose and xyloglucan. Pectin consists of a linear homogalacturonan backbone, the so-called 'smooth region', interrupted by rhamnogalacturonan which is branched and constitutes the 'hairy region'. These pectic polysaccharides can be found as a wide range of acidic polymers, both in composition and in distribution of the constituent blocks. Homogalacturonans, mostly homogeneous in composition, are formed by a(l-4) linked D-galacturonic acid residues in varying degrees of methylation, while rhamnogalacturonans are rich in rhamnose and commonly include several neutral saccharides such as arabinans, galactans and arabinogalactans as side chains. The industrial processing of pectic polysaccharides has been traditionally accomplished by chemical means, they are extracted, purified and modified using inorganic acids and bases, however the increasing availability of specific pectin modifying enzymes is already enabling a more effective and precise treatment of these substrates. Enzymatic extraction and modification may enhance the traditional uses of pectins as gelling and stabilising agents and extend their range of applications in both foods and pharmaceuticals. An excellent compendium of the advances in this area has recently been published, edited by Visser and Voragen, 'Pectins and Pectinases' compiles the scientific contributions to the meeting organised by Wageningen Agricultural University in 1995. The focus of the current paper is on the more recent structural studies of enzymes and the implications for molecular evolution and substrate binding.