Pectin methylesterase

Other polysaccharides of primary cell walls.-A complex mixture of enzymes including endopolygalacturonase, pectin methylesterase, and/or pectin lyase solubilizes a mixture of polysaccharides from the primary cell walls of fruits [57-64]. Food scientists have referred for some 15 years to this mixture of polysaccharides as the hairy region to describe the highly branched character of the polysaccharides in the fraction and to emphasize the contrast to unbranched homogalacturonan. The recent discovery of rhamnogalacturonan hydrolase [65,66], which selectively cleaves the backbone of RG-I, led to the realization that the hairy... 

These acidic molecules might result from either the lack of PMT activity or the action of PME, known to be present in most plant cell walls. Pectin methyltransferases and pectin methylesterases extracted from active and resting cells were therefore characterized. 

Pitkanen, K., Heikinheimo, R. and Pakkanen, R. (1992) Purification and characterization of Erwinia chrysanthemi B374 pectin methylesterase produced by Bacillus subtilis. Enzyme Microb TechnoHA, 832-836. 

Erwinia chrysanthemi synthesizes and secretes a large number of pectinases. The major pectinases include a pectin methylesterase PemA and five isoenzymes of endo-pectate lyases PelA, PelB, PelC, PelD and PelE. In addition, secondary pectinases were identified a pectin methylesterase PemB, two endo-pectate lyases PelL and PelZ, an exo-pectate lyase PelX and an exopolygalacturonase, PehX. The regulation of pectinase synthesis is very complex and dependent on many environmental conditions. It is induced by pectin catabolic products and affected by growth phase, catabolite repression, osmolarity, iron or oxygen starvation... 

Recently, a gene coding for a novel pectin methylesterase, has been cloned (19). This gene, pemB, codes for a 433 amino add protein induding a N-terminal sequence of 21 amino adds which presents the characteristics of lipoprotein... 

Homology between PemA and PemB is quite low (19). Thus, it seems unlikely that the presence in E. chrysanthemi of two pern genes results firom a recent duplication of an ancestral gene as proposed for pel genes. The six regions conserved in bacterial or plant pectin methylesterases are present in PemA and PemB (19, 21). Since the central regions II, III, IV and V are more conserved than regions I and VI, they are more probable candidates to be involved in the catalytic site. 

In E. chrysanthemi, the pectate lyase, polygalacturonase and pectin methylesterase activities are induced in the presence of PGA (26, 37). The inducer is not the polymer itself but some breakdown products, initially generated by the... 

Tieman, D.M, Harriman,R.W, Ramamohan,G and Handa,A.K (1992) An antisense pectin methylesterase gene alters pectin chemistry and soluble solids in tomato fiuit. The Plant Cell. 4. 667-679. ... 

Role of pectin methylesterase in tomato fruit ripening and quality attributes of processed tomato juice... 

Among the five different species of Azospirillurn, only A. irakeme shows clearly pectinolytic activity on solid and in liquid medium. Moreover, this species can grow under non-diazotrophic as well as diazotrophic conditions when pectin is the sole carbon source (Khammas and Kaiser, 1991). Khammas and Kaiser (1991) analysed the pectinolytic activity of seven A. irakense isolates, and gave evidence for the presence of two types of pectinolytic enzymes. All strains tested have inducible Ca dependent pectate lyase activity. Six strains, showed also pectin methylesterase activity. So far, none of the corresponding enzymes have been purified. 

Figure 2. Comparison of Phaseolus vulgaris pectin methylesterase cDNA clones PE3M (cv. Masai), PE3V PE2V (cv. Verona) and genomic clone PEIV (cv. Verona). Homology boxes are printed in bold [6]. Numbers refer to the aa sequence of the full length cDNA clone PE3M.

Castaldo, D., Lovoi, A., Quagliuolo, L., Servillo, L., Balestrieri, C. and Giovane, A. 1991. Orange juices and concentrates stabilization by a proteic inhibitor of pectin methylesterase. J. Food Sci. 56 1632-1634. 

Moustacas, A.-M. Nari, J. Borel, M. Noat, G. Ricard, J. 1991. Pectin methylesterase, metal ions and plant cell-wall extension. The role of metal ions in plant cell-wall extension. Eur. J. Biochem. 279 351-354. 

Pectins from different tissue zones, namely epidermis, the outer parenchyma, the parenchyma of the Ccirpels zone, the carpels and the core line, were isolated firom alcohol-insoluble solids (AIS. In both zones of parenchyma, the cell-wall material represented about 80% of the total cell-wall material from the whole fruit. The pectins from the outer parenchyma accounted for 70% of the total. However, there was no change in galacturonic acid concentration. The enzymatic solubilisation of tissues or AIS was higher in the parenchyma zones than in the others. Nevertheless, the depolymerisation of the soluble pectins from parenchyma zones with an endopolygacturonase required the action of pectin methylesterase. The depoiymerisation of pectins from the other zones, however, did not. 

Figure 4. Content of galacturonic acid in the soiuble fraction after hydrolysis of AIS with polygalacturonases [a], with polygalacturonases and pectin methylesterase [b], with pectinases and cellulases [c].

Figure 1 indicates that pectin methyltransferase (PMT) activity from freeze-thawed microsomes measured without exogenous substrate was maximal at neutral pH (6.5 to 7.5). When exogenous pectic substrates of various DE had been added, similar optimal neutral pH was observed, and the activity was slightly stimulated (1.2 to 1.8 times). A second optimal pH occured at pH 5.5, but in the presence of low methylated pectin (DE 0.1). As suggested by Lineweaver and Ballou [8] to explain the behaviour of another pectic enzyme -i.e. pectin methylesterase (PME), the mobility and the activity of PMT might be influenced by the presence of polyanionic substrates. On the other hand, the existence of several forms of pectin methyltransferase in flax microsomes might be responsible for such variations of the activity. 

Pectin methylesterase B of Erwinia chrysanthemi, the first pectinase characterise(l as a membrane lipoprotein. 

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