Endo-enzymes

Enzyme.—Polygalacturonase (PG) was obtained from a culture of Rhizopus nigricans using Citrus pectin as carbon source [11,12], The enzyme used for oligouronides obtention was the residual activity after a thermal treatment (100°C, 60 sec) of the native Rh. nigricans endo and exoPG, since the endo-enzyme was thermoresistant while the exo-enzyme was thermolabile [13]. 

In principle, linker groups are polymer-enlarged versions of blocking functions used in regular solution-phase chemistry. Therefore, enzymatic transformations that may be employed for the removal of protecting groups in solution, in principle may also open up alternative opportunities for releasing compounds from polymeric supports. The linkers developed so far can be divided into exo- and endo-linkers (Fig. 10.1) cleavable by exo- respectively endo-enzymes, as proposed by Flitsch et al. [6]. 

Galacturonanases in the parenchyma have been shown to be wall-bound in peach (both endo- and exo-enzymes),625 apple (exo-enzyme only),621 tomato (endo-enzyme),499 and pear (unspecified activity).641 Increases in the level of polygalacturonase activity during ripening have been demonstrated in tomato,499,642-644 avocado,615,633 peach645 date,636 cucumber,631,646 and pear,641 and, in tomato,644 avocado,615 and cucumber,631 this increase is associated with the respiratory climacteric following a transient burst of ethylene production. In the mutant rin tomato fruit, which does not ripen, this increase in galacturonanase does not occur.644... 

Enzymes depolymerizing polysaccharides may have an endo or an exo action pattern, and may hydrolyze, or cleave by elimination. Both the conformation of the polysaccharide and the active site of the enzyme need to be considered in the enzyme-glycan interaction. endo-Enzymes split, by a random type of depolymerization, glycosidic bonds situated internally in... 

The endo-enzymes attack polysaccharide chains internally, with decreasing activity as chain length rises. Endoxylanases (EC 3.2.1.8) are specific for /3-(l —>4)-xylopyranose polymers, (i.e., the back-... 

All catalyze depolymerization by a beta-elimination reaction like that which occurs during base-catalyzed depolyraerization. Pectin lyases, all of which are endo-enzymes, catalyze beta-eliminations at esterified D-galacturonlc acid units (See Figure 1) (68). Pectate lyases catalyze beta-eliminations at nonesterifled D-galacturonic acid units. Both exo- and endo-pectate lyases are T[Pg.10]

One particularly difficult task has been the detection of endo-acting enzymes in preparations of exo-enzymes. Enzymes of the latter type are particularly useful in structural analysis for example, in the removal of all nonreducing end-residues in a polysaccharide susceptible to the action of the enzyme. When chains of susceptible residues are present, these are degraded by the endwise action of the enzyme until a non-susceptible linkage or residue is encountered. In either instance, measurement of the degree of hydrolysis may give useful results. However, the presence of an endo-enzyme is likely to make such measurements meaningless, as its lack of requirement for a chain end may render some residues susceptible to action of exo-enzyme, for example, residues in the interior chains or backbone of a polysaccharide, which otherwise would not have been attacked. Even traces of endo-enzyme are sufficient to create difficulties. 

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