Splitting of glycosidic bonds

Besides ester formation another effect of phosphoric acid is hydrolytic splitting of glycosidic bonds. This effect is more pronounced for celluloses with a higher DP and depends also on the amount of acid used for impregnation and the pretreatment temperature. 

Figure 2 Splitting of glycosidic bonds in pectin by hydrolysis (polygalacturonase) and by -elimination (pectate lyase and pectin lyase).

The mechanism of action of alkylating agents is complex. Adenine and guanine are easily alkylated. Guanine is alkylated primarly at position 7 and adenine at position 3. The reaction produces an exceedingly labile glycosidic bond. Splitting of this bond leads to depurination. 

The group of pectic enzymes includes pectinesterase, which catalyzes the de-esterification of pectin, and depolymerizing enzymes which catalyze the splitting of glycosidic a-( 1 — 4) bonds of the D-galacturonan chain of the pectin molecule. 

In the course of degradation of nonesterified and low-esterified substrates by pectate lyases, the glycosidic bonds can be split either at random or terminally, starting from the reducing end of the molecule. On the other hand, in the course of splitting of highly esterified substrates by pectin lyases, only the random-action pattern is operative. So far, no pectin lyase is known that can split the glycosidic bonds terminally. 

For endopectate lyases, the rate of cleavage of glycosidic bonds and the affinity of the enzyme for the substrate depend on the chain length,240,241 as with endo-D-galacturonanase. The frequency of splitting of bonds 2 and 3 in tetra(D-galactosiduronic acid) is different. Endopectate lyase of Bacillus polymyxa splits240 bond 3 1.4 times faster than bond 2. 

This enzyme is also known as trans-elimi-nase it splits the glycosidic bonds of a glu-curonide chain by trans elimination of hydrogen from the 4- and 5-positions of the glucuronide moiety. The reaction pattern is presented in Figure 10-8. The glycosidic bonds in pectin are highly susceptible to this reaction. The pectin lyases are of the endo-type and are obtained exclusively from fila-... 

Purine nucleotides are modified by a variety of enzymes, including those that attack the phosphate group and those that split the glycosidic bond. In addition to these, there are changes in the bases in which the amino groups of adenylic acid and guanylic acid are removed by specific deaminases. With these cases excepted (i.e., the formation of inosinic and xanthylic acids), degradation of purines occurs after hydrolysis to yield the free base. 

Acid hydrolysis of the coenzymes splits the glycosidic bond to the bases, as well as the pyrophosphate bridge. The products are thus nicotinamide, ribose-5-phosphate, and adenine. 

Table I lists the bond cleavage frequencies for PGII. G4 is exclusively split in 1-3 mode while reduced G4 was not hydrolysed. G5 is cleaved in the 1-4 and 2-3 mode at 67 % and 33 % respectively, while reduced G5 is only split into reduced G2 and G3. The reduced G6 is not cleaved in the 1-5 mode, while reduced G2 and reduced G3 are readily formed. The non-reduced G6 is cleaved in 1-5, 2-4 and 3-3 modes yielding equimolar product pairs as was also seen for the cleavage of G4 and G5. These data demonstrate that cleavage of the glycosidic bond occurs from the reducing end.

For three endo-D-galacturonanases, isolated from the culture filtrate of Coniothyrium diplodiella,114-116 that differed in the extent of degradation of sodium pectate, a 50% decrease of viscosity of the substrate solution corresponded to the splitting of 3, 4, and 10% of the glycosidic bonds, respectively. A similar difference was found between two endo-D-galacturonanases isolated from tomatoes117 one... 

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