Substituent-cleaving enzymes

Since many proteases are specific in cleaving peptide linkages adjacent to substituent groups, we decided to prepare substituted polymers, anticipating that the introduction of the substituents would make the polymers more degradable. Methylated, benzylated, and hydroxylated polymers were prepared and their biodegradations have been studied. Compared to the unsubstituted polymers we found the substituted polymers more susceptible to attack by enzymes and microorganisms. (4, 6)... 

Trichoderma (9-10). Much less is known about the concurrent production of the enzymes which cleave substituent groups of the xylan polymer. The presence of acetyl xylan esterases (11,12) and a-glucuronidases (13-15) in xylanolytic enzyme systems has only recently been pointed out. Although a-arabinosidases have mainly been studied as arabinan-degrading enzymes (16), they have also been shown to release arabinose from xylans (17). 

Biotransformation can he viewed as a toolbox to produce polysaccharides with targeted properties [7-10]. One can use hydrolase enzymes to lower the molecular weight. One can use enzymatic reactions to add or modify a substituent on a polysaccharide. One can add a monosaccharide or a second polysaccharide to produce a larger molecule. Some polysaccharides contain proteins or fatty acids, and these can he cleaved or otherwise altered to produce different properties. 

First attempts to predict the selectivity of enzymes are dated back to 1964 when Prelog described an empirically determined rule for the addition of hydrogen to ketones by the yeast Culvaria lunata [13]. In 1991 Kazlauskas published the hydrolysis of acetates of secondary alcohols by Pancreatic cholesterol esterase, Pseudomonas cepacia and Candida rugosa and formulated the widely applicable Kazlauskas rule according to which esters of secondary alcohols with a specific substitution pattern of large (L) and medium (M) substituents are cleaved faster than the corresponding enantiomer [14]. 

A variety of pyrimidine dimer-cleaving photochemical model systems have been developed to aid in the study and elucidation of the DNA-PL reaction mechanism (752, 158, 159). Direct excitation of pyrimidine dimers does not occur on ultraviolet or visible irradiation, suggesting that enzyme cofactors might be involved in a photosensitization process 146). Model systems utilizing free flavin derivatives have recently been described 137, 160). Rokita and Walsh have demonstrated that lumiflavin, 5-deazariboflavin, and 8-methoxy-7,8-didemethyl-7V °-ethyl-5-deazaflavin (Scheme 31) are effective photosensitizers for the thymine dimer cleavage reaction 160). These reactions utilized cis-syn-thymine dimer as substrate with irradiation at the Xmax of the flavin derivative, under strict anaerobic conditions and high pH. Flavin derivatives that contain electron-rich substituents at the 8-position, such as 8-hydroxy-5-deazariboflavin,... 

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