Debranching enzyme other enzymes

A starch-debranching isoamylase, obtained from sugary 1 (sul) maize, has been cloned by James et al.70 and found to have 32% sequence identity with Pseudomonas isoamylase. Other isoamylases or starch debranching enzymes have been isolated from Cytophaga sp.,71 Streptomyces sp.,72 Flavobacterium sp.,73 a yeast, Lipomyces kononenkoae,74 potato tubers,75 B. circulans76 and an alkaline isoamylase with a pH optimum of 9 from an alkalophilic Bacillus sp.77... 

Jespersen, H. M., MacGregor, A., Henrissat, B., Sierks, M. R., and Svensson, B. 1993. Starch-and glycogen-debranching and branching enzymes prediction of structural features of the catalytic (/S/a)g-barrel domain and evolutionary relationship to other amylolytic enzymes. J. Protein Chem. 12,781-805. 

As mentioned earlier, the 2, 5 -phosphodlester bond in excised Introns is hydrolyzed by a debranching enzyme, yielding a linear molecule with unprotected ends that can be attacked by exonucleases (see Figure 12-9). The predominant nuclear decay pathway is 3 —>5 hydrolysis by 11 exonucleases that associate with one another in a large protein complex called the exosome. Other proteins in the complex Include RNA hell-cases that disrupt base pairing and RNA-proteln interactions that would otherwise Impede the exonucleases. Exosomes also function in the cytoplasm as discussed later. In addition the exosome appears to degrade pre-mRNAs that have not been properly spliced or polyadenylated. It is not yet clear how the exosome recognizes improperly processed pre-mRNAs. 

A C. reinhardtii mutant lacking D-enzyme activity has been characterized and has been shown to have significantly lower levels of starch. Other enzymes involved in starch metabolism such as ADP-Glc PPase, granule-bound and soluble starch synthase, BE, phosphorylase, a-glucosidase, amylases, and debranching enzyme activities were not affected. The starch content in the mutant was about 6-13% of wild type and there was an excessive accumulation of maltooligosaccharides up to a polymer size of 16 glucose units. 

Defects in other enzymes of glycogen metabolism also exist. Cori s disease is caused by a genetic defect in the debranching enzyme. As a result, individuals who have this disease cannot completely degrade glycogen and thus use their glycogen stores very inefficiently. 

One of the most significant items of information, particularly for glycogens and amylopectins, comes from a comparison of the effects of isoamylase and pullulanase and it may be used to classify, very readily, a polysaccharide as an amylopectin or a glycogen. It is now known that carefully isolated glycogens, undegraded during extraction, are virtually unattacked by pullulanase, " but are completely debranched by isoamylase. On the other hand, amylopectins are almost completely debranched by both debranching enzymes. ... 

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