Arabinosyl transferase enzyme

The antimicrobial action of ethambutol, like that of isoniazid, is specific for mycobacteria, suggesting a target in the unique components of the mycobacterial cell wall. Cells treated with ethambutol accumulate an isoprenoid intermediate, decaprenyl-arabinose which is the source ofarabinose in the arabinogalactan polymer. This suggests that ethambutol blocks assembly of the arabinogalactan through inhibition of an arabinosyl transferase enzyme. 

Ethambutol is a water-soluble, heat-stable compound that acts by inhibition of arabinosyl transferase enzymes that are involved in cell wall biosynthesis. Nearly all strains of M tuberculosis and M. kansasii and most strains of Mycobacterium avium-intracellulare are sensitive to ethambutol. Drug resistance relates to point mutations in the gene (EmbB) that encodes the arabinosyl transferases that are involved in mycobacterial cell wall synthesis. 

Besides stimulating the production and release of certain hydrolases, GA can also retard enzyme activity. Consistent with the observation that GA induces degradation of aleurone cell walls is the observation that GA inhibits the synthesis of pentosan (arabinose and xylose) components of the wall. Between 4 and 16 h after GA addition to isolated aleurone tissue there is a marked decline in pentosan biosynthesis, in part due to the reduced activity of a membrane-bound arabinosyl transferase, and perhaps also of xylosyl transferase [62]. Changes in such membrane-bound enzymes could reflect a shift in the role of membranes from the production of cell wall components to the production, or packaging of hydrolases. 

A large number (>50) of glycosyltransferases and other types of transferase are required to synthesize pectic polysaccharides [20]. The activities of several glycosyltransferases have been identified in cell-free membrane preparations from plants. However, only a few of these enzymes have been partially characterized and shown to be involved in pectin biosynthesis. These include D-galacturonosyl transferase, galactosyl transferase, arabinosyl transferase, and apiosyl transferase [20]. An enzyme that catalyzes the methylesterification of homogalacturonan (homogalatur-onan methyltransferase) has been partially characterized [20]. There are also reports that plants contain methyltransferases that catalyze the methylesterification of RG-I and RG-II [25]. No 0-acetyl transferase involved in pectin biosynthesis has been characterized nor have the enzymes that catalyze the addition of phenolic acids to pectins [20]. 

---