Steroidal glycosides substrates

Plant sterols are products of primary metabolism, but they may also be regarded as direct precursors of many secondary plant metabolites, such as the cardiac glycosides, saponins and steroid alkaloids. All of the compounds mentioned share the same basic skeleton therefore, the accumulation of a particular compound can only be achieved if (1) enzymes with a high degree of substrate specificity are involved in their biosynthesis, (2) metabolites can be channelled efficiently to the respective pathways and (3) products can be transported, sequestered and/or stored in specific compartments. 

Fig. 3.2 Classification of "metabolism ESTs. Tentative unique genes (TUGs) from each species that clustered into the metabolism category (see Fig. 3.1J were further subdivided. Here, carbohydrate incorporates all glycoside hydrolases, lipid includes lipases, B-oxidation and steroid metabolizing enzymes, protein includes predominantly proteases and their precursors, and amino acid includes all enzymes involved in the interconversion of amino acids. Esterase includes those esterase-like enzymes with unknown substrates, and other contains those TUGs that do not sort into other categories. These include predominantly oxidoreductases and purine/pyrimidine metabolizing enzymes.

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