Biosynthetic enzymes primary-secondary metabolism

This approach assumes some knowledge, or at least some presumption, of the general biosynthetic pathway by which the metabolite is formed. It is usually possible to make a good guess at the biosynthetic origins of a molecule, particularly when biosynthetic information is available on a structurally similar molecule, and these theories can be tested by the use of isotopically labeled precursors. It also relies on the fact that the biosynthetic enzymes are not so specific as to exclude all but the natural precursor. Fortunately, the enzymes of secondary metabolism tend to be less specific than those of primary metabolism. 

To summarize, it is possible to make use of the various biosynthetic, secondary metabolic, and primary metabolic pathways and enzymes associated with particular organisms by giving them the natural product starting matenal to work with (see Note 7). 

It is generally assumed that many metabolic enzymes do not function in isolation, but interact physically, or are in close proximity with, other enzymes that partieipate in eommon pathways (227 28). Structures composed of interacting proteins involved in specific metabolic processes are known as metabolic complexes, metabolie ehannels, metabolons, or multi-enzyme complexes. A metabolic channel is a eomplex of two or more enzymes that facilitates the direct transfer of biosynthetic intermediates between the catalytic sites of sequential enzymes in a biosynthetic pathway. Theoretically, metabolic channels provide several advantages that promote efficient eellular metabolism. The direct transfer of a pathway-intermediate from one enzyme to another maintains a high local substrate concentration, which avoids the dilution of intermediates released into the cytoplasm. Enzyme complexes also eliminate competition from other enzymes for the same substrate, increase the stability of intermediates, and minimize the deleterious effects of cytotoxic intermediates. Several investigations of the interactions between pathway enzymes have hinted at the importance of metabolic channels in primary and secondary metabolism (228,229). 

The biosynthetic pathways are universal in plants and are responsible for the occurrence of both primary metabolites (carbohydrates, proteins, etc.) and secondary metabolites (phenols, alkaloids, etc.). Secondary compounds were once regarded as simple waste products of a plant s metabolism. However, this argument is weakened by the existence of specialist enzymes, strict genetic controls and the high metabolic requirements of these componnds (Waterman and Mole 1994). Today most scientists accept that many of these componnds serve primarily to repel grazing animals or destrnctive pathogens (Cronquist 1988). 

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