Metabolism connections between pathways

Nevertheless, there are many questions still open because of problems to detect enzyme activities corresponding to each step of the pathway. The model of biosynthesis pathway was put together by studying the metabolism of exogenously applied intermediates in cell cultures of various origins and combining these results with data of native brassinosteroid patterns. It is more or less accepted that there are three pathways in parallel, the early and the late C6 oxidation pathway, as well as the 24/ -epimers follow ing the same route. Some observations in the analysis of native brassinosteroid patterns suggest a possible connection between the pathways. It was shown that seeds of Arabidopsis contain castasterone and 24-epi-brassinolide [34]. Also members of both 24-epimers, brassinolide and 24-epi-brassinolide were detected in tomato seeds [Winter, unpublished]. 

The fact that there are these severe disease entities which originate from the lack of both factors—and can be prevented by either of them—indicates that there must be close connections between the functions of these two agents in intermediary metabolism. The possibility that alternate pathways of metabolism, for instance, electron transfer, are catalyzed by the two factors has been discussed in previous publications (Schwarz, 1958a). One may also ask whether the effect of Factor 3/selenium may not be primarily associated with the requirements of the cell wall for energy or active transport. Any working hypothesis has to take into consideration... 

The vitamins included under this heading are all soluble in water and most of them are components of coenzymes (see Table 5.3). Although the mechanism of action in this role is known, the connection between the observed deficiency symptoms and the failure of the metabolic pathways is not always clear. 

Fig. 5.3 Examples of connections between epigenetics and metabolic pathways. (Abbreviations a-KT a-ketoglutarate AcCoA acetyl coenzyme A AcsCSl acetyl-CoA synthase 1 ACL ATP-citrate lyase ETC electron-transport chain FAD flavin adenine dinucleotide GSH glutathione IDH isocitrate dehydrogenase LDH lactate dehydrogenase NAD nicotinamide adenine dinucleotide SAM5-adenosyl methionine TCA tricarboxylic acid cycle)...

Flavors are formed from major plant constituents under genetic control. Each metabolic pathway is connected to other metabolic pathways. As direct products of a metabohc pathway, or as a result of interactions between pathways or end products, a host of volatile compounds are produced which contribute to the flavor (aroma) of a ripe fruit. The precursors of some aroma compounds are presented in Table 4.1 [7],... 

Amplification of the oxidation-reduction potential is not the only cause for diversity of secondary metabolites in angiosperms. In order to understand the phenomenon more fully it is necessary to remember also the best known of the evolutionary trends the gradual substitution of woody forms by herbaceous ones that operate in several lineages. Reduced utilization for the production of lignins and condensed tannins should cause initially a surplus of cinnamate, raw material for the biosynthesis of allelochemicals of primitive angiosperms. The continuing decrease in the importance of the shikimate pathway and the connection between primary, and secondary metabolism requires that the transition from the woody to the herbaceous habit be accompanied in the long run by a curtailment of shikimate precursors for the biosynthesis of allelochemicals. Let us look now into the evidence for these postulates. 

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