Plants, common metabolic reactions

Figure 5. The Most Common Metabolic Reactions in Plants...

The enzymes most commonly studied In plant defense reactions can be divided Into two major groups (lytic enzymes and enzymes associated with plant phenolic metabolism) and a lesser third group of generally unrelated enzymes (Table T). Discussion of some of the salient Information regarding the roles and action of these enzymes when plants are Infected by pathogens follows. Instances where these enzymes are thought not to play roles In plant defense are also presented. 

It is evident from the scheme shown in Fig. 3.21 that the chemical structures of pesticides are quite diverse they undergo various physico-chemical effects in the environment after application (solar radiation, heat, air, soil, water) as well as being subjected to various metabolic transformations in plants, microorganisms, insect and animals. Common metabolic transformations are schematically surveyed in Table 3.20, which shows that primarily oxidation, hydrogenation, reduction and hydrolytic reactions are concerned. Also among the individual chemical compounds mutual chemical reactions take place. Some examples of photochemical reactions of pesticides in water are presented in Figs 3.22 to 3.26. Biochemical reactions of DDT and DDE are shown in Fig. 3.27. The number of individual chemical species is hence significantly multiplied in the hydrosphere due... 

Fatty acids with odd numbers of carbon atoms are rare in mammals, but fairly common in plants and marine organisms. Humans and animals whose diets include these food sources metabolize odd-carbon fatty acids via the /3-oxida-tion pathway. The final product of /3-oxidation in this case is the 3-carbon pro-pionyl-CoA instead of acetyl-CoA. Three specialized enzymes then carry out the reactions that convert propionyl-CoA to succinyl-CoA, a TCA cycle intermediate. (Because propionyl-CoA is a degradation product of methionine, valine, and isoleucine, this sequence of reactions is also important in amino acid catabolism, as we shall see in Chapter 26.) The pathway involves an initial carboxylation at the a-carbon of propionyl-CoA to produce D-methylmalonyl-CoA (Figure 24.19). The reaction is catalyzed by a biotin-dependent enzyme, propionyl-CoA carboxylase. The mechanism involves ATP-driven carboxylation of biotin at Nj, followed by nucleophilic attack by the a-carbanion of propi-onyl-CoA in a stereo-specific manner. 

The oxidation of aciy lic acid can be rationalized in terms of the endogenous catabolism of propionic acid, in which acrylyl coenzyme A is an intermediate. This pathway is analogous with fatty acid 3-oxidation, common to all species and, unlike the corresponding pathway in plants, does not involve vitamin 8,2. 3-Hydroxypropionic acid has been found as an intennediate in the metabolism of acrylic acid in vitro in rat liver and mitochondria (Finch Frederick, 1992). The CO2 excreted derives from the carboxyl carbon, while carbon atoms 2 and 3 are converted to acetyl coenzyme A, which participates in a variety of reactions. The oxidation of acrylic acid is catalysed by enzymes in a variety of tissues (Black Finch, 1995). In mice, the greatest activity was found in kidney, which was five times more active than liver and 50 times more active than skin (Black et al., 1993). 

Microbes and plants synthesize aromatic compounds to meet their needs of aromatic amino acids (L-Phe, L-Tyr and L-Trp) and vitamins. The biosynthesis of these aromatics [69] starts with the aldol reaction of D-erythrose-4-phosphate (E4P) and phosphoenolpyruvate (PEP), which are both derived from glucose via the central metabolism, into DAHP (see Fig. 8.13). DAHP is subsequently converted, via a number of enzymatic steps, into shikimate (SA) and eventually into chorismate (CHA, see later), which is the common intermediate in the biosynthesis of the aromatic amino acids [70] and vitamins. 

The mammalian alkaloids are formed from aromatic amino acids and their metabolically derived amines by reaction with carbonyl substrates at physiological pH. The reaction is catalyzed by acid and is commonly referred to as the Pictet-Spengler cyclization (I0a,b,II). Winterstein and Trier in 1910 proposed that the Pictet-Spengler reaction might be of significance in the biosynthesis of benzylisoquinoline alkaloids in plants (5a). The carbonyl compounds participating in the Pictet-Spengler synthesis of mammalian alkaloids are aldehydes and a-keto acids, which are produced... 

---