Biosynthetic enzymes phenylpropanoids

Table 6.1 Citrus phenylpropanoid and core flavonoid biosynthetic enzymes...

The 4-coumarate CoA ligase (4CL EC 6.2.1.12) enzyme activates 4-coumaric acid, caffeic acid, ferrulic acid, and (in some cases) sinapic acid by the formation of CoA esters that serve as branch-point metabolites between the phenylpropanoid pathway and the synthesis of secondary metabolites [46, 47]. The reaction has an absolute requirement for Mg " and ATP as cofactors. Multiple isozymes are present in all plants where it has been studied, some of which have variable substrate specificities consistent with a potential role in controlling accumulation of secondary metabolite end-products. Examination of a navel orange EST database (CitEST) for flavonoid biosynthetic genes resulted in the identification of 10 tentative consensus sequences that potentially represent a multi-enzyme family [29]. Eurther biochemical characterization will be necessary to establish whether these genes have 4CL activity and, if so, whether preferential substrate usage is observed. 

In a recent study (54), we showed increased activities of two enzymes of the general phenylpropanoid pathway, PAL and 4-coumarate CoA lig-ase, as well as one enzyme of the specific pathway of lignin biosynthesis, cinnamy 1-alcohol dehydrogenase (CAD), in resistant plants at the time of the hypersensitive host cell death. On the other hand, decreased activities were observed at the same time with susceptible host plants (54). Furthermore, we showed that the well known increase in peroxidase activities, which is strong in resistant and only weak in susceptible plants (55-58), is at least partly due to the increased activity of the lignin biosynthetic pathway (54,59). 

Fig. 1. Simplified diagram of the phenylpropanoid and flavonoid biosynthetic pathways. Enzymes that catalyze the reactions are placed on the left-hand side, and transcription factors on the right-hand side of the arrows. Both transcription factors for which their control over the enzymatic steps has been genetically proven, as well as transcription factors that have been shown to interact with promoters of the structural genes, are shown. PAL Phenylalanine ammonia lyase C4H cinnamate 4-hydroxylase 4CL 4-coumaroyl-coenzyme A ligase CHS chalcone synthase CHI chalcone-flavanone isomerase F3H flavanone 3(3-hydroxylase DFR dihydroflavonol 4-reductase AS anthocyanin synthase UFGT UDP glucose-flavonol glucosyl transferase RT anthocyanin rhamnosyl transferase...

Fig. 11.1 Simplified diagram of the flavonoid biosynthetic pathway, starting with the general phenylpropanoid metabolism and leading to the main types of flavonoids. Only a few examples are illustrated of the large variety of flavonoids that arise through modification at different positions (not indicated or shown as R). Enzymes catalysing some key reactions are indicated by the following abbreviations PAL, phenylalanine ammonia-lyase CHS, chalcone synthase CHI, chalcone isomerase DFR, dihydroflavonol reductase F3H, flavanone 3-hydroxylase F3 5 H, flavonoid 3 5 -...

Fig. 1. Biosynthetic pathway and the enzymes involved in the biosynthesis of salicylic acid and 2,3-dihydroxybenzoic acid in plants, via the phenylpropanoid pathway and in microorganisms, via the chorismate/isochorismate pathway.

In the phenylpropanoid pathway, the first biosynthetic step leading to SA is a deamination of phenylalanine to CA which is catalyzed by phenylalanine ammonia lyase (PAL). This enzyme is induced by a range of biotic and abiotic stress conditions and is a key regulator in the phenylpropanoid pathway, which yields a variety of phenolics among others involved in structural and defense-related functions [42]. In recent years, PAL and its corresponding genes have been subject of numerous studies in various plant species [43 7]. 

Since the work of Zenk, a series of contributions has appeared, containing biosynthetic proposals. Ferulic acid is an important intermediate on t e thetic pathway to phenylpropanoids. Starting from phenylalanine, pound is formed on membrane-associated multi-enzyme complexes-cally at every intermediate stage further routes may branch off, leading o rivatives of benzaldehyde and finally to vaniUin. [150,151]... 

Fig. 1. Presumed biosynthetic pathway of glyceollin and its structural isomers in soybean. The sequence comprises the enzymes of general phenylpropanoid metabolism ...

The phenylpropanoid biosynthetic route is induced by stress. In this way, the enzymes of phenylalanine ammonia-lyase and ciimamate-4-hydroxylase are induced at transcriptional level as a defense mechanism against different damages... 

---