Arabidopsis branching enzymes

Figure 6.1 Major branch pathways of flavonoid biosynthesis in Arabidopsis. Branch pathways, enzymes, and end products present in other plants but not Arabidopsis are shown in light gray. Abbreviations cinnamate-4-hydroxylase (C4H), chalcone isomerase (CHI), chalcone synthase (CHS), 4-coumarate CoA-ligase (4CL), dihydroflavonol 4-reductase (DFR), flavanone 3-hydroxylase (F3H), flavonoid 3 or 3 5 hydroxylase (F3 H, F3 5 H), leucoanthocyanidin dioxygenase (LDOX), leucoanthocyanidin reductase (LCR), O-methyltransferase (OMT), phenylalanine ammonia-lyase (PAL), rhamnosyl transferase (RT), and UDP flavonoid glucosyl transferase (UFGT).

Fisher, D. K., Gao, M., Kim, K.-N., Boyer, C. D., and Guiltinan, M. J. 1996b. Two closely related cDNAs encoding starch branching enzyme from Arabidopsis thaliana. Plant Mol. Biot. 30, 97-108. 

Schwartz, S. H., X. Q. Qin et al. (2004). The biochemical characterization of two carotenoid cleavage enzymes from Arabidopsis indicates that a carotenoid-derived compound inhibits lateral branching. J. Biol. Chem. 279(45) 46940 16945. 

Capsaicinoids are synthesized by the condensation of vanillylamine with a short chain branched fatty acyl CoA. A schematic of this pathway is presented in Fig. 8.4. Evidence to support this pathway includes radiotracer studies, determination of enzyme activities, and the abundance of intermediates as a function of fruit development [51, 52, 57-63], Differential expression approaches have been used to isolate cDNA forms of biosynthetic genes [64-66], As this approach worked to corroborate several steps on the pathway, Mazourek et al. [67] used Arabidopsis sequences to design primers to clone the missing steps from a cDNA library. They have expanded the schema to include the biosynthesis of the key precursors phenylalanine and leucine, valine and isoleucine. Prior to this study it was not clear how the vanillin was produced, and thus the identification of candidate transcripts on the lignin pathway for the conversion of coumarate to feruloyl-CoA and the subsequent conversion to vanillin provide key tools to further test this proposed pathway. 

Interestingly, Arabidopsis CHS, CHI, and F3H form a multi-enzymatic complex with each other as well as with the next enzyme in the pathway, DFR.15,16 There is a possibility that flux into different branches of the pathway could be directed through the formation of distinct complexes, with CHI playing more of a central structural role rather than a catalytic role, or maybe both and, thus, explain its essential nature in Arabidopsis.16 However, the maize CHS, CHI, and DFR enzymes can complement the Arabidopsis tt4, tt5, and tt3 mutations, respectively, despite the modest sequence identity between the monocot and dicot enzymes.17 More studies are necessary to establish whether the formation of a flavonoid metabolon is a curiosity of Arabidopsis or a general property of this biosynthetic pathway. 

In the second approach, herbicide-resistance mutations in the Arabidopsis ALS gene were studied in E. coli. To do this, wild type and mutant Arabidopsis genes were functionally expressed in E. coli, such that the plant genes complemented a branched chain amino acid auxotrophy in the bacteria (Smith et al. 1989, PNAS in press). ALS enzyme assays on extracts prepared from E. coli expressing the mutant Arabidopsis gene indicated that the mutant enzyme is resistant to sulfonylurea herbicides but is sensitive to the imidazolinone herbicide imazaquin. This selective... 

Comprehensive studies are now required, however, to determine the feedback properties of all of the enzymes leading to Phe (1)/Tyr (2) biosynthesis, including the individual Arabidopsis ADTs and the yet to be provisionally identified aminotransferases both in vivo and in vitro. This is a particularly interesting and important question, considering that feedback-insensitive isoforms exist for the other branch point enzymes DAHP synthase, CM, and AS. 

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