Adenylate isopentenyltransferases

Origin of aromatic cytokinins and metabolic pathways participating in their biosynthesis has not yet been identified and a possible implication of adenylate isopentenyltransferase has not been examined. 

Bios5mthetic pathways of naturally occurring cytokinins are illustrated in Fig. 29.5. The first step of cytokinin biosynthesis is the formation of A -(A -isopentenyl) adenine nucleotides catalyzed by adenylate isopentenyltransferase (EC 2.5.1.27). In higher plants, A -(A -isopentenyl)adenine riboside 5 -triphosphate or A -(A -isopentenyl)adenine riboside 5 -diphosphate are formed preferentially. In Arabidopsis, A -(A -isopentenyl)adenine nucleotides are converted into fraws-zeatin nucleotides by cytochrome P450 monooxygenases. Bioactive cytokinins are base forms. Cytokinin nucleotides are converted to nucleobases by 5 -nucleotidase and nucleosidase as shown in the conventional purine nucleotide catabolism pathway. However, a novel enzyme, cytokinin nucleoside 5 -monophosphate phosphoribo-hydrolase, named LOG, has recently been identified. Therefore, it is likely that at least two pathways convert inactive nucleotide forms of cytokinin to the active freebase forms that occur in plants [27, 42]. The reverse reactions, the conversion of the active to inactive structures, seem to be catalyzed by adenine phosphoiibosyl-transferase [43] and/or adenosine kinase [44]. In addition, biosynthesis of c/s-zeatin from tRNAs in plants has been demonstrated using Arabidopsis mutants with defective tRNA isopentenyltransferases [45]. 

Another derivative is the zeatin. Zeatin belongs to the plant-growth hormones, cytokinins. Cytokinins are adenine derivatives with isoprenoid side chain [108, 109]. The rate-limiting step of tra/is-zeatin biosynthesis is catalyzed by adenylate-isopentenyltransferase (cytokinin synthase) [110]. The plant enzyme (EC 2.5.1.27) utilize ADP, which is different from the microbial one (EC 2.5.1.112) which recognizes ATP/or ADP [111, 112]. Afterward, the adenylate-isopentenyl converts to the active tra/is-zeatin by a single hydroxylation reaction (cytokinin tra/is-hydroxylase). On the opposite site, cis-zeatin biosynthesis starts with a different transferase, tRNA-dimethylallyl-transferase (EC 2.5.1.75) [113-116], which mediates the transfer of an isopentenyl group of DMAPP to adenine of tRNAs. 

The second and main biosynthetic pathway of cytokinins in plants is de novo synthesis from phosphorylated adenosine and a precursor of the side chain. Fig. (2). The first and for a long time the only known genes encoding cytokinin de novo synthesizing enzyme, adenylate dimethylallyltransferase (EC 2.5,1.27) also called isopentenyltransferase... 

---