Isopentenyl-AMP

In some cases, an isoprene residue is used as an element to modify molecules chemically. One example of this is N -isopentenyl-AMP, which occurs as a modified component in tRNA. 

This enzyme [EC 2.5.1.27], also called 2-isopentenyl-diphosphate AMP A -isopentenyltransferase and cyto-kinin synthase, catalyzes the reaction of AMP with A -isopentenyl diphosphate to yield pyrophosphate and A -(A -isopentenyl)adenosine 5 -monophosphate. 

In spite of their biological and agricultural importance, only in the last decade the basic molecular mechanism of their biosynthesis and signal transduction have been elucidated. The first step in the isoprenoid CK biosynthesis is the M-prenylation of adenosine-5 -phosphate (AMP, ADP, or ATP) with DMAPP or hydroxymethylbutenyl diphosphate (HMBDP) catalyzed by adenosine phosphate-isopentenyltransferase (IPT). Plant IPTs preferentially utilize ATP or ADP as the isoprenoid acceptors to form isopentenyl riboside 5 -triphosphate (iPRTP) and... 

The low affinity of the AtIPTs for AMP implies that most of the natural isopentenyl riboside 5 -monophosphate (iPRMP) is formed by dephosphorylation of iPRDP and iPRTP, phosphorylation of isopentenyl riboside (iPR) by adenosine kinase, and conjugation of a phosphoribosyl moiety to iP by adenine PT [266]. Arabidopsis CYP735A1 and CYP735A2, cytocrome P450 monooxygenases, encode CK hydroxylases that catalyze tZ biosynthesis via the iPRMP-dependent pathway. The reduction of the double bond in the tZ side chain, catalyzed by a zeatin reductase, forms 6-(4-hydroxy-3-methylbutylamino)purine, whose trivial name is DZ. cZ and tZ can be enzymatically interconverted by zeatin cis-trans isomerase. 

Taya et al. (1978) have reported that a cell-free system from the slime mold Dictyostelium discoideum catalyzes the condensation of DMAPP with AMP to produce isopentenyl adenine, a cytokinin. (An organic chemist would probably call this dimethylallyl adenine, but both terms are correct, and isopentenyl adenine is firmly established in the plant physiology literature.) They have emphasized that this is a direct synthesis, not involving nucleic acid breakdown, and suggest that the same system may operate in higher plants. 

The decrease of resonance causes bond polarization. The stronger this polarization, the easier electrophilic or nucleophilic substitution of a substance or the greater its capacity to substitute. It is, therefore, reactive and is termed activated . Activated phosphates are, for example, acyl phosphates and acyl AMP derivatives (C 1.2), sugar phosphates (C 6), and isopentenyl pyrophosphate (D 6). 

Alkylations and other types of transformation may, however, also proceed with free purines. In Dictyostelium for instance, dimethylallyl pyrophosphate condenses with AMP to produce isopentenyl adenine. In mammalian tissue 3-methyladenine is formed from adenine, and in plants theobromine and caffeine... 

Experiments demonstrating incorporation of radioactive precursors, commonly adenine, into cytokinins in plant tissues have been used to examine the de novo pathway of cytokinin biosynthesis [15,16,22 and references therein]. Some of these in vivo labelling experiments have highlighted the importance of cytokinin nucleotides as the primary products of biosynthesis [19]. Further confirmation for this proposal was provided by the in vitro work which demonstrated that a cell free preparation from a slime mold catalyzed the formation of iPA 5 -monophosphate from 5 -AMP and IPP [26]. Neither adenine nor adenosine acted as a substrate for the enzyme, A--IPP AMP-A--isopentenyl transferase, a prenyl transferase commonly termed cytokinin synthase. The enzyme has subsequently been purified from a number of higher plant sources [7, 11]. In addition it has been established that cytokinin (and auxin) production in crown gall tissues is under the control of T-DNA, and genes have been identified which directly code for the production of these phytohormones [17]. 

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