Adenosine nucleosidase

Fig. 1. Ethylene biosynthesis. The numbered enzymes are (1) methionine adenosyltransferase, (2) ACC (l-aminocyclopropane-l-carboxylic acid) synthase, (3) ethylene forming enzyme (EFE), (4) 5 -methylthio-adenosine nucleosidase, (5) 5 -methylthioribose kinase. Regulation of the synthesis of ACC synthase and EFE are important steps in the control of ethylene production. ACC synthase requires pyridoxal phosphate and is inhibited by aminoethoxy vinyl glycine EFE requires 02 and is inhibited under anaerobic conditions. Synthesis of both ACC synthase and EFE is stimulated during ripening, senescence, abscission, following mechanical wounding, and treatment with auxins.

Cytokinins, N6-substituted derivatives of adenine, are important in the regulation of many processes in plant tissues. Chism et al. (1984) reported an HPLC assay measuring the hydroxylation of 6-(3-methylbut-2-enylamino)purine (IPA) and 6-(3-methylbut-2-enylamino)-8-hydroxypurine by cytokinin nucleosidases and adenosine nucleosidases. 

Nucleotidase [15] followed by adenosine nucleosidase [16] are expected to be the enzymes responsible for the step-by-step conversion of the cytokinin nucleotide to the base iPA. Both of these reactions may proceed also in the opposite direction, and in this case they are catalysed by adenosine phosphorylase (ribosylation of iPA, [17]) and adenosine kinase (phosphorylation of iPAR, [18-20]). These enzymes are common in the mutual conversions of adenine and purine metabolites (reviewed in [21]) and their properties have been summarised by [22]. These enzyme activities seem to be the key for understanding the fate of C-labelled adenine (Ade) and adenosine (Ado) in feeding experiments [summarised by 23]. 

Generally, all conversions in the biosynthetic direction, i.e. iPARMP— iPAR— iPA (catalysed by 5 -nucleotidase, (EC 3.1.3.5), and adenosine nucleosidase, (EC 3.2.2.7), respectively, c/. Fig. 2) may also proceed in the opposite direction, i.e. base-— nucleoside — nucleotide (catalysed by adenosine phosphorylase and adenosine kinase, respectively). All these enzymes require both Ade and iPA or Ado and iPAR, respectively, as substrates. They were characterised in wheat germ [15-18] and lupin seeds [19]. Interestingly, no K, -constants were reported for Z-type cytokinins (see summary in [22]). However, as seen in H-labelled Z-derivatives feeding experiments, Z-type cytokinins are also interconverted in a similar way [82,121,122]. Moreover, the specificity of these enzymes is not too strict with respect to the side chain configuration and one may speculate that this complex may function for most if not all native cytokinins [21,81]. 

The equilibrium of Eq. (12) is strongly in favor of AdoHcy synthesis (de la Haba and Cantoni, 1959 Poulton and Butt, 1976 Guranowski and Pawelkiewicz, 1977). However continuing hydrolysis of AdoHcy can proceed when the reaction products are maintained at low concentrations (de la Haba and Cantoni, 1959 Poulton and Butt, 1976). Homocysteine can be converted to methionine by tetrahydropteroyltriglutamate methyltransferase (Section III,B). Adenosine can be converted to adenine by adenosine nucleosidase (E.C. 3.2.2.7), the activity of which closely correlates with that of... 

Figure 6.63 Isoprenoid cytokinin biosynthesis pathway in Arabidopsis. IPT, phosphate-isopentenyl transferase AK, adenosine kinase 1, phosphatase 2,5 -ribonucleotide phosphohydrolase 3, adenosine nucleosidase 4, purine nucleoside phosphorylase 5, zeatin cis-trans isomerase and 6, zeatin reductase.

Fig. 29.2 Caffeine is produced from xanthosine derived from four routes (1) inosine-5 -monophosphate (IMP) originating from de novo purine synthesis (de novo route), (2) adenosine released from the 5-adenosyhnethionine (SAM) cycle, (3) the cellular adenosine nucleotide pool (AMP route), and (4) the guanine nucleotide pool (GMP route). Enzymes AMPDA AMP deaminase, APRT adenine phosphorihosyltransferase, AK adenosine kinase, ARN adenosine nucleosidase, GRD guanine deaminase, IMPDH IMP dehydrogenase, 5 NT 5 -nucleotidase...

Data yielded from this type of experiment are shown in Figure 4-18. The same type of assay may also be used for 5 -nucleosidase catalyzing the reaction AMP-------- adenosine + P,. 

The AMP nucleosidase reaction was studied by TS analysis with and without adenosine 5 -triphosphate (ATP) as an allosteric activator (Table An... 

Transition-state structures computed from multiple kinetic isotope-effect, data have provided important information on the catalytic mechanisms of glycosylases. This is exemplified by the refined transition-state structures derived by Schramm and his associates and used to probe the hydrolytic reactions catalyzed by nucleosidases and the ADPR (adenosine diphosphori-bosyl) transferase toxins of Vibrio cholerae and Corynebacterium diphthe-... 

Similarly, adenosine is a true inducer of purine nucleosidase. But at a level of 0.5% its repressing effect is great enough to prevent the appearance of enzyme. Use of ribonucleic acid (RNA) or of the phosphate ester of adenosine (AMP) by the fungus leads to a low but continuous supply of adenosine and a resultant high enzyme concentration (Reese, 1968). 

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]. 

Schramm, V. L., 1974, Kinetic properties of allosteric adenosine monophosphate nucleosidase from Azotobacter vinelandii, J. Biol. Chem. 249 1729. 

H. NAD Nucleosidase. NAD nucleosidase, or sometimes simply called NADase, is present in some snake venoms. It hydrolyzes the nicotinamide V-ribosidic linkage of NAD. The products of this reaction are nicotinamide and adenosine diphosphate ribose. 

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