Inosine residues

Cyclic Dimer Configurations. In 8-bromoinosine [BRINOSlOj the inosine residues are self-associated (Fig. 17.8 a). This is a rare example of self-association in the crystal structures of the nucleosides. Surprisingly, in uridine [BEURID10] (Fig. 17.8b) the CH- 0=C hydrogen bond plays the same role as the NH 0=C interaction in 8-bromoinosine. 

The signal of the quaternary carbon atom C-4 of inosine can be easily distinguished from that of C-2 by proton off-resonance decoupling. Comparison of the spectrum of inosine with the spectral data of 8-deuterioinosine and 6-thioinosine leads to the unequivocal signal assignment of the base residue of this nucleoside [749] (see Table 5.22). 

The phosphodiester bonds of xanthylic acid in deaminated RNA were scarcely split by RNase U2 (30). The susceptibility of purine nucleotide residues to RNase U2 decreases in the order of A>G>I X, indicating that the phosphodiester bonds of adenylic acid and inosinic acid without a keto group at the position of purine base are more sensitive to RNase U2 than those of guanylic acid and xanthylic acid. The resistance of TNP-RNA to RNase U2 may be also attributed to the steric hindrance by a larger substituent at 2-amino groups of guanylyl residues, as with RNase T, (SO). 

Phosphorylation of XLVII with phosphorus oxychloride in pyridine solution, followed by hydrolysis to remove the methyl and isopropylidene residues, gave D-ribose 5-phosphate (XLVIII) which, as its barium salt, was found to be identical with the barium salt of the D-ribose phosphate from inosinic acid. By way of further confirmation of the structure of D-ribose 5-phosphate, Levene, Harris and Stiller129 showed that in methanolic hydrogen chloride solution both the natural and synthetic material mutarotated in a manner characteristic of a sugar which can form only a furanoside. 

IMP consists of inosine to which a phosphate group is attached at C-5 of the ribose residue. Thus, its structure is... 

In inosine, the NH2 at C(2) on guanine is removed, thereby reducing the hydrogen-bonding capability by a donor group with two functional hydrogens. 8-Bromoinosine is a rare example where the self-association of the purine residues occurs in the crystal structure of a nucleoside. 

On phosphorylation of isopropylidene-inosine, followed by hydrolysis of the acetone residue, muscle inosinic acid (which, as will be seen later, is definitely known to be 5-phospho-inosine) is formed. This confirms the above formulations for isopropylidene-inosine and inosine. 

Robert Holley first determined the base sequence of a tRNA molecule in 1965, as the culmination ul 7 years of effort, Indeed, his study of yeast alanyl-tRNA provided the first complete sequence of any nucleic acid. This adapter molecule is a single chain of 76 ribonucleotides (Figure 30.2). The 5 terminus is phosphorylated (pCi), whereas the 3 terminus has a free hydroxyl group. T he amino acid-attachment site is the 3 -hydroxyl group of the adenosine residue at the 3 terminus of the molecule. The sequence 5 - IGC-3 in the middle of the molecule is the anticodon, where I is the purine base inosine. It is complementary to 5 -GCC-3, one of the codons for alanine. 

Fig. 5.6. Quantitative primer extension analysis of RNA editing after RT-PCR. The illustration shows an A to inosine (I) conversion, but the method can be used for any base change. The RT-PCR will convert the inosine to a G. A 32P end-labelled (asterisk) primer is annealed 3 of the edited residue at a distance without any G s between the 3 -end of the primer and the edited site. Extension in the presence of ddCTP, dATP, dTTP and dGTP produces two bands depending on whether the transcript is edited or nonedited.

The structure of purine ribonucleosides has recently been studied by proton-magnetic resonance, and the conformation of the D-ribofuranosyl residue in adenosine and inosine has been determined by an analysis of proton-magnetic resonance data C-2 is considered to be out of the plane defined either by C-1, 0, and C-4 or by C-1, 0, C-3, and C-4 and is pointing on the same side as the C-4—C-5 bond. A similar study of deoxy-ribonucleosides suggests that the ring-oxygen atom and, possibly, C-1 of this sugar moiety may be twisted out of the plane of the five-membered... 

This concept has been extended. Thus the trione (696) rapidly and irreversibly inactivates human erythrocyte nucleoside phosphorylase (PNPase), which catalyzes the reversible phosphorylation of inosine and guanosine to the respective bases and ribose 1-phosphate. Inhibitors of this enzyme have several potential medical applications, for example, in the prevention of foreign tissue rejection, in the treatment of gout and malaria, and for the potentiation of antineoplastic nucleosides. Mechanistically the 5,8-dione (quinone) (696) enters the enzyme active site. An active-site nucleophilic residue subsequently converts the quinone moiety to a hydroquinone by reductive addition (701). The resulting hydroquinone affords an alkylating quinone methide species by elimination of HCl (702) and then traps a second nucleophilic enzyme residue by a Michael type reaction (703). Cross-linking of the active site rationalizes the observed potency <91B8480>. 

Transfer of D-glucopyranosyl residues is ten times faster from adenosine 5-(a-D-glucopyranosyl pyrophosphate) than from the uridine derivative. The D-glucopyranosyl pyrophosphates of inosine, cytidine, and guanosine are not active as substrates for the synthesis of starch. Since adenosine 5-(a-D-glucopyranosyl pyrophosphate) and a pyrophosphorylase catalyzing its synthesis have been found in plant material,it seems likely that this a-D-glucosyl nucleotide is a precursor of starch in vivo. 

The bases are modified at the same time the endonucleolytic cleavage reactions are occurring (see Fig. 14.20, circle 3). Three modifications occur in most tRNAs (1) Uracil is methylated by S-adenosylmethionine (SAM) to form thymine (2) one of the double bonds of uracil is reduced to form dihydrouracil , and (3) a uracil residue (attached to ribose by an A-glycosidic bond) is rotated to form pseudouridine, which contains uracil linked to ribose by a carbon-carbon bond, (see Fig. 14.17). Other, less common but more complex, modifications also occur and involve bases other than uracil. Of particular note is the deamination of adenosine to form the base inosine. 

Arastu-Kapur S, Ford E, Ullman B et al. Functional analysis of an inosine-guanosine transporter from Leishmania donovani The role of conserved residues, asparute 389 and arginine 393. J Biol Chem 2003 278(35) 33327-33333. 

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