Adenylate formation from adenosine

In the preceding sections the conversion of purines and purine nucleosides to purine nucleoside monophosphates has been discussed. The monophosphates of adenosine and guanosine must be converted to their di- and triphosphates for polymerization to RNA, for reduction to 2 -deoxyribonucleoside diphosphates, and for the many other reactions in which they take part. Adenosine triphosphate is produced by oxidative phosphorylation and by transfer of phosphate from 1,3-diphosphoglycerate and phosphopyruvate to adenosine diphosphate. A series of transphosphorylations distributes phosphate from adenosine triphosphate to all of the other nucleotides. Two classes of enzymes, termed nucleoside mono-phosphokinases and nucleoside diphosphokinases, catalyse the formation of the nucleoside di- and triphosphates by the transfer of the terminal phosphoryl group from adenosine triphosphate. Muscle adenylate kinase (myokinase)... 

Direct deamination of AMP to IMP by adenylate aminohydrolase (EC 3.5.4.6) occurs primarily in mammalian systems. It plays little, if any, role in bacteria where conversion usually occurs indirectly via deamination of adenine or adenosine [113,114]. Little is known about regulation at these levels, although adenosine deaminase is inducible by its substrate [114] and mutants lacking it have been obtained [115], Another indirect conversion in bacteria involves the regeneration of AlCAR, in the eventual conversion of phosphoribosyl-ATP (PR-ATP) to histidine. The AlCAR so obtained reenters the biosynthetic pathway and IMP is produced [33]. This pathway is regulated by histidine which exerts a profound feedback inhibition at the level of PR-ATP formation from ATP [116]. 

Adenyl cyclase The enzyme (also known as adenylate, or adenylyl cyclase) that catalyses the formation of the second messenger cyclic adenosine-.l A -monophosphate (cAMP) from ATP following the activation of a Gs protein-coupled receptor. 

Adenylate cyclase. The enzyme that catalyzes the formation of cyclic 3, 5 -adenosine monophosphate (cAMP) from ATP. 

The aminoacyl adenylates react rapidly with amino acids to yield peptides39 , under physiological conditions in aqueous solution at room temperature. At pHs higher than 7, the maximum is attained at pH 10 where the extent of the polymerization of peptides from alanyl adenylate is about 60 % 40). The aminoacyl adenylate in aqueous solution undergoes an intra- or inter-molecular rearrangement with the formation of 2 (3 )-aminoacyl ester of adenosine 40). 

Jhe synthesis of proteins, as characterized by the in vitro incorporation of amino acids into the protein component of cytoplasmic ribonu-cleoprotein, is known to require the nonparticulate portion of the cytoplasm, ATP (adenosine triphosphate) and GTP (guanosine triphosphate) (15, 23). The initial reactions involve the carboxyl activation of amino acids in the presence of amino acid-activating enzymes (aminoacyl sRNA synthetases) and ATP, to form enzyme-bound aminoacyl adenylates and the enzymatic transfer of the aminoacyl moiety from aminoacyl adenylates to soluble ribonucleic acid (sRNA) which results in the formation of specific RNA-amino acid complexes—see, for example, reviews by Hoagland (12) and Berg (1). The subsequent steps in pro-... 

Adenylate cyclase is considered as a second messenger that catalyzes the formation of cAMP (cyclic adenosine monophosphate) from ATP this results in alterations in intracellular cAMP levels that change the activity of certain enzymes—that is, enzymes that ultimately mediate many of the changes caused by the neurotransmitter. For example, there are protein kinases in the brain whose activity is dependent upon these cyclic nucleotides the presence or absence of cAMP alters the rate at which these kinases phosphorylate other proteins (using ATP as substrate). The phosphorylated products of these protein kinases are enzymes whose activity to effect certain reactions is thereby altered. One example of a reaction that is altered is the transport of cations (e.g., Na+, K+) by the enzyme adenosine triphosphatase (ATPase). 

A.7.1 Esterification of Acids using Carbodiimides. The formation of anhydrides from carboxylic acids, thiocarboxylic acids, sulfonic acids and phosphorous acids are discussed in Section 2.4.S.2. In this section only special cases of anhydride formation are described. Mixed anhydrides of amino acids and adenylic acid are produced from the corresponding acids using DCC as the condensation agent. ° Mixed anhydrides not containing amino acids, such as butyryl adenate, adenosine 5 -phosphosulfate and p-nitrophenyl-thymidine-5-phosphate are also obtained. 

The three tissue enzymes known to participate in formation of the phosphate esters are (1) thiaminokinase (a pyro-phosphokinase), which catalyzes formation of TPP and adenosine monophosphate (AMP) from thiamine and adenosine triphosphate (ATP) (2) TPP-ATP phosphoryl-transferase (cytosoHc 5"-adenylic kinase)which forms the triphosphate and adenosine diphosphate from TPP and ATP and (3) thiamine triphosphatase, which hydrolyzes TPP to the monophosphate. Although thiaminokinase is widespread, the phosphoryl transferase and membrane-associated triphosphatase are mainly in nervous tissue. 

The measurement of TSH was originally based on bioassays such as the stimulation of colloid droplet formation in the guinea pig thyroid gland and the release of labeled thyroidal iodide into mouse blood. These early in vivo bioassays, however, were of limited sensitivity and precision and were not applicable to the measurement of TSH in unfractionated serum. Most TSH bioassays have involved the in vitro stimulation of thyroid cyclic adenosine monophosphate (cAMP) or adenylate cyclase activity. The rat FRTL-5 thyroid cell line is an example of a particularly convenient and precise assay system. Unfortunately, such methods require purification and concentration of TSH from serum before assay. Sensitive detection of TSH in unfractionated serum is possible using a cytochemical bioassay, but this procedure is technically difficult and time-consuming. At present, immunoassay is the procedure of choice for the measurement of serum TSH in the clinical laboratory. 

Cyclic adenosine monophosphate (cAMP) activates PKA, which in turn phosphor-ylates Cx43 in rat cardiomyocytes.33 Increases in the cAMP concentration increase electrical conductance between paired cardiomyocytes31,34,35 and increase cell permeability - assessed as dye transfer - in non-cardiomyocytes.36 38 Apart from increased cell-cell conductance and permeability, cAMP also increases the extent of gap junction formation.36 38 Increased cAMP concentration results from its enhanced production following stimulation of adenyl cyclase or from inhibition of phosphodiesterase III secondary to an increased concentration of cyclic guanosine monophosphate (cGMP).39-41... 

Activation of the LH receptor results in an increase of intracellular cyclic adenosine monophosphate (cAMP) levels via activation of a G protein and adenylate cyclase. In the presence of elevated concentrations of cAMP, cholesterol esterase activation occurs. This enzyme catalyzes the cleavage of cholesterol esters to free cholesterol, v/hich is then converted in mitochondria to pregnenolone as described previously. The formation of progesterone from pregnenolone is catalyzed by 5-ene-3p-hydroxysteroid dehydrogenase and 3-oxosteroid-4,5-isomerase (steps c and d in Fig. 46.3). 

In muscle phosphorylase, the passage from the inactive to the active form is stimulated only by epinephrine. (It has been suggested that in the liver fluke. Fasciola hepatica, serotonin stimulates phosphorylase. In this fluke, serotonin also stimulates phosphofruc-tokinase, probably through the intermediate of 3, 5 -cyclic adenylic acid.) In liver, both epinephrine and glucagon stimulate the activity of the phosphorylase. In liver, the activation has been demonstrated in vivo in cell-free preparations epinephrine does not act directly on phosphorylase, but it stimulates the formation of 3, 5 -cyclic adenylic acid, which in turn catalyzes the conversion of inactive phosphorylase to active phosphorylase in the supernatant. The cyclic nucleotide is formed from ATP in the presence of a particulate enzyme (adenine cyclase), which is found in heart, muscle, liver, brain, and other organs. The reaction requires magnesium, and, as can be expected, 3, 5 -cyclic adenosine accumulates when the medium contains fluoride. 

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]. 

If adenosine (adenine riboside) is formed by either of the two mechanisms discussed above, the formation of adenylic acid (AMP) and the pyrophosphates depends upon the introduction of a phosphate group. Kinases for the formation of riboside 5 -phosphates from the riboside and ATP are known, Another reaction sequence (reaction b) has also been found for the formation of nucleotides (IV). Individual enzymes have been found for handling orotic acid (a pyrimidine precursor) and adenine... 

A reaction similar in type to that described above has been demonstrated in liver extracts by Wajzer and Baron for inosine-3 -phosphate synthesis from hypoxanthine and ribose-3-phosphate. The formation of the mononucleotide, adenylic acid, by the phosphorylation of adenosine by adenosinetriphosphate has also been described. The significance and integration of these different reactions remains a major problem for future effort. 

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