Adenosine kinase, reaction catalyzed

Although purine nucleosides are intermediates in the catabolism of nucleotides and nucleic acids in higher animals and humans, these nucleosides do not accumulate and are normally present in blood and tissues only in trace amounts. Nevertheless, cells of many vertebrate tissues contain kinases capable of converting purine nucleosides to nucleotides. Typical of these is adenosine kinase, which catalyzes the reaction... 

As an example of enzyme action, look in Figure 24.11 at the enzyme hexose kinase, which catalyzes the reaction of adenosine triphosphate (ATP) with glucose to yield glucose-6-phosphate and adenosine diphosphate (ADP). The enzyme first binds a molecule of ATP cofactor at a position near the active site, and glucose then bonds to the active site with its C6 hydroxyl group held rigidly in position next to the ATP molecule. Reaction ensues, and the two products are released from the enzyme. 

One of the major features of solid tumors and even small deposits of tumor tissue is deficiency in the level of oxygen, because of an inadequate vascular supply. The adenosine elevation in response to hypoxia is not exclusive to tumor tissues, but, in this context, the adenosine elevation is localized to the tumor microenvironment, since the surrounding tissue is normally oxygenated. Adenosine is generated mainly by two enzymatic systems intra- or extracellularly localized 5 -nucleoti-dases and cytoplasmic S-adenosylhomocysteine hydrolase. The processes of adenosine elimination in the cell involve reactions catalyzed by adenosine deaminase and adenosine kinase (Shryock and Belardinelli 1997) yielding inosine or 5 -AMP,... 

Analogs can be used in another way. Consider the development of an assay procedure for adenosine kinase, the enzyme that catalyzes the primary reaction Ado + ATP —> AMP + ADP. Problems will arise during the assay of this activity in crude extracts, since other enzymes may be present that can form AMP directly from ATP. 

Figure 10.10 AMP was formed from adenosine and ATP in a reaction catalyzed by adenosine kinase. After a 10-minute incubation, adenylate deaminase was added to the reaction mixture, and samples were taken and analyzed by HPLC Samples were analyzed every S minutes.

Adenosine can also be salvaged to AMP by a phosphorylation reaction catalyzed by adenosine kinase. The reaction requires ATP, which is converted to ADP during the reaction. 

Adenosine kinase is one of a family of nucleoside kinases that are widely found in animal tissues, microorganisms, and plants. This enzyme catalyzes reaction (22), the phosphorylation of the 5 -hydroxyl group of adenosine by MgATP. 

Reactions (35a) and (35b) are catalyzed by galactose-1-P uridylyltransferase and UDPglucose pyrophosphorylase, respectively reactions (36a) and (36b) are catalyzed by nucleoside diphosphate kinase and adenylate kinase, respectively and reactions (37a) and (37b) are catalyzed by nucleoside phosphotransferase and adenosine kinase, respectively. 

Kinases are enzymes that transfer a phosphate group from adenosine triphosphate (ATP), or other trinucleotide, to a number of biological substrates, such as sugars or proteins. They are part of a larger family of enzymes known as group transferases, but are limited to phosphate transfers. A typical reaction catalyzed by a kinase (e.g., hexokinase) is the phosphorylation of glucose upon its entry into a cell... 

Phosphoenolpyruvate carboxykinase is induced. Oxaloacetate produces PEP in a reaction catalyzed by PEPCK. Cytosolic PEPCK is an inducible enzyme, which means that the quantity of the enzyme in the cell increases because of increased transcription of its gene and increased translation of its mRNA. The major inducer is cyclic adenosine monophosphate (cAMP), which is increased by hormones that activate adenylate cyclase. Adenylate cyclase produces cAMP from ATP. Glucagon is the hormone that causes cAMP to rise during fasting, whereas epinephrine acts during exercise or stress. cAMP activates protein kinase A, which phosphorylates a set of specific transcription factors (CREB) that stimulate transcription of the PEPCK gene (see Chapter 16 and Pig. 16.18). Increased synthesis of mRNA for PEPCK results in increased synthesis of the enzyme. Cortisol, the major human glucocorticoid, also induces PEPCK. 

Fig. 41.10. Salvage of bases. The purine bases hypoxanthine and gnanine react with PRPP to form the nucleotides inosine and gnanosine monophosphate, respectively. The enzyme that catalyzes the reaction is hypoxanthine-gnanine phosphoribosyltransferase (HGPRT). Adenine forms AMP in a reaction catalyzed by adenine phosphoribosyltransferase (APRT). Nucleotides are converted to nucleosides by 5 -nucleotidase. Free bases are generated from nncleosides by purine nucleoside phosphorylase. Deamination of the base adenine occurs with AMP and adenosine deaminase. Of the purines, only adenosine can be directly phosphorylated back to a nucleotide, by adenosine kinase.

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]. 

W. A. Blattler and J. R. Knowles (1979), Stereochemical course of phospho-kinases. The use of adenosine [y-(5 )- 0, 0, 0] triphosphate and the mechanistic consequences for the reactions catalyzed by glycerol kinase, hexokinase, pyruvate kinase and acetate kinase. Biochemistry 18, 3927-3933. 

According to more recent observations, cytallene (lid) is readily phosphorylated with ATP and 6,000-fold purified dCyd kinase from human leukemic spleen cells. In addition, cytallene (lid) but neither adenallene (11c) nor ddAdo inhibit phosphorylation of dCyd and dAdo catalyzed with dCyd kinase. The efficiency of phosphorylation of lid is lower than that of ddCyd. Results obtained with human T lymphoblastoid CEM cell line deficient in adenosine (Ado) kinase indicate that the latter enzyme may be important for phosphorylation of ddAdo. However, another study20 shows that ddAdo is not efficiently phosphorylated in a reaction catalyzed by Ado kinase from human thymus. It should also be noted that ddAdo is a substrate for dCyd kinase from the latter source whereas neither racemic adenallene (11c) nor / -enantiomer (31) inhibit phosphorylation of dAdo and dCyd catalyzed by the respective kinases from calf and rabbit thymus.21... 

It appears that in the formation of F-ATP from 2-F-adenosine, the enzyme adenosine kinase catalyzes the rate-limiting reaction. 

The activation of adenylyl cyclase enables it to catalyze the conversion of adenosine triphosphate (ATP) to 3 5 -cyclic adenosine monophosphate (cAMP), which in turn can activate a number of enzymes known as kinases. Each kinase phosphorylates a specific protein or proteins. Such phosphorylation reactions are known to be involved in the opening of some calcium channels as well as in the activation of other enzymes. In this system, the receptor is in the membrane with its binding site on the outer surface. The G protein is totally within the membrane while the adenylyl cyclase is within the membrane but projects into the interior of the cell. The cAMP is generated within the cell (see Rgure 10.4). 

Phosphorylation and dephosphorylation Phosphorylation reactions are catalyzed by a family of enzymes called protein kinases that use adenosine triphosphate (ATP) as a phosphate donor. Phosphate groups are cleaved from phosphorylated enzymes by the action of phosphoprotein phosphatases (Figure 5.18). 

Some of the reactions of PO3- parallel enzymatic reactions promoted by adenosine triphosphate (ATP). Pyruvate kinase catalyzes the equilibration of ATP and pyruvate with adenosine diphosphate (ADP) and phosphoenol pyruvate (11,12). In a formal sense, this reaction resembles the preparations of enol phosphate (eqs. 6 and 7). Cytidine triphosphate synthetase catalyzes the reaction of uridine triphosphate with ammonia to yield cytidine triphosphate (13). In a formal sense, this reaction resembles the replacement of the ester carbonyl group of ethyl acetate by the nitrogen of aniline (eq. 8). 

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). 

Kinases and sulfotransferases utilize similar substrates and catalyze similar reactions. Both transfer anionic groups (Scheme 14.9). Both enzyme classes are capable of binding adenosine-based substrates. Sulfotransferases bind 3 -phospho-adenosine-5 -phosphosulfate (PAPS) (35) as a sulfate donor and kinases bind adenosine-5 -triphosphate (ATP) (36) as a phosphoryl donor. 

Figure 8 Simplified diagram of a signaling cascade that involves NE, BDNF, and CREB after NE acts on the postsynaptic fi-noradrenergic receptor. NE couples to a G protein (Gas), which stimulates the production of cAMP from adenosine triphosphate (ATP). This reaction is catalyzed by adenylate cyclase (AC). cAMP activates protein kinase A (PKA). Inside the cell, PKA phosphorylates (P) the CREB protein, which binds upstream from specific regions of genes and regulates their expression. BDNF is one target of cAMP signaling pathways in the brain. CRE, cyclic AMP regulatory element ER, endoplasmic reticulum, [reprinted from Reference 76 with permission of the author and the publisher, Canadian Medical Association].

Enzymes are highly specific and usually catalyze only one type of reaction. Some enzymes show absolute specificity. For example, pyruvate kinase catalyzes the transfer of a phosphate group only from phosphoenolpyruvate to adenosine diphosphate during glycolysis (Chapter 13). Examples of enzymes that show less specificity are ... 

In brief, enzymes are proteinaceous substances that catalyze biochemical reactions, and kinases constimte a subclass of the enzymes classified as transferases, which transfer functional groups. In turn, a kinase can be described as a phosphoryl-transfer enzyme utilizing ATP (adenosine triphosphate) or described as transferring a phosphate group from a nucleoside triphosphate to another molecule, according to the Academic Press Dictionary of Science and Technology. (Examples of some kinases and their actions occur in Figure 3.1 therein.)... 

ATP can transfer energy to another molecule in a variety of ways (Figure 3.10.2). The most common way is the transfer of a phosphate group to the molecule that requires energy (Fogiel, 1999). This reaction liberates adenosine diphosphate (ADP), and is known as a phosphorylation reaction. It is usually catalyzed by enzymes called kinases. If two phosphate groups are transferred, adenosine monophosphate (AMP) remains. 

These reactions are catalyzed by the enzymes nucleoside monophos-phokinase and nucleoside diphosphokinase, respectively. Note that these reactions are reversible, so that ATP may be synthesized at the expense of GTP or another nucleoside triphosphate. The precursor ADP (adenosine diphosphate) may also be synthesized from the reaction of AMP with ATP, catalyzed by the enzyme adenylate kinase ... 

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