Adenylosuccinate lyase

It is likely that ara-HxMP similarly exerts its antiviral activity in the form of the triphosphate, ara-HxTP, since ara-HxTP inhibits HSV-1 DNA polymerase (49). Another possible explanation of the antiviral activity of ara-HxTP is that it is metaboHcaHy converted to ara-AMP. In fact, it has been shown at Wellcome Research Laboratories that ara-HxMP is a substrate for adenylosuccinate synthetase, and that the resulting arabinofuranosyladenylosuccinate is cleaved to ara-AMP by adenylosuccinate lyase (1). The selective action of ara-A against HSV appears to be a consequence of the preferential inhibition of ara-ATP against HSV-1 and HSV-2 polymerases. Ara-ATP also inhibits normal cellular DNA polymerases, which may be the reason for its cellular toxicity. Also, it has been observed that ara-A is incorporated uniformly throughout the HSV-1 genome, which may result in defective viral DNA (50). 

Attention has been drawn to the potential of phosphoric acid anhydrides of nucleoside 5 -carboxylic acids (14) as specific reagents for investigating the binding sites of enzymes. For example, (14 B = adenosine) inactivates adenylosuccinate lyase from E. coli almost completely, but has little effect on rabbit muscle AMP deaminase. The rate of hydrolysis of (14) is considerably faster than that of acetyl phosphate, suggesting intramolecular assistance by the 3 -hydroxyl group or the 3-nitrogen atom. 

Mutations in the gene for adenylosuccinate lyase (ASL), inherited as an autosomal recessive disorder in purine metabolism, are associated with severe mental retardation and autistic behavior, but apparently not self-mutilation [10, 11]. This enzyme catalyzes two distinct reactions in the de novo biosynthesis of purines the cleavages of adenylosuccinate (S-Ado) and succinylaminoimidazole carboxamide ribotide (SAICAR), both of which accumulate in plasma, urine and cerebrospinal fluid of affected individuals [12]. Measurements of these metabolites in urine... 

Stone, R. L., Aimi, J. and Barshop, B. A. el al. A mutation in adenylosuccinate lyase associated with mental retardation and autistic features. Nat. Genet. 1 59-63,1992. 

Sivendran, S., Patterson, D., Spiegel, E., McGown, I., Cowley, D. and Colman, R. F. Two novel mutant human adenylosuccinate lyases (ASLs) associated with autism and characterization of the equivalent mutant Bacillus subtilis ASL. /. Biol. Chem. 279 53789-53797, 2004. 

Marinaki, A. M., Champion, M., Kurian, M. A. et al. Adenylosuccinate lyase deficiency - first British case. Nucleosides Nucleotides Nucleic Acids 23 1231-1233, 2004. 

Castro, M., Perez-Cerda, C., Merinero, B. et al. Screening for adenylosuccinate lyase deficiency clinical, biochemical and molecular findings in four patients. Neuropediatrics 33 186-189, 2002. 

Adenylosuccinate lyase, malyl coenzyme A lyase , and NADase are reported to have this mechanism. It should also be recognized that the termed ordered Uni Bi is often used to refer to enzyme-catalyzed reactions which are actually ping pong Bi Bi mechanisms in which water is the second substrate. See Multisubstrate Mechanisms... 

AMP AMINOHYDROLASE ADENYLOSUCCINATE LYASE ADENYLOSUCCINATE SYNTHETASE Adenylyl cyclase,... 

Table 7.1.4 Concentration range of purine and pyrimidine metabolites in urine (pmol/mmol creatinine) from patients. ADA Adenosine deaminase, APRT adenine phosphoribosyltransferase, ASA adenylosuccinate lyase, DHP dihydropyrimidinase, DPD dihydropyrimidine dehydrogenase, HGPRT hypoxanthine-guanine phosphoribosyltransferase, PNP purine nucleoside phosphorylase, TP thymidine phosphorylase, UMPS uridine monophosphate synthase, / -UP fi-ureidopropionase... 

Other enzymes that catalyze elimination reactions that produce fumarate are aspartate ammonia-lyase (aspartase),63 argininosuccinate lyase (Fig. 24-10, reaction g),64/65 and adenylosuccinate lyase... 

Adenylate kinase 303, 654, 655s fold 658, 659 functions of 655 Adenylate system 302-304 storage of energy 302 5 -Adenylic acid. See AMP Adenylosuccinate lyase 685 Adenylylation 545... 

IMP does not accumulate in the cell but is converted to AMP, GMP, and the corresponding diphosphates and triphosphates. The two steps of the pathway from IMP to AMP (fig. 23.11) are typical reactions by which the amino group from aspartate is introduced into a product. The 6-hydroxyl group of IMP (tautomeric with the 6-keto group) is first displaced by the amino of aspartate to give adenylosuccinate, and the latter is then cleaved nonhydrolytically by adenylosuccinate lyase to yield fumarate and AMP. In the condensation of aspartate with IMP, cleavage of GTP to GDP and phosphate provides energy to drive the reaction. 

Didanosine is a synthetic purine nucleoside analog that inhibits the activity of reverse transcriptase in HIV-1, HIV-2, other retroviruses and zidovudine-resistant strains. A nucleobase carrier helps transport it into the cell where it needs to be phosphorylated by 5 -nucleoiidase and inosine 5 -monophosphate phosphotransferase to didanosine S -monophosphate. Adenylosuccinate synthetase and adenylosuccinate lyase then convert didanosine 5 -monophosphate to dideoxyadenosine S -monophosphate, followed by its conversion to diphosphate by adenylate kinase and phosphoribosyl pyrophosphate synthetase, which is then phosphorylated by creatine kinase and phosphoribosyl pyrophosphate synthetase to dideoxyadenosine S -triphosphate, the active reverse transcriptase inhibitor. Dideoxyadenosine triphosphate inhibits the activity of HIV reverse transcriptase by competing with the natural substrate, deoxyadenosine triphosphate, and its incorporation into viral DNA causes termination of viral DNA chain elongation. It is 10-100-fold less potent than zidovudine in its antiviral activity, but is more active than zidovudine in nondividing and quiescent cells. At clinically relevant doses, it is not toxic to hematopoietic precursor cells or lymphocytes, and the resistance to the drug results from site-directed mutagenesis at codons 65 and 74 of viral reverse transcriptase. 

There is a fifth bifunctional enzyme which catalyzes reactions 8 and 12 of the purine pathway (Fig. 15-16) but adenylosuccinate lyase has one active site with dual specificity, catalyzing both reactions (SA1CAR—> AICAR, sAMP—> AMP Fig. 15-16). All 14 enzymatic activities of Fig. 15-16 are cytosolic and there is a variety of evidence for association of subsets of these activities in vivo. The existence of a pathway particle or metabolon" for de novo purine biosynthesis in intact cells has been proposed. 

In muscle, a unique nucleotide reutilization pathway, known as the purine nucleotide cycle, uses three enzymes myoadenylate deaminase, adenylosuccinate synthetase, and adenylosuccinate lyase. In this cycle, AMP is converted to IMP with formation of NH3, and IMP is then reconverted to AMP. Myoadenylate deaminase deficiency produces a relatively benign disorder of muscle... 

The purine nucleotide cycle of muscle consists of the conversion of AMP —> IMP AMP and requires AMP deaminase, adenylosuccinate synthetase, and adenylosuccinate lyase (Figure 27-24). Flux through this cycle increases during exercise. Several mechanisms have been proposed to explain how the increase in flux is responsible for the maintenance of appropriate energy levels during exercise (Chapter 21). 

Less dramatic conformational changes are involved in cleft closure or closure of lids over active sites in many enzymes, including triosephosphate isomerase, hexokinase, phosphonatase, and adenylosuccinate lyase." Such motions sequester active sites, preventing escape of reactive intermediates or inadvertent reaction with the solvent, and often help to orient substrates and/or catalytic residues. 

Adenylosuccinate lyase catalyzes the following reaction in the de novo biosynthesis of AMP Adenylosuccinate <=> Fumarate + AMP... 

In addition to salvaging purines, most cells interconvert adenine and guanine nucleotides. Inosine monophosphate (IMP), is the common intermediate. IMP is converted into AMP by a two-step reaction catalyzed by adenylosuccinate synthetase and adenylosuccinate lyase. Guanine nucleotides are formed in a two-step reaction in which IMP is converted into xanthine monophosphate (XMP) and then aminated to GMP. Both GMP and AMP can be reconverted into IMP. Mammalian cells can also deaminate adenosine to inosine and guanine to xanthine (Fig. 6.1). 

Leishmania adenylosuccinate synthetase has a narrow substrate specificity but accepts several IMP analogs which include allopurinol ribonucleotide (34). The GMP reductase from L. donovani is quite different from the human GMP reductase (35) and IMP analogs are more potent inhibitors for it. Other leishmanial enzymes that have been investigated include IMP dehydrogenase (36), nucleoside hydrolase and phos-phorylase activities (37,38), adenosine kinase (39), nucleotidases (40) and the adenylosuccinate lyase (34). 

Individual enzymes of purine salvage are similar to those of Leishmania. PRTase activities were found for adenine, hypoxanthine, and guanine in the three forms (43). As in Leishmania, there is also a separate xanthine PRTase. Nucleoside kinase activities were found for adenosine, inosine, and guanosine (43), nucleoside hydrolase activities for inosine and guanosine and a nucleoside phosphorylase activity for adenosine. There are both nucleoside hydrolase and phosphorylase activities in epimastigotes (44,45). The adenylosuccinate synthetase and adenylosuccinate lyase are essentially identical to those found in L. donovani (46). 

Spector, T., Jones, T. E. and Elion, G. B. (1979) Specificity of adenylosuccinate synthetase and adenylosuccinate lyase from Leishmania donovani. Selective amination of an antiprotozoal agent. J. Biol. Chem. 254 8422-8426. 

Spector, T., Berens, R. L. and Marr, J. J. (1982) Adenylosuccinate synthetase and adenylosuccinate lyase from Trypanosoma cruzi, specificity studies with potential chemotherapeutic agents. Biochem. Pharmacol. 31 225-229. 

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