Nucleosides enzymic reactions

ADP as a substrate in enzyme reactions, ADENYLATE KINASE (or MYOKINASE) ATP SYNTHASE CREATINE KINASE NUCLEOSIDE DIPHOSPHATE KINASE PHOSPHOGLYCERATE KINASE PYRUVATE KINASE RIBONUCLEOTIDE REDUCTASE SULFATE ADENYLYLTRANSFERASE (ADP) [ADP]/[ATP] ratio,... 

V. Enzymic Reactions of Glyeosyl Esters of Nucleoside Pyrophosphates.362... 

The purpose of this Chapter is to summarize the present knowledge concerning the isolation, structure, preparation, and chemical reactions of glycosyl esters of nucleoside pyrophosphates. These aspects have been considered only briefly in the articles cited. Another important topic, which has not been treated previously in a comprehensive manner from a chemical viewpoint, is the mechanism of the enzymic reactions of these compounds, and the specificity of their interaction with the corresponding enzymes. These topics will also be considered here. 

The analogous conversion of the uridine derivative 107d has been observed in extracts of tobacco leaves136 and Chlorella cells,137 although the enzyme has not yet been purified. The enzymic reactions described here are frequently used for preparation of the 6-deoxy-hexosyl-4-ulose esters of nucleoside pyrophosphates that are inacces-... 

The key intermediates in the biosynthesis of 6-deoxy sugars are the nucleoside 6-deoxyhexosyl-4-ulose diphosphates (7), formed through enzymic reactions catalyzed by NDP-sugar 4,6-dehydratases (EC 4.2.1.45-47) from primary glycosyl nucleotides. These reactions were observed... 

Studies on the kinetic behaviour of nucleoside and nucleotide complexes are less common than those on structural aspects. This arises because of the rapid rates of the formation and dissociation reactions, requiring NMR or temperature-jump relaxation measurements. The number of species that can coexist in solution also hinders interpretation. The earlier kinetic studies have been reviewed by Frey and Stuehr.127 Two important biological reactions of the nucleotides are phosphoryl and nucleotidyl group transfers. Both reactions are catalytic nucleophilic reactions and they both require the presence of a divalent metal ion, in particular Mg2+. Consequently, one of the main interests has been in understanding the catalytic mechanism of the metal ion involvement. This has mainly involved studies on related non-enzymic reactions.128... 

As enzymic reactions are in theory reversible, an analogue of a nucleoside triphosphate (e.g. I or II) may be formed in which the S,Y phosphoryl residues of the triphosphate moiety are replaced by PAA or PFA residues. We have prepared the ATP analogue of PAA. This compound has, not unexpectedly, no effect on enzymes such as hexokinase which transfer the Y phosphoryl residue of ATP to a substrate. This ATP analogue is also not a substrate RNA polymerase from E. aoli or from influenza virus. Furthermore, the dTTP analogue is not a substrate for the DNA polymerase of HSV (7). 

The -amino groups of lysyl residues serve as attachment sites of a number of coenzymes in proteins (e.g. biotin in pyruvate carboxylase, pyridoxal phosphate in phosphorylase, lipoic acid in lipoate acetyl-transferase) and form covalent intermediates in several enzymic reactions (e.g. transaldolase, aldolase, etc.). Discussion of all of these naturally-occuring derivatives of lysine will not be attempted in this treatise, but the investigator using chemical modification of proteins should be aware of their possible presence and effect on the results of his experiments. It should be noted that e-N-phospholysine has been reported in nucleoside diphosphate kinase (Walinder 1968). 

The relationship between the rate of tautomeric equilibrium of formycin and its enzymic reactions has shown that the former is faster so that the rate of any process which is specific to the minor tautomer is not limited by the rate of tautomerism. Several references to C-bonded nucleosides appear in Chapter 3. 

The use of enzymes as precolumn and post-column modification reagents for HPLC has been investigated. For example, nucleoside phosphorylase will catalyze the reaction of orthophosphate and inosine to form hypox-anthine and ribose-6-i)hosphate. After running the enzyme reaction, separation of the hypoxanthine formed from the substrate inosine by HPLC permits the determination of phosphate in complex matrices. Creatine kinase (CK) isozymes have been separated by HPLC and detected using the luciferase bioluminescence reaction. 

FIGURE 23.6 The PEP carboxykinase reaction. GTP formed in this reaction can be converted to ATP by nucleoside diphosphate kinase, althongh liver cells in some species may not contain this enzyme. 

While mammahan cells reutilize few free pyrimidines, salvage reactions convert the ribonucleosides uridine and cytidine and the deoxyribonucleosides thymidine and deoxycytidine to their respective nucleotides. ATP-dependent phosphoryltransferases (kinases) catalyze the phosphorylation of the nucleoside diphosphates 2 "-de-oxycytidine, 2 -deoxyguanosine, and 2 -deoxyadenosine to their corresponding nucleoside triphosphates. In addition, orotate phosphoribosyltransferase (reaction 5, Figure 34-7), an enzyme of pyrimidine nucleotide synthesis, salvages orotic acid by converting it to orotidine monophosphate (OMP). 

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