Nucleotides enzymic conversion

Fio. 7.—Tentative Mechanism for the Enzymic Conversion of 3-Deoxy-n-aro5mo-heptulosonic Acid 7-Phoaphate to 6-Dehydroquinic Acid. (DPN denotes diphosphopyridine nucleotide and DPNH is reduced diphosphopyridine nucleotide.)... 

The fourth step in the de novo synthesis of pyrimidine nucleotides—the conversion of dihydroorotic acid to orotic acid—is catalyzed by dihydroorotic acid dehydrogenase. The enzyme, located on the cytosolic side of the inner membrane of mitochondria, is a target for antitumor agents. 

Metabolism of Sterols. It is interesting that a heat-stable sterol carrier protein (SCP) has been detected in the protozoan Tetrahymena pyriformis. This protozoan-SCP (P-SCP) was required, in addition to oxygen and pyridine nucleotides, for conversion of cholesterol into cholesta-5,7,22-trien-3p-ol by the protozoan microsomal enzymes (A - and A -dehydrogenase). It is interresting that both protozoan-SCP and liver-SCP are interchangeable in cholesterol biosynthesis by liver enzymes and the oxidation of cholesterol to the triene by protozoan enzymes. The effect of numerous hypocholesteraemic compounds on the cyclization of squalene to the pentacyclic triterpenoid tetrahymanol has also been studied in Tetrahymena. ... 

The oxidizing enzymes involved in the conversion of purines to ureides have been well studied and are described in the next subsection. Little attention has, however, been paid to the hydrolytic enzymes. Conversion of nucleotides to ureides by nodule tissue or cell-fiee extracts or organelle preparations thereof implies that an efficient hydrolytic system is present. A study by Christensen and Jochimsen (1983) identified a 50-fold excess in levels of 5 -nucleotidase in soybean nodules over nodules of Pisum sativum. Similar differences were found for levels of the enzyme in other organs of the two plants. Levels of purine nucleosidase were, however, not significantly different between the two species, although levels in soybean were somewhat higher in all organs. 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

Pyrimidine 5 -nucleotidase (P5N) is a unique enzyme that was recognized from studies of families with relatively common hemolytic disorders. The enzyme catalyzes the hydrolytic dephosphorylation of pyrimidine 5 -nucleotides but not purine nucleotides. The role of this enzyme is to eliminate RNA and DNA degradation products from the cytosol during erythroid maturation by conversion of nucleotide monophosphates to diffusible nucleosides. P5N is inhibited by lead, and its activity is considered to be a good indicator of lead exposure (PI). 

The DszB enzyme encoded by the nucleotide sequence of ORF-2 catalyzes the conversion of HPBS to 2-hydroxybiphenyl and inorganic sulfur. 

The answer is c. (Katzung, p 933.) Resistance to thioguanine occurs because of an increase in alkaline phosphatase and a decrease in hypoxanthine-guanine phosphoribosyl transferase. These enzymes are responsible, respectively, for the increase in dephosphorylation of thiopurine nucleotide and the conversion of thioguanine to its active form, 6-thioinosinic acid. 

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)... 

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