ATP phosphoribosyltransferase

Many proteins have structures related to those of aminoacyl-tRNA synthetases.282 283 For example, asparagine synthetase A functions via an aspartyl-adenylate intermediate (Chapter 24, Section B), and its structure resembls that of aspartyl-tRNA synthetase.284 The his G gene of histidine biosynthesis (Fig. 25-13) encodes an ATP phosphoribosyltransferase with structural homology to the catalytic domain of histidyl-tRNA synthetase.284 The reason is not clear, but some aminoacyl-tRNA synthetases, especially the histidyl-tRNA synthetase, are common autoantigens for the inflammatory disease polymyosititis.285 286... 

Evidence for the presence of the enzymes of the histidine pathway in plants appears to be limited to the work of Winter et al. (1971a) who demonstrated the presence of ATP-phosphoribosyltransferase, the first enzyme of the pathway, imidazole glycerolphosphate dehydratase and histidinol phosphatase in extracts from the shoots of barley, oats, and peas, and to the unpublished observations of Davies (see Davies, 1971) on the presence of histidinol dehydrogenase in rose tissue culture cells. The specific activity of ATP-phosphoribosyltransferase was greatest in peas and oats and least in barley. The enzymes from oats and barley were thermolabile losing activity after 30 min at 37°C. The specific activities of imidazole glycerolphosphate dehydratase were very low but it was possible to purify the enzyme to some extent. The values for imidazole glycerolphosphate for the barley enzyme was 0.6 mM which compares with values for jthe yeast and bacterial enzymes of 0.3 and 0.4 mM, respectively. Histidinolphosphatase was purified 20-fold but the authors considered that two phosphatases were still present. 

Fig. 2. The pathway of histidine biosynthesis. Enzymes a, ribosephosphate pyrophos-phokinase E.C. 2.7.6.1 b, ATP-phosphoribosyltransferase, E.C. 2.4.2.17 c, phosphoribosyl-AMP cyclohydrolase, E.C. 3.5.4.19 d, N-(5 -phospho-D-ribosylforminino)5-amino-l-(5"-phos-phoribo yl)-4-imidazole carboxamide isomerase, E.C. 5.3.1.16 e, glutamine amidotransferase f, imidazolglycerolphosphate dehydratase E.C. 4.2.1.19 g, histidinol-phosphate aminotransferase E.C. 2.6.1.9 h, histidinol phosphatase, E.C. 3.1.3.15 i, histidinol dehydrogenase, E.C. 1.1.1.23.

Consistent with this circumstantial evidence is the effect of histidine in inhibiting isolated ATP-phosphoribosyltransferase (Wiateret al., 1971a). The enzyme from fresh extracts of pea and oat shoots was inhibited by about 50% in the presence of 0.01 mM histidine, however, when the pea enzyme was stored at 4°C overnight the sensitivity of the razyme increased to 80% although the activity in the absence of histidine dropped. The results could be explained by the inactivation overnight of an insensitive form of the enzyme and more studies are required on the control mechanisms of this enzyme. These preUminary studies have however shown that the plant enzyme is of greater sensitivity than the enzyme from Saccharomyces cerevisiae (inhibited 50% by 0.06 mM histidine). 

As far as the evidence available is concerned there is no reason to suppose that histidine biosynthesis occurs in a different manner to that worked out for bacteria and fungi. However given the actual weight of evidence available this is not a very firm conclusion. Similarly the limited results obtained suggest that the pathway is feedback regulated by histidine at ATP-phosphoribosyltransferase. No information is available on any crosspathway regulation, neither with respect to other amino acids nor to nucleotide bases, or on the localization of the pathway within the plant cell. 

ATP phosphoribosyltransferase 2 phosphoribosyl-ATP pyrophosphatase 3 phosphoribosyl-AMP cyclohydrolase 4 isomerase 5 formyltransferase, inosine monophosphate cyclohydrolase 6 adenylosuccinate synthetase 7 adenylosuccinate lyase 8 adenylate kinase 9 imidazole glycerol phosphate dehydratase 10 histidinol phosphate aminotransferase 11 histidinol phosphatase 12 histidinol dehydrogenase... 

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). 

Nicotinate phosphoribosyltransferase catalyzes the formation of nicotinate mononucleotide (NaMN) and pyrophosphate from 5-phosphoribosyl-a-D-pyrophosphate (PRibPP) and free nicotinic acid. The reaction requires the hydrolysis of ATP to ADP. 

Figure 9.87 Elutions of the nicotinate phosphoribosyltransferase (N-PRTase) assay solution through a /xBondapak C18 column after various enzyme incubation times. The incubation mixture contained 5 mM MgCl2,100 y,M nicotinate, 75 tiM ATP, 30 /iM PRibPP, and 25 yu.L of 4 mg/mL N-PRTase in 50 mM Tris-HQ (pH 8). Elution conditions 5 yuL sample injection volumes, 0.7 mL/min flow rate, 25 m M (NH))P04 (pH 8) elution buffer, 25°C. (From Hanna and Sloan, 1980.)...

At this stage, orotate couples to ribose, in the form of 5-phosphoribosyl-l-pyrophosphate (PRPP), a form of ribose activated to accept nucleotide bases. PRPP is synthesized from ribose-5-phosphate, formed by the pentose phosphate pathway, by the addition of pyrophosphate from ATP. Orotate reacts with PRPP to form orotidylate, a pyrimidine nucleotide. This reaction is driven by the hydrolysis of pyrophosphate. The enzyme that catalyzes this addition, pyrimidine phosphoribosyltransferase, is homologous to a number of other phosphoribosyltransferases that add different groups to PRPP to form the other nucleotides. Orotidylate is then decarboxylated to form uridylate (IMP), a major pyrimidine nucleotide that is a precursor to RNA. This reaction is catalyzed by orotidylate decarboxylase. 

The pathway in bacteria is regulated by a complex repression system involving the products of several genes including those concerned with the production and charging of tRNA" (see Umbarger, 1978). Feedback inhibition of ATP-5 phosphoribosyltransferase also occurs. 

Tissues were homogenized in phosphate buffer and sonicated, and the supernatant was used for electrophoresis. Samples were run in phosphate buffer, pH 8.5, on cellulose acetate paper for 2 hours at 4 and at 0.5 mA/cm. PRPP synthetase was located on the paper by a radiochemical assay formation of PRPP from ribose-5-phosphate and ATP was coupled to inosinic acid (IMP) synthesis by the addition to the reaction mixture of labelled hypoxanthine and partially purified hypoxan-thine-guanine phosphoribosyltransferase (HGPRT). 

Quinolinate decarboxylation and conversion to nicotinic acid mononucleotide is catalysed by quinolinate phosphoribosyltransferase, a rate-limiting enzyme in the conversion of tryptophan to NAD the reaction requires Mg and is negatively regulated by nicotinamide. Next the transfer of adenylate from ATP by an intermediate of nicotinamide/nicotinate-mononucleotide-adenyl-transferases isoenzymes (NMNAT, see below) yields nicotinic acid adenine... 

PP-ribose-P is a sugar phosphate which is synthesized from ATP and ribose-5-phosphate in a reaction requiring magnesium and inorganic phosphate catalyzed by PP-ribose-P synthetase (Figure 1). The PP-ribose-P formed is a substrate in the first and probable rate-limiting reaction of purine synthesis de novo which is catalyzed by PP-ribose-P amidotransferase (PAT). In addition, PP-ri-bose-P is a substrate in the purine phosphoribosyltransferase reactions which constitute a pathway for the salvage of purine bases. 

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