Histidinol phosphatase

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dehydratase and histidinol phosphatase in extracts from the shoots of barley, oats, and peas, and to the unpublished observations of Davies (see Davies, 1971) on the presence of histidinol dehydrogenase in rose tissue culture cells.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.

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... 

The conversion of histidinol phosphate to histidine requires removal of the phosphate group free histidinol was the first precursor of histidine established in genetic studies. The hydrolysis of histidinol phosphate is catalyzed by a phosphatase that has recently been purified by Ames from Neurospora extracts. This enzyme is active with histidinol phosphate as a substrate, but has very little, if any, activity with a number of phosphate esters, including the other imidazole compounds involved in histidine biosynthesis. At least one less specific phosphatase occurs in Neurospora histidinol phosphate is also hydrolyzed nonspecifically, but the activity of the nonspecific hydrolytic enzyme is slow compared with the specific histidinol phosphatase. The nonspecific reaction may account for the slow growth of mutants deficient in the specific enzyme. The specific enzyme is quite insensitive to beryllium and chelating agents, which inhibit nonspecific phosphate hydrolysis. 

Fig. 4. Histidine tnosynthess. Intermediates PRPP, phosphoribosyl pyrophosidiate PR-ATP, JV-phoqjhoribosyl-ATP PR-AMP, A/ -phosphoribosyl-AMP PR-FAIC-RN, phosphoribosyl for-mimino-S-amino imidazole carboxamide ribonucleotide PRu-FAIC-RN, phosphoribulosyl for-mimino-5-aniino imidazole carboxamide ribonucleotide IGP, imidazole glycerol phosphate lAP, imidazole acetol idrosphate HP, histidinol (dioqrhate HOL, histidinol. Enzymes (I) ATP phos-phoribosyltransferase (EC 2.4.2.17) (2) phosphoribosyl-ATP pyrophosphohydrolase (EC 3.6.1.31) (3) idiospharibosyl-AMP cyciohydrolase (EC 3.5. A19) (4) PR-FAIC-RN isomerase (EC S.3.1.16) (S) glutamine amidotransferase (6) cyclase (7) imidazole glycerol phosphate dehydratase (EC 4.2.1.19) (8) histidinol-phosphate aminotransferase (EC 2.6.1.9) (9) histidinol phosphatase (EC 3.1.3.15) (10) histidinol dehydrogenase (EC 1.1.1.23).

HISTIDINOL-PHOSPHATE PHOSPHATASE HISTIDYL-tRNA SYNTHETASE HISTONE KINASE Hofmann rule,... 

GLYCOGEN PHOSPHORYLASE HISTIDINOL-PHOSPHATE PHOSPHATASE myo-INOSITOL 1-MONOPHOSPHATASE... 

SL-histidinol phosphate aminotransferase histidinol phosphate phosphatase ( histidinol dehydrogenase... 

Values are for the specific activity of histidinol phosphate phosphatase relative to the level in the parent strain grown on minintal salts-glucose. The data are taken from Klopotowski and Ames [1171 and Roth et al. [15]. 

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