Histidinol dehydrogenase, histidine

HISTAMINE N-METHYLTRANSFERASE HISTIDINE AMMONIA-LYASE DEHYDROALANINE HISTIDINE DECARBOXYLASE Histidine decarboxylase reduction, BOROHYDRIDE REDUCTION HISTIDINOL DEHYDROGENASE... 

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dehydratase and histidinol phosphatase in extracts from the shoots of barley, oats, and peas, and to the unpublished observations of Davies (see Davies, 1971) on the presence of histidinol dehydrogenase in rose tissue culture cells.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

It has not been possible to fractionate histidinol dehydrogenase into component enzymes, so it is presumed that a single enzyme catalyzes both steps of the over-all reaction. In addition histidinal was synthesized and found to serve as a substrate for the enzyme. 

The most active source of histidinol dehydrogenase was obtained from a histidinol-adapted soil organism Arthrobacter histidindovorana). Extracts of this was purified about twentyfold. Subsequently an enzyme preparation from brewers yeast was purified about a hundredfold by the usual protein purification procedures. Even so the bacteria] preparation had twice the qiecific activity of the yeast enz3une. The dehydrogenase was found to be present in various bacteria and in yeast, but was absent in E. coli mutants unable to convert histidinol to histidine. 

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 is oxidized to histidine by a dehydrogenase that appears to catalyze both steps required. - The most definitive studies on this dehydrogenase were carried out with preparations of Arlhrobacter hi i-dinolovorans, a soil organism isolated by an enrichment technique using histidinol as a carbon and nitrogen source. Similar enzymes occur in E. coli and yeast. Two equivalents of DPNH are produced in this reaction. The reduction of the first mole of DPN by these enzymes should result in the formation of the aldehyde, histidinal. This compound, however, has not been detected in incubation mixtures, even when aldehyde binders were added. Histidinal, an unstable compound in neutral solution, does serve as a substrate for the dehydrogenase, with both DPN... 

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