The hisB gene, found in the enterobacteria (such as E. coli), in Campylobacter jejuni and in Xylella/Xanthomonas encodes a protein involved in catalysis of two step in histidine biosynthesis (the sixth and eight step), namely the bifunctional Imidazoleglycerol-phosphate dehydratase/histidinol-phosphatase. The former function (EC 4.2.1.19), found at the N-terminal, dehydrated d-erythroimidazoleglycerolphosphate to imidazoleacetolphosphate, the latter function (EC 3.1.3.15), found at the C-terminal, dephosphorylates l-histidinolphosphate producing histidinol. The firth step is catalysed instead by histadinolphosphate aminotransferase (encoded by hisC) The peptide is 40.5kDa and associates to form a hexamer (unless truncated) In E. coli hisB is found on the hisGDCBHAFI operon The phosphatase activity possess a substrate ambiguity and overexpression of hisB can rescue phosphoserine phosphatase (serB) knockouts.
Reactions hisB-N
D-erythro-1-(imidazol-4-yl)glycerol 3-phosphate ⇌ {\displaystyle \rightleftharpoons } 3-(imidazol-4-yl)-2-oxopropyl phosphate + H2O hisB-C
L-histidinol phosphate + H2O ⇌ {\displaystyle \rightleftharpoons } L-histidinol + phosphate
Non-fusion protein in other species HIS3 from Saccharomyces cerevisiae is not a fused IGP dehydratase and hisidinol phosphatase, but an IGPD only (homologous to hisB-N). Whereas HIS2 is the HP (analogous to hisB-C, called hisJ in some prokaryotes).
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