HisB

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.^[1]

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.^[2][3][4]

The firth step is catalysed instead by histadinolphosphate aminotransferase (encoded by hisC)^[5]

The peptide is 40.5kDa and associates to form a hexamer (unless truncated)^[6]

In E. coli hisB is found on the hisGDCBHAFI operon^[7]

The phosphatase activity possess a substrate ambiguity and overexpression of hisB can rescue phosphoserine phosphatase (serB) knockouts.^[8]

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).