HisB explained

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, found at the N-terminal, dehydrated d-erythroimidazoleglycerolphosphate to imidazoleacetolphosphate, the latter function, 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

\rightleftharpoons 3-(imidazol-4-yl)-2-oxopropyl phosphate + H₂OhisB-C

L-histidinol phosphate + H₂O

\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).

Notes and References

1. Brilli . M. . Fani . R. . 10.1007/s00239-003-2547-x . Molecular Evolution of hisB Genes . Journal of Molecular Evolution . 58 . 2 . 225–237 . 2004 . 15042344. 2004JMolE..58..225B . 1684458 .
2. Parker95: Parker, A.R., Moore, J.A., Schwab, J.M., Davisson, V.J. (1995). "Escherichia coli Imidazoleglycerol Phosphate Dehydratase: Spectroscopic Characterization of the Enzymic Product and the Steric Course of the Reaction." Journal of the American Chemical Society.
3. Grisolia . V. . Carlomagno . M. S. . Bruni . C. B. . Cloning and expression of the distal portion of the histidine operon of Escherichia coli K-12 . Journal of Bacteriology . 151 . 2 . 692–700 . 1982 . 10.1128/jb.151.2.692-700.1982 . 6284708 . 220310.
4. Grisolia . V. . Riccio . A. . Bruni . C. B. . Structure and function of the internal promoter (hisBp) of the Escherichia coli K-12 histidine operon . Journal of Bacteriology . 155 . 3 . 1288–1296 . 1983 . 10.1128/jb.155.3.1288-1296.1983 . 6309747 . 217827.
5. Keseler . I. M. . Collado-Vides . J. . Santos-Zavaleta . A. . Peralta-Gil . M. . Gama-Castro . S. . Muñiz-Rascado . L. . Bonavides-Martinez . C. . Paley . S. . Krummenacker . M. . Altman . T. . Kaipa . P. . Spaulding . A. . Pacheco . J. . Latendresse . M. . Fulcher . C. . Sarker . M. . Shearer . A. G. . MacKie . A. . Paulsen . I. . Gunsalus . R. P. . Karp . P. D. . EcoCyc: A comprehensive database of Escherichia coli biology . 10.1093/nar/gkq1143 . Nucleic Acids Research . 39 . Database issue . D583–D590 . 2010 . 3013716 . 21097882.
6. Rangarajan . E. S. . Proteau . A. . Wagner . J. . Hung . M. -N. . Matte . A. . Cygler . M. . 10.1074/jbc.M604916200 . Structural Snapshots of Escherichia coli Histidinol Phosphate Phosphatase along the Reaction Pathway . Journal of Biological Chemistry . 281 . 49 . 37930–37941 . 2006 . 16966333. free .
7. Alifano . P. . Carlomagno . M. S. . Bruni . C. B. . Location of the hisGDCBHAFI operon on the physical map of Escherichia coli . Journal of Bacteriology . 174 . 11 . 3830–3831 . 1992 . 10.1128/jb.174.11.3830-3831.1992 . 1592835 . 206079.
8. Patrick . W. M. . Quandt . E. M. . Swartzlander . D. B. . Matsumura . I. . Multicopy Suppression Underpins Metabolic Evolvability . 10.1093/molbev/msm204 . Molecular Biology and Evolution . 24 . 12 . 2716–2722 . 2678898 . 2007 . 17884825.