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Histidinol Dehydrogenase
Histidinol dehydrogenase (HDH) is an enzyme that catalyzes the last step in the histidine biosynthetic pathway, which converts L-histidinol to L-histidine with a L-histidinaldehyde intermediate. This primordial pathway was found in bacteria, archaebacteria, fungi, and plants. HDH has been one of the most studied enzyme biochemically and genetically throughout time.[1]
HDH is encoded by the structural gene hisD in Brucellosis, commonly known as Maltafeve. Brucellosis is a bacterial disease transmitted by having contact with infected animals. HDH being encoded by hisD is essential for intramacrophagic replication because it provides a novel target for the development of anti-Brucella agent.[2] Because HDH is absent from mammals, it has become an attractive target for inhibition as part of the herbicide development.[1]
General Information
Gene Name: hisD [3]
Organism: Escherichia coli (strain K12) [3]
Classification: Oxidoreductase [4]
Length: 434 Residues [3]
Molecular Weight: 46107.65 Da [5]
Isoelectric Point: 5.06 [5]
Chains: , [4]
Ligands: glycerol (GOL), selenomethionine (MSE), sulfate ion (SO4)[4]
Structure
The crystal structure of histidinol dehydrogenase can be determined by x-ray crystallography. The overall structure is 48% helical (20 helices; 211 residues) and 16% beta sheet (15 strands; 73 residues).[6]
HDH functions as a homodimer, but it consists of two monomers. The presence of Zn2+ cation is required per monomer. Each HDH monomer is made of four domains, two larger domains and two smaller domains. The two larger domains make up the globule and the two smaller domains make up the extending tail. The intertwined dimer is thought to result from domain swapping. The two domains presents a similar incomplete Rossmann fold, which suggests an ancient event of gene duplication. Residues from both monomers form the active site. The active site (residue His-327) participates in acid-base catalysis [1]
Related Structures: 1KAE and 1KAR
[1]
Figure 1. (A) Stereo view of the monomer. Domains: 1, blue; 2, green; 3, orange; 4, magenta. L-histidinol, NAD, and the Zn2� are shown as ball-and-sticks. ( B) Domain 1. Rossmann fold shown in blue, V-shaped pairs of helices (residues 25 –103) connected by a linker that forms the sixth strand are in cyan. (C) Domain 2. Rossmann fold (green) in similar orientation as B. Strand-helix hairpin completes the �-sheet (residues 1–24, magenta). (D) Topology diagram. Secondary structure elements are numbered consecutively. The chain meanders between domains in the order 2 –1-3–1-2–1-3– 4. (E) HisD dimer with one molecule colored as in A and the other shown in pale colors. Zn2� atoms and NAD bound to each monomer (red) define the position of the active site.
Sequence of HDH [6]
Figure. 2 This is the full sequence of histidinol dehydrogenase.
Enzymatic Mechanism
Figure 3. This bifunctional enzyme converts L-histidinol to L-histidine through a L-histidinaldehyde intermediate. His-327 and Glu-326 are the two main active sites of HDH.[3]
The reaction above is as follows:
-1 proton and 1 hydride are abstracted from L-histidinol by His-327 (B1). NAD+ accepts hydride.
-L-histdinol becomes L-histidinaldehyde (sp2)
-Reduced NADH cofactor leaves and then is replaced by another NAD+
-Water is activated by Glu-326 (B2) and makes a nucleophilic attack on the reactive carbon.
-Concurrently, His-327 (B3) donates its proton to the aldehyde oxygen
-Repeat step 1 and then it leads to the formation of L-histidine [1]
Implications or Possible Applications
Brucellosis, commonly known as Maltafever, is the most widespread bacterial zoonosis worldwide. It is a bacterial disease of human beings transmitted by contact with infected animals or infected meat or milk products. It causes fever and headaches. Its causative agent, Brucella spp., is a facultative intracellular pathogen developed inside the host’s macrophages.
The absence of a vaccine for humans and the appearing resistance of Brucella spp. to anti-biotic chemotherapy points to the necessity to develop new therapeutic strategies to eradicate this reemerging pathogen. The virulome analysis of Brucella suis shows that genes involved in the biosynthesis of amino acids are essential for the virulence of the bacteria.
Inhibition of its enzymatic activity with specific inhibitors will prevent intramacrophagic multiplication of Brucella. Histidinol dehydrogenase is an amino acid biosynthetic enzyme, which can provide a novel target for the development of anti-Brucella agents. Histidinol dehydrogenase has no counterpart in mammalians; therefore, it constitutes a therapeutic target for the development of an anti-infectious treatment against intracellular pathogens.[7]
References
- ↑ 1.0 1.1 1.2 1.3 1.4 http://www.pnas.org.prox.lib.ncsu.edu/content/99/4/1859.full.pdf
- ↑ http://aac.asm.org/content/51/10/3752.full.pdf+html
- ↑ 3.0 3.1 3.2 3.3 http://www.uniprot.org/uniprot/P06988#section_terms
- ↑ 4.0 4.1 4.2 http://oca.weizmann.ac.il/oca-bin/ocashort?id=1K75
- ↑ 5.0 5.1 http://www.topsan.org/Proteins/BSGI/1k75
- ↑ 6.0 6.1 http://www.rcsb.org/pdb/explore/remediatedSequence.do?structureId=1K75&bionumber=1
- ↑ http://pubs.rsc.org.prox.lib.ncsu.edu/en/content/articlepdf/2011/md/c1md00146a
