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Acta Crystallogr D Biol Crystallogr. 2014 Mar;70(Pt 3):811-20. doi: 10.1107/S1399004713033609. Epub 2014 Feb 22.

Structural characteristics of alkaline phosphatase from the moderately halophilic bacterium Halomonas sp. 593.

Author information

  • 1Quantum Beam Science Directorate, Japan Atomic Energy Agency, 2-4 Shirakata-shirane, Tokai, Ibaraki 319-1195, Japan.
  • 2Applied and Molecular Microbiology, Faculty of Agriculture, Kagoshima University, 1-21-24 Korimoto, Kagoshima 890-0065, Japan.
  • 3College of Medicine, Florida State University, 1115 West Call Street, Tallahassee, FL 32306-4300, USA.

Abstract

Alkaline phosphatase (AP) from the moderate halophilic bacterium Halomonas sp. 593 (HaAP) catalyzes the hydrolysis of phosphomonoesters over a wide salt-concentration range (1-4 M NaCl). In order to clarify the structural basis of its halophilic characteristics and its wide-range adaptation to salt concentration, the tertiary structure of HaAP was determined by X-ray crystallography to 2.1 Å resolution. The unit cell of HaAP contained one dimer unit corresponding to the biological unit. The monomer structure of HaAP contains a domain comprised of an 11-stranded β-sheet core with 19 surrounding α-helices similar to those of APs from other species, and a unique `crown' domain containing an extended `arm' structure that participates in formation of a hydrophobic cluster at the entrance to the substrate-binding site. The HaAP structure also displays a unique distribution of negatively charged residues and hydrophobic residues in comparison to other known AP structures. AP from Vibrio sp. G15-21 (VAP; a slight halophile) has the highest similarity in sequence (70.0% identity) and structure (C(α) r.m.s.d. of 0.82 Å for the monomer) to HaAP. The surface of the HaAP dimer is substantially more acidic than that of the VAP dimer (144 exposed Asp/Glu residues versus 114, respectively), and thus may enable the solubility of HaAP under high-salt conditions. Conversely, the monomer unit of HaAP formed a substantially larger hydrophobic interior comprising 329 C atoms from completely buried residues, whereas that of VAP comprised 264 C atoms, which may maintain the stability of HaAP under low-salt conditions. These characteristics of HaAP may be responsible for its unique functional adaptation permitting activity over a wide range of salt concentrations.

KEYWORDS:

alkaline phosphatase; halophilic enzymes

PMID:
24598750
PMCID:
PMC3949524
DOI:
10.1107/S1399004713033609
[PubMed - indexed for MEDLINE]
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