title: Crystal structures of a halophilic archaeal malate synthase from Haloferax volcanii and comparisons with isoforms A and G.
abstract: BACKGROUND
  Malate synthase, one of the two enzymes unique to the glyoxylate cycle, is found in all three domains of life, and is crucial to the utilization of two-carbon compounds for net biosynthetic pathways such as gluconeogenesis. In addition to the main isoforms A and G, so named because of their differential expression in E. coli grown on either acetate or glycolate respectively, a third distinct isoform has been identified. These three isoforms differ considerably in size and sequence conservation. The A isoform (MSA) comprises ~530 residues, the G isoform (MSG) is ~730 residues, and this third isoform (MSH-halophilic) is ~430 residues in length. Both isoforms A and G have been structurally characterized in detail, but no structures have been reported for the H isoform which has been found thus far only in members of the halophilic Archaea.
  
  
  RESULTS
  We have solved the structure of a malate synthase H (MSH) isoform member from Haloferax volcanii in complex with glyoxylate at 2.51 Å resolution, and also as a ternary complex with acetyl-coenzyme A and pyruvate at 1.95 Å. Like the A and G isoforms, MSH is based on a β8/α8 (TIM) barrel. Unlike previously solved malate synthase structures which are all monomeric, this enzyme is found in the native state as a trimer/hexamer equilibrium. Compared to isoforms A and G, MSH displays deletion of an N-terminal domain and a smaller deletion at the C-terminus. The MSH active site is closely superimposable with those of MSA and MSG, with the ternary complex indicating a nucleophilic attack on pyruvate by the enolate intermediate of acetyl-coenzyme A.
  
  
  CONCLUSIONS
  The reported structures of MSH from Haloferax volcanii allow a detailed analysis and comparison with previously solved structures of isoforms A and G. These structural comparisons provide insight into evolutionary relationships among these isoforms, and also indicate that despite the size and sequence variation, and the truncated C-terminal domain of the H isoform, the catalytic mechanism is conserved. Sequence analysis in light of the structure indicates that additional members of isoform H likely exist in the databases but have been misannotated.
authors.affiliation: Department of Physical Science, Southern Utah University, Cedar City, UT 84720-2470, USA.
authors.forename: Colten D
authors.initials: CD
authors.lastname: Bracken
authors.forename: Amber M
authors.initials: AM
authors.lastname: Neighbor
authors.forename: Kenneth K
authors.initials: KK
authors.lastname: Lamlenn
authors.forename: Geoffrey C
authors.initials: GC
authors.lastname: Thomas
authors.forename: Heidi L
authors.initials: HL
authors.lastname: Schubert
authors.forename: Frank G
authors.initials: FG
authors.lastname: Whitby
authors.forename: Bruce R
authors.initials: BR
authors.lastname: Howard
mesh_terms: D000105:Acetyl Coenzyme A / Q000737:chemistry; D019843:Archaeal Proteins / Q000737:chemistry*; D001665:Binding Sites; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D019622:Haloferax volcanii / Q000201:enzymology*; D008292:Malate Synthase / Q000737:chemistry*; D020033:Protein Isoforms / Q000737:chemistry; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary
chemical_list: D019843:Archaeal Proteins; D020033:Protein Isoforms; D000105:Acetyl Coenzyme A; D008292:Malate Synthase