Crystal structures of a halophilic archaeal malate synthase from Haloferax volcanii and comparisons with isoforms A and G

• Published in: BMC structural biology Vol. 11; no. 1; p. 23
• Main Authors: Bracken, Colten D, Neighbor, Amber M, Lamlenn, Kenneth K, Thomas, Geoffrey C, Schubert, Heidi L, Whitby, Frank G, Howard, Bruce R
• Format: Journal Article
• Language: English
• Published: England BioMed Central Ltd 10-05-2011; BioMed Central
• Subjects: Acetyl Coenzyme A - chemistry; Archaeal Proteins - chemistry; Binding Sites; Catalytic Domain; Crystallography, X-Ray; Crystals; Gene expression; Genetic aspects; Haloferax volcanii - enzymology; Halophilic bacteria; Malate Synthase - chemistry; Physiological aspects; Protein Isoforms - chemistry; Protein Structure, Quaternary; Protein Structure, Tertiary; Structure; Transferases; United States
• ISSN: 1472-6807; 1472-6807
• DOI: 10.1186/1472-6807-11-23

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.
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.
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.