X-ray Structures of the Maltose–Maltodextrin-binding Protein of the Thermoacidophilic Bacterium Alicyclobacillus acidocaldarius Provide Insight into Acid Stability of Proteins

Abstract

Maltose-binding proteins act as primary receptors in bacterial transport and chemotaxis systems. We report here crystal structures of the thermoacidostable maltose-binding protein from Alicyclobacillus acidocaldarius, and explore its modes of binding to maltose and maltotriose. Further, comparison with the structures of related proteins from Escherichia coli (a mesophile), and two hyperthermophiles (Pyrococcus furiosus and Thermococcus litoralis) allows an investigation of the basis of thermo-and acidostability in this family of proteins.The thermoacidophilic protein has fewer charged residues than the other three structures, which is compensated by an increase in the number of polar residues. Although the content of acidic and basic residues is approximately equal, more basic residues are exposed on its surface whereas most acidic residues are buried in the interior. As a consequence, this protein has a highly positive surface charge. Fewer salt bridges are buried than in the other MBP structures, but the number exposed on its surface does not appear to be unusual. These features appear to be correlated with the acidostability of the A. acidocaldarius protein rather than its thermostability.An analysis of cavities within the proteins shows that the extremophile proteins are more closely packed than the mesophilic one. Proline content is slightly higher in the hyperthermophiles and thermoacidophiles than in mesophiles, and this amino acid is more common at the second position of b-turns, properties that are also probably related to thermostability. Secondary structural content does not vary greatly in the different structures, and so is not a contributing factor.

Structural and Thermodynamic Analyses of Solute-binding Protein from Bifidobacterium longum Specific for Core 1 Disaccharide and Lacto-N-biose I

“…trehalose/maltose-binding protein and cyclo/maltodextrin-binding proteins) from other sources have also been determined (33,(45)(46)(47) (49), and liganded open (50) states have been reported. In the case of GL-BP, we could not obtain crystals without ligands even after extensive crystallization screening.…”

Characterization of a Thermoacidophilic l -Arabinose Isomerase from Alicyclobacillus acidocaldarius : Role of Lys-269 in pH Optimum

“…However, since random directed evolution methods require highthroughput screening systems, limitations to improving enzyme properties for many industries still remain (20). As an alternative, three-dimensional (3-D) structure-based rational engineering has been also extensively performed (15,19,34,38). Nonetheless, although recent progress has been made using both methods in engineering enzymes that can operate at a lower pH, attempts to rationally engineer enzymes have been limited to those enzymes whose structures are known (35).…”

“…To dates, several acidic proteins from Alicyclobacillus acidocaldarius, a thermoacidophilic bacterium that grows optimally at 50 to 60°C and at pH 3 to 4 (7,42), has been investigated for pH-related properties (9,10,33,34,41). For example, although AIs from Bacillus species such as B. subtilis and B. halodurans and from Geobacillus stearothermophilus are highly conserved in terms of amino acid sequences (Ͼ60%), they showed different pH optima (23).…”

Role of Water in Ligand Binding to Maltose-Binding Protein: Insight from a New Docking Protocol Based on the 3D-RISM-KH Molecular Theory of Solvation

“…3D-RISM-KH calculations were performed in 0.00029 g/cm 3 sodium chloride aqueous solution at temperature 25°C( ≈298 K). The water density was set to 0.99983 g/cm 3 and the dielectric constant to 78.5. The solute−solvent interaction potential comprises the Coulomb and Lennard−Jones interactions.…”

“…2,3 The structure of MBP has been resolved in X-ray and NMR spectroscopy experiments both for the liganded (holo) and unliganded (apo) states. 2−12 These studies revealed that MBP has two structural domains connected by the hinge region with the carbohydrate binding site located in the cleft between the domains ( Figure 1).…”