Structural adaptations that modulate monosaccharide, disaccharide, and trisaccharide specificities in periplasmic maltose-binding proteins.
Periplasmic binding proteins comprise a superfamily that is present in archaea, prokaryotes, and eukaryotes. Periplasmic binding protein ligand-binding sites have diversified to bind a wide variety of ligands. Characterization of the structural mechanisms by which functional adaptation occurs is key to understanding the evolution of this important protein superfamily. Here we present the structure and ligand-binding properties of a maltotriose-binding protein identified from the Thermus thermophilus genome sequence. We found that this receptor has a high affinity for the trisaccharide maltotriose (K(d)<1 microM) but little affinity for disaccharides that are transported by a paralogous maltose transport operon present in T. thermophilus. Comparison of this structure to other proteins that adopt the maltose-binding protein fold but bind monosaccharides, disaccharides, or trisaccharides reveals the presence of four subsites that bind individual glucose ring units. Two loops and three helical segments encode adaptations that control the presence of each subsite by steric blocking or hydrogen bonding. We provide a model in which the energetics of long-range conformational equilibria controls subsite occupancy and ligand binding.
Department of Biochemistry, Duke University Medical Center, Durham, NC 27710, USA.
Matthew J
MJ
Cuneo
Anita
A
Changela
Lorena S
LS
Beese
Homme W
HW
Hellinga
D000595:Amino Acid Sequence; D001665:Binding Sites; D002352:Carrier Proteins / Q000737:chemistry*; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D004187:Disaccharides / Q000378:metabolism*; D019791:Guanidine / Q000494:pharmacology; D008024:Ligands; D058619:Maltose-Binding Proteins; D008969:Molecular Sequence Data; D009005:Monosaccharides / Q000378:metabolism*; D033902:Periplasmic Binding Proteins / Q000737:chemistry*; D011489:Protein Denaturation / Q000187:drug effects; D017433:Protein Structure, Secondary; D016415:Sequence Alignment; D013379:Substrate Specificity / Q000187:drug effects; D016963:Thermus thermophilus / Q000378:metabolism*; D017186:Titrimetry; D014312:Trisaccharides / Q000378:metabolism*
D002352:Carrier Proteins; D004187:Disaccharides; D008024:Ligands; D058619:Maltose-Binding Proteins; D009005:Monosaccharides; D033902:Periplasmic Binding Proteins; D014312:Trisaccharides; D019791:Guanidine