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Maltose binding protein

from Escherichia coli

To highlight described properties click the boxes . Move the molecule anytime with the mouse - if the view is messed up, clicking the large boxes will restore a convenient view.
The maltose binding protein MalE (MBP) is part of the maltodextrin transport system in E. coli. The maltodextrins diffuse through the LamB pore located in the outer membrane into the periplasm. Here the sugar is bound to MalE which prevents it to diffuse back outside. MalE passes the sugar to a complex of MalF and MalG in the cytoplasmic membrane where active tranport is mediated by the ATPase MalK.

MBP is a protein of Mr = 40622 with an ellipsoid shape measuring 30 x 40 x 65 Å. Here MBP is shown with bound maltotriose . There are two domains in the protein (domain I and domain II) with similar secondary structure: central pleated sheets are surrounded by helices . The domains are connected by three bridges , between the domains is a deep groove in which the sugar is bound . All hydroxyl groups of the sugar are fixed by sidechains of the protein by hydrogen bonds . Some water molecules are part of the hydrogen bridge network :

hydrogen bridges with distances (Å)
names of amino acids

The amino acids involved in these hydrogen bonds are mainly in domain I. Besides the hydrogen bridges there are numeous van der Waals-contacts between sugar and protein :

van der Waals-electron clouds
names of amino acids
rotation

A tight interaction of aromatic amino acids with sugar rings is typical for sugar binding proteins. In MBP these amino acids are located mainly in domain II.

The bound maltodextrin is almost completely surrounded by the protein, a small part of the surface of the sugar is accessible from outside (10,5Å2 out of 206Å2) . The opening in the protein is so small thast the sugar cannot diffuse out (binding constant 1,6 x 10-7 M). A comparison of the structures of MBP with or without bound sugar demonstrates the movability of the domains: only after binding the maltotriose the groove narrows down to engulf the sugar.


       

The amount of change in conformation depends on the chain length of the bound maltodextrin. Monomeric glucose is not bound at all. Upon binding oligomeric glucose there are no hydrogen bridges towards the glycosidic bonds; monomeric glucose has hydroxyl groups in the corresponding positions, which can't find hydrogen bridge partners in the protein. Hydroxyl groups are more polar than ether bridges.

For the transport across the cytoplasmic membrane the sugar has to be set free from the binding protein. This requires a conformational change in MBP which is induced by a contact with MalG. From genetic experiments the contact area between MalE and MalG is known . These groups of amino acids are on both sides of the binding groove for the sugar so the conformational change allows to discern the sugar bound from the free state of MalE.

MalE is also involved in chemotaxis towards maltodextrins. Target protein for this function in the cytoplasmic membrane is Tar (Taxis to Aspartate and away from some Repellents). Genetical experiments allowed to identify another group of amino acids involved in contacts between MalE and Tar .

Literature:
FA Quiocho, Atomic structures of periplasmic binding proteins and the high-affinity active transport systems in bacteria, Phil. Trans. R. Soc. Lond. B 326 (1990) 341-352
JC Spurlino et al, The 2.3-Å resolution structure of the maltose- or maltodextrin-binding protein, a primary receptor of bacterial active transport and chemotaxis, J. Biol. Chem. 266 (1991) 5202-5219
FA Quioche et al, Extensive features of tight oligosaccharide binding revealed in high-resolution structures of the maltodextrin transport/chemosensory receptor, Structure 5 (1997) 997-1015
M Ehrmann et al, The ABC maltose transporter, Mol. Microbiol. 29 (1998) 685-694



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