Structural study of hinge bending in L-arabinose-binding protein.

The L-arabinose-binding protein of Escherichia coli is a periplasmic component of the bacterial L-arabinose transport system. The three-dimensional structure of the molecule has been determined by x-ray diffraction and shown to have two globular domains and a connecting hinge. Theoretical study of the flexibility of the hinge using computer simulation showed that the hinge is quite permissive in that only moderate increases in the internal energy are required for opening the cleft where the L-arabinose-binding site is located. In this study, the structural changes that accompany the hinge bending are analyzed. The results show that bending-induced stresses are accommodated by coupled action of covalent and noncovalent forces within the protein molecule. Strains in internal coordinates (bond lengths, bond angles, and torsional angles) are distributed throughout the hinge region after structural relaxation. The pattern of structural changes within a hinge strand upon bending and relaxation depends in large degree on its geometric relationship with the bending axis (e.g. distance and orientation) and the atomic packing of its immediate environment. The distributed structural changes result in a characteristic zigzag pattern for the directional change at each residue in the hinge strands.