Ok. Gasymov et al., STRUCTURAL-CHANGES IN HUMAN TEAR LIPOCALINS ASSOCIATED WITH LIPID-BINDING, Biochimica et biophysica acta. Protein structure and molecular enzymology, 1386(1), 1998, pp. 145-156
Structural and conformational changes in tear lipocalins were detected
in association with ligand binding and release. Circular dichroism me
asurements demonstrated that ligand binding induces beta structure for
mation, aromatic side chain asymmetry, and a more rigid state in tear
lipocalins (TL). The exposure of the tyrosyl component is less in apo-
TL than in holo-TL. The sole tryptophan residue, Trp(17), is buried in
both holo- and apo-TL. The steady state exposure of Trp(17) is the sa
me in holo- and apo-TL, but the dynamic exposure is two-fold greater i
n apo-TL. Maneuvers to unfold the protein with urea or incubation in a
n acidic environment resulted in increased exposure of aromatic amino
acids. Electron paramagnetic resonance studies verified that lipids ar
e liberated from TL in an acidic environment. Acidic pH promotes confo
rmational changes in TL involving aromatic residues, particularly the
conserved residue Trp(17). These changes are associated with lipid rel
ease. The liberation of lipid from the cavity of TL under acidic condi
tions involves a molten globule state of the protein. We postulate tha
t TL, exposed to the steep surface pH gradient that exists at lipid-aq
ueous interfaces, would release lipid in association with a molten glo
bule transition. The data suggest a plausible regulatory mechanism for
lipid delivery from lipocalins at the tear film surface. (C) 1998 Els
evier Science B.V. All rights reserved.