|
Enkephalins and beta-casomorphins (opioid peptides) were found to bind in a
variety of conformations to a human light chain (Bence-Jones) dimer from a
patient (Mcg) with amyloidosis. The peptides were diffused into crystals of the
protein and their positions, relative occupancies and modes of binding were
determined at 2.7 A resolution by difference Fourier analyses. Collectively, the
opioid peptides occupied practically all of the available space in the concave,
internal parts of the binding region, as well as flat or convex external
surfaces around the rim of the binding cavity. Suitable ligands ranged in size
from four to seven residues. As many as five residues could be accommodated
inside the binding region, and there was space for at least four residues on the
external surfaces. External binding was influenced by solvent effects and local
packing interactions among adjacent protein molecules in the crystal lattice. In
the enkephalin series the presence of amino-terminal tyrosine was necessary, but
not sufficient for binding. [Met]-enkephalin, a pentapeptide, showed two
different modes of binding in overlapping subsites. In one subsite, preferred
over the second in a ratio of 1.3:1.0, the side chain of amino-terminal tyrosine
penetrated through the floor of the main cavity to lodge in the deep binding
pocket about 20 A from the entrance. The remainder of the peptide spanned the
length of the main cavity in an extended conformation. In the second subsite the
amino end was restricted to the main cavity and the peptide backbone turned
abruptly upward at residue 3 to interact with external surfaces. An (Arg-6,
Phe-7) heptapeptide extension of [Met]-enkephalin entered the deep pocket and
assumed an extended conformation in the main cavity like the pentapeptide. Its
last two residues flattened against the external surfaces. [Leu]-enkephalin and
its analogues displayed a combination of internal and external binding like
[Met]-enkephalin in its secondary subsite. Enkephalin analogues with D-amino
acids in position 2 generally adopted conformations which were more convoluted
than those in the L-isomers. Moreover, external interactions tended to be more
prominent in the D-derivatives. The beta-casomorphin-7 heptapeptide penetrated
into the deep pocket and traversed the main cavity in as extended a conformation
as the presence of two proline residues would permit. On removal of the ligand
there was an unexpected hysteresis effect involving permanent structural
alterations in the walls of the binding region. beta-casomorphins-4 and -5 were
bound in the main cavity with the carboxyl ends protruding from the
entrance.(ABSTRACT TRUNCATED AT 400 WORDS)
|
