Conformational energy studies of beta-sheets of model silk fibroin peptides. I. Sheets of poly(Ala-Gly) chains.
A new model structure is proposed for the silk I form of the crystalline domains of Bombyx mori silk fibroin and the corresponding crystal form of poly(L-Ala-Gly). It was deduced from conformational energy computations on stacked sheet structures of poly(L-Ala-Gly). The novel sheet structure contains interstrand hydrogen bonds but is composed of anti-parallel polypeptide chains whose conformation differs from that of the antiparallel beta-sheets that constitute the silk II structure. The strands of the new sheet have a two-residue repeat, in which the Ala residues adopt a right-handed and the Gly residues a left-handed sheet-like conformation. The computed unit cell is orthorhombic, with cell dimensions a = 8.94 A, b = 6.46 A, and c = 11.26 A. The model accounts for most spacings in the observed fiber x-ray diffraction patterns of silk I and of the silk-I-like form of poly(L-Ala-Gly), and it is consistent with nmr and ir spectroscopic data. As a test of the computations, the well-established beta-sheet structure of silk II and the corresponding form of poly(L-Ala-Gly) have been reproduced. The computed energies for the two forms of poly(L-Ala-Gly) indicate that the silk-II-like form is more stable, by about 1.0 kcal/mol per residue. The main difference between the two structures is the orientation of the Ala side chains of neighboring strands in each sheet. In the Pauling-Corey beta-sheet and in the silk II form, referred to as an "in-register" structure, the Ala side chains of every strand point to the same side of a sheet. In the silk I structure, referred to as "out-of-register," the side chains of Ala residues in adjacent strands point to opposite sides of the sheet.
Fossey
S A
SA
Baker Laboratory of Chemistry, Cornell University, Ithaca, New York 14853-1301.
Némethy
G
G
Gibson
K D
KD
Scheraga
H A
HA
D000595:Amino Acid Sequence; D005349:Fibroins / Q000737:chemistry*; D008433:Mathematics; D008958:Models, Molecular; D008962:Models, Theoretical; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry*; D011108:Polymers; D011487:Protein Conformation; D013816:Thermodynamics
D010455:Peptides; D011108:Polymers; C034500:poly(alanylglycine); D005349:Fibroins