The geometry of alpha-sheet: Implications for its possible function as amyloid precursor in proteins.

alpha-sheet has been proposed as the main constituent of the prefibrillar intermediate during amyloid formation. Here the helical parameters of the alpha-sheet strand are calculated from average main-chain dihedral angles reported from molecular dynamics simulations. It is an almost linear polypeptide that forms a right-handed helix of about 100 A diameter, with 100 residues and a rise of 30 A per turn. The strands are curved but untwisted, which implies that neighboring strands need not coil to make interstrand hydrogen bonds. This suggests that compared to beta-sheets in native folded proteins, alpha-sheets can be larger and stack more easily to create extensive 3D blocks. It is shown that alpha-sheet is related to a category of structures termed "mirror" structures. Mirror structures have repetitive pairs of main-chain dihedral angles at residues i and i+1 that satisfy the condition phi(i) (+1) = -psi(i), psi(i) (+1) = -phi(i). They are uniquely identified by the two orientations of their peptide planes, specified by phi(i) and psi(i). Their side chains point alternately in opposite directions. Interestingly, their conformations are insensitive to phi(i) and psi(i) in that the pseudo dihedral angle formed by four consecutive C(alpha) atoms is always close to 180 degrees . There are two types: "beta-mirror" and "alpha-mirror" structure; beta-mirror structures relate to beta-sheet by small peptide plane rotations, of less than 90 degrees , while alpha-mirror structures are close to alpha-sheet and relate to beta-sheet by approximately 180 degrees peptide plane flips. Most mirror structures, and in particular the alpha-mirror, form wide helices with diameters 50-70 A. Their gentle curvature, and therefore that of the alpha-sheet, arises from the orientation of successive peptide units causing the difference in the bond angles at the C and N atoms of the peptide unit to gradually change the direction of the chain.