[Super-secondary structure of beta-proteins].

A structural unit common for all the known beta-proteins as well as for some others is considered in the paper. The simplest variant of this unit consists of four consecutive beta-strands denoted as a, b, c and d. Three of the strands, a, b and d, form an antiparallel beta-sheet, and the fourth, c, lies in the other layer of a layered protein sandwich structure. The strands, b and d, are parallel in the beta-sheet and together with the c-strand fold into a right-handed bcd-superhelix which is analogous to the superhelix formed by beta alpha beta regions. An a-strand is found between the strands b and d and antiparallel to them. Such locally ordered regions of proteins are described here as abcd-units. In all the known proteins the abcd-unit was found to be always at the edge of a protein bilayer. The remaining strands of a molecule or a domain are always located on the same side of the abcd-unit where the d-strand lies. It is shown that the packing of beta-strands in the beta-proteins depends on the packing of the other strands. The structural approach suggested here allows the packing of beta-strands in three-dimensions to be obtained if the abcd-unit is taken as the embryo of folding.