Size and folding in globular proteins.

We have modeled protein folding by packing a unified length of regular structural elements (alpha-helices and beta-sheets) into a 'cube'. In a globular protein with m alpha-helices and n beta-strands, this unified length is expressed in units of heptapeptides in alpha-helices, and in units of tripeptides in beta-strands. Calculations using published data show that a 4-helix bundle (m = 4, n = 0) has at least 2 x 2 x 2 helical heptapeptides; the 16-strand beta-barrel of porin (m = 0, n = 16) is at most 4 x 4 x 4 tripeptides in beta-strands. Compact, recurring protein modules with mixed helices and beta-strands are the ones that actually acquire a geometrically quasi-spherical, or cubic, shape.