Size and folding in globular proteins

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