Folding minimal sequences: the lower bound for sequence complexity of globular proteins.

Alphabet size and informational entropy, two formal measures of sequence complexity, are herein applied to two prior studies on the folding of minimal proteins. These measures show a designed four-helix bundle to be unlike its natural counterparts but rather more like a coiled-coil dimer. Segments from a simplified sarc homology 3 domain and more than 2000000 segments from globular proteins both have lower bounds for alphabet size of 10 and for entropy near 2.9. These values are therefore suggested to be necessary and sufficient for folding into globular proteins having both rigid side chain packing and biological function.