<p>This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.</p> <p>Structural and functional complexity of proteins is dramatically reduced to a simple linear picture when the laws of polymer physics are considered. A basic unit of the
protein structure is a nearly standard closed loop of 25–35 amino acid residues, and
every globular protein is built of consecutively connected closed loops. The physical
necessity of the closed loops had been apparently imposed on the early stages of
protein evolution. Indeed, the most frequent prototype sequence motifs in prokaryotic
proteins have the same sequence size, and their high match representatives are found
as closed loops in crystallized proteins. Thus, the linear organization of the closed
loop elements is a quintessence of protein evolution, structure and folding.</p>
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Comp Funct Genomics. 2002 Dec; 3(6):525-534