Protein interactions. IV. Aggregation of globular proteins

Many globular proteins are known to polymerize. It is pointed out that a monomer protein molecule containing a single polypeptide chain is asymmetric, and that a single pair of complementary combining regions on such a molecule would permit it to attach itself to equivalent molecules in such a way as to form a helical structure, giving rise to fibrils. If the pitch of the helix is zero or nearly zero the helix degenerates into a circle, and a small polymer (dimer, trimer) is formed. A reasonable amount (about 5°) of freedom of angular motion in the bond between adjacent molecules would permit a helical fibril of large diameter to change its length by as much as a factor of two, and would also permit tubular aggregates with walls one, two, or three molecules thick to be formed. It is suggested that some of the observed properties of globular proteins can be accounted for in this way.