Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Globular Proteins: Structure and Function of Hemoglobin
Polypeptide chains fold at an extremely high rate

In living Cells, Proteins are synthesized from Amino Acids at an extremely high rate. For example, in E. coli cells, a fully biologically active protein molecule containing 100 amino acid residues can be assembled in 5 s at 37 ˚C. However, calculations show that if a polypeptide chain of 100 amino acid residues were to randomly sample all possible angles of rotation around each single bond of the backbone until it "found" its native biologically active conformation, it would take at least 10⁵⁰ years!

Thus, proteins cannot attain their correct conformation by folding in a completely random, trial-and-error fashion. There must be shorter, more direct pathways. We do not know precisely how or by what pathway The process of spontaneous protein folding occurs—whether it begins at one end of the chain, in the middle, or at several points simultaneously. However, the spontaneous folding of polypeptide chains into their correct tertiary Structure must clearly be highly cooperative. This means that if some minimal segment of the chain folds correctly, it greatly increases the probability that the remaining Regions of the chain will fold properly.