Protein folding and protein refolding.
The functional three-dimensional structure of proteins is determined solely by their amino acid sequences. Protein folding occurs spontaneously beginning with the formation of local secondary structure concomitant with a compaction of the molecule. Secondary structure elements subsequently interact to form subdomains and domains stabilized by tertiary interactions. Disulfide bond formation, and cis-trans isomerization of X-Pro peptide bonds, as the rate-limiting folding reactions, are enzymatically catalyzed during protein folding in the cell. Although folding of domains is fast enough to occur cotranslationally in vivo, such vectorial folding on the ribosome is not essential for attainment of the native structure of a protein. Slow steps on the pathway to the functional protein structure are docking reactions of domains, association of subunits, or reshuffling reactions at the oligomer level. Aggregation as a competing side reaction is prevented, and the kinetic partition between competing polypeptide folding and translocation reactions is regulated by chaperone proteins binding to incompletely folded polypeptides.