Folding occurs step-wise with several intermediates unfolded/secondary structure/domains/molten globule/native tertiary structure. The steps include, a collapsed structure (molten globule) occurs very quickly. steps between molten globule and native tertiary structure usually occur slowly (a) intermediates are isolatable (b) multiple pathways are possible. Folding is driven by hydrophobic forces. Proteins can self assemble but in vivo folding is facilitated by proteins. Chaperones are binding proteins which assist folding (a) chaperones cause misfolded protein to unfold rather than aggregate (b) many chaperones require ATP hydrolysis for activity (c) more than one chaperone may act simultaneously and sequentially in the folding of a single protein (d) chaperones are specific for specific protein synthesis pathways (cytosolic vs. mito. vs. endoplasmic reticulum). Enzymes catalyze kinetically slow steps infolding (a) cis-trans prolyl isomerase (i) both cis and trans peptide bonds to proline naturally occur (ii) the isomerization of peptidyl-proline bonds may be slow (b) protein disulfide isomerase (i) catalyzes disulfide bond formation and isomerization. Denaturation is unfolding 1. Requires some input to overcome hydrophobic forces (a) heat (b) denaturant (urea or guanidinium) and requires reductant to reduce disulfide bridges to sulfhydryls.
