Not all proteins follow this folding model: can randomly fold different secondary structures, these secondary structures start to interact with each other and make globular proteins, this happens alone and on its own. Peptidyl prolyl cis-trans isomerase: polypeptides are synthesized with most x-pro peptide bonds in the trans conformation, most proteins are in the trans form, ~10% of these are in the cis conformation in globular proteins; mostly in beta turns. Uncatalyzed cis-trans isomerization is extremely slow for proline: removing the proline leads to much faster folding times in vitro. Peptidyl prolyl cis-trans isomerase: converts proline between cis and trans isomers, although the trans form is low energy, the reaction is driven by the. Increases speed greatly ability of the protein to now fold correctly: remember, very few prolines require this! The heat shock response is unregulated to counter sever stress: pretreat: heat at low temp for a little bit enables heat shock response.