I'm guessing its because of the specific interactions that take place between the amino acid sequence- like the disulphide bonds that can only occur between cysteine and methionine because they are the only aas with sulphur in their structure... - Rachel Bell

So in short, there actually isn't many ways that it can fold.. - Rachel Bell

Not all proteins are able to fold back by themselves. Some proteins need the help of other specialized substances to help them fold back. These are called ceprons (or something spelt like that). It is true that the probability of proteins folding back to their exact same shape is low. However, for those proteins which auto-renature, this is because of their structure dictating the shape. For instance, the hydrophobic parts of a protein will never be exposed out in the aqueous surrounding, so this omitts a great number of folding ways and increases the probability of original shape. - Abdulkhaliq Alsaadi from iPhone

Chaperones not 'ceprons' - Dr Chris Willmott from iPod

Thanks Rachel and Abdulkhaliq! - Hasan Mohamed

The chaperones only help the proteins to fold back after they have suffered from heat shock and have been unfolded. - Hasan Mohamed

@Hasan not correct - SOME proteins always require the help of chaperones, the alternative name "heat shock proteins" reflects the fact that they were first identified after cells were exposed to high temperatures but it doesn't follow that the ONLY time they are needed is after heat shock - Dr Chris Willmott