MadSci Network: Biochemistry

Re: Why are prions, which are proteins, not denatured by heat??

Date: Mon Apr 2 19:27:24 2001
Posted By: Mark Sullivan, Medical Student
Area of science: Biochemistry
ID: 981575849.Bc

Hi Hector,
     Prions are a very interesting topic because there is still a lot of 
debate over whether a protein can initiate changes in other proteins to 
cause disease or if these proteins are just manifestations of another 
underlying process.  Anyway, prions are known to be responsible for 
diseases such as Bovine Spongiform Encephalitis(mad-cow), Creutzfeldt-
Jakob disease, and Scrapies.  What essentially happens is that an animal 
eats something that contains the prion from a similar animal.  This prion 
is acid, heat, UV, and other denaturing agent resistant due to it's very 
hearty tertiary structure(which I will get to in a minute).  It somehow 
finds normal precursor proteins, called PrP for short, and converts them 
to Prions.  These guys then accumulate in tissue, the effects of which 
especially apparant when it involves the brain because neurological 
symptoms arise like Mad-cow disease.  
     Tertiary structure as you may know has to do with folding of a 
protein into a 3-D shape.  This shape depends on things like primary 
stucture(amino acid sequence) and secondary structure (alpha helix aka 
twists and beta pleated sheets aka turns) as well as the type of 
environment it is in(water, lipid).  What makes a protein hardy is the 
number of crosslinks it has, especially by cysteine residues that form 
disulfide bonds.  Let's take curly hair for example.  It has a lots of 
disulfide bonds that hold the structure of all those proteins in their 
normal shape.  It is fairly resistant to heat(barring the extreme) and 
such, but when we add something like beta-mercaptoethanol the disulfide 
bonds break and we can straighten the hair out.  Other factors like 
hydrophobic regions also aid to hold tertiary structure and unless you 
remove the insulting environmental water those regions of the protein will 
do their best to exlude water through electrochemical interactions.  
     Prions are just very good at resisting all those insulting factors 
that would otherwise denature it.  It is a little more complex than what I 
have said above, but that is the basic idea.  For more, and better, 
information on prions I suggest you check out this article found in the 
journal called PNAS (or Proceedings of the National Acadamy of Science).  
It is by Stanley B. Prusiner who is at the forefront of prion research.  
It is an overview of prions, with a lot of technical info but I am sure 
that you'll have a better understanding of these interesting proteins 
after reading this.  You can find it in Vol. 95, Issue 23, pages 13363-
13383, November 10, 1998.  It is simply titled Prions.  Good reading.

Mark Sullivan

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