RNase stability

Rimas Rimantas.Plaipa at gf.vu.lt
Mon Feb 19 05:09:26 EST 2001


Nazli Syed wrote:
> 
>  RNases have 4 disulphide bonds which makes them resistant to
> denaturation to high temperatures and pH.
> Disulphide bonds render resistance to RNase unfolding.
> 
> Nazli
> 
> ramesh wrote:
> 
> > HI!
> >
> > can anyone tell me why RNases have evolved as such stable enzymes.
> > They r
> > known to be active across such a wide range of pH and temperatures.
> > I am
> > unable to come up with a valid explanation for this. I would
> > certainly
> > appreciate help in this regard
> > thanx
> >

Denaturation have nothing to do with stability of RNAses. They are small
enzymes (~100 aa) and they renature immediately when the solution is
coooled. Some of them don't have disulphide bonds but are thermostable
equaly well (one such RNase without Cys - barnase - has been (and
probably still are) used by Fersht for protein folding studies). 

So because they cannot be denatured the only way to kill them is
chemical degradation. And disulphide bonds are actualy their weakest
point (is neutral/slighly alkaline media) - they degrade through
reduction (if reducing agents are present) , -SS- shufling (if free SH
are present) or beta-elimination (which eventualy splits polipeptide
chain and completely inactivates enzyme). 
If no disulphide bonds are present in the molecule the main mechanism of
degradation is deamidation of Asn/Gln which is slower and not necessary
inactivates enzymatic activity immediately. Deamidation is also the main
way of degradation in acidic media.

The kintics have been investigated in:
S.E. Zale and A.M. Klibanov (1986 Sep 23 #19) Biochemistry 25,
5432-5444. Why does ribonuclease irreversibly inactivate at high
temperatures?

-- 
Rimas


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