Prions

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Not all infectious diseases are transmitted by bacteria or viruses. Some neurological diseases, such as Creutzfeldt-Jaakob disease (CJD) or mad cow disease are in fact caused by agents called Prions, which are of similar size to viruses but are made up of only protein [1] These diseases are can be called Prion diseases or transmissible spongiform encephalpathies (TSE). [2] 

Prions have these characteristics:

  1.  "The transmissible agent consists of aggregated forms of a specific protein" [3].
  2. These protein aggregates cannot be degraded by the agents that degrade most other proteins [4].
  3. "The protein is largely or completely derived from a cellular protein called PrP, that is normally present in the brain" [5].

So an aggregated form of a protein (that is already present in the brain) is the infectious agent in prion diseases [6].

The pathalogic mechanism of prions is often associated with structural change. Exogenous prions cause the endogenous host proteins to undergo a structural change, rendering them functionless or harmful.

References

  1. J. M. Berg et al. (2007) p 53, Biochemistry, Sixth edition, New York, W.H. Freeman and Company
  2. http://www.who.int/bloodproducts/tse/en/
  3. J. M. Berg et al. (2007) p 53, Biochemistry, Sixth edition, New York, W.H. Freeman and Company
  4. J. M. Berg et al. (2007) p 53, Biochemistry, Sixth edition, New York, W.H. Freeman and Company
  5. J. M. Berg et al. (2007) p 53, Biochemistry, Sixth edition, New York, W.H. Freeman and Company
  6. J. M. Berg et al. (2007) p 54, Biochemistry, Sixth edition, New York, W.H. Freeman and Company
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