Infectious disease agents Membrane cell

The abnormal deposits found in the brains of CJD victims consist of an abnormal isoform of PrP. Prion protein is normally found in cells. Detailed structural studies show that normal cellular PrP (PrP ) is a soluble protein whose conformation is rich in a-helices with very little P-sheet. The PrP protein extracted from the brains of CJD victims (i.e., PrP ) is identical in primary amino acid sequence to the normal PrP (PrP ). However, PrP has a much greater content of P-sheet conformation with little a-helical structure. Thus PrP is neurotoxic because of its three-dimensional structure. When the prion protein is predominantly in an a-helical conformation it is nontoxic when the prion protein is predominantly in a P-sheet conformation, it kills neurons. The prion protein is thus made neurotoxic not by its amino acid composition but by its conformation. This concept is both fascinating and terrifying. Prion diseases are transmissible thus prions are infectious agents. However, prions are not like bacteria or viruses, or other infectious microbes—they are simply protein molecules. Prions are not microbes with cell membranes and nucleic acids they are not living things. Indeed, prions are not even infectious molecules, they are infectious molecular shapes. 

This group of diseases is characterized by the accumulation of a specific isoform of the normal prion protein (PrP), termed PrPSc, which seems to be the main infectious agent. PrPSc is derived by a conformational change from the isoform of the prion protein PrPC. PrPSc is a copper-binding glycoprotein attached to the cell membrane of neuronal cells [21]. The PrPSc form is protease resistant, has a beta-sheet structure and possesses a tendency to polymerization [22]. 

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