Alzheimer's and "mad-cow" diseases are unique in that their infectious agents
are not viruses or germs, but rather proteins. The brains of patients who suffered
from Alzheimer's or cows that died of "mad-cow" disease show deposits of abnormal
tissue called amyloid plaques. The primary component of these plaques is a protein
called prion protein or PrP. Chemical and biochemical analysis showed that there
was no difference in composition or primary structure between the normal, cellular
form of PrP (PrPC, shown at right) and the disease form of PrP (PrPSc).
Further analysis showed that PrPC can change into PrPSc when
two of the α helices (shown in green) change into ß sheets.
This ß sheet can then induce a similar change in another molecule of PrPC and
hydrogen bond to it. The PrPScs then polymerize and come out of solution,
forming the plaques found in Alzheimer's patients and mad cows. How the plaques
cause the symptoms of the diseases is still not clear, but the prion protein
holds the unique distinction of causing a disease solely through a small alteration
in secondary structure.