'Normal' PrP^C has a single cystine bond (cysteine amino acid [cys-SH] bound to another cysteine to form cys-S-S-cys). This is an intermolecular cystine bond (both cysteines provided by the some protein molecule). The 'normal' structure of PrP^C is shown in white in the left of the above figure (white arrow indicates position of cystine bond). Such bonds are common to secreted proteins and those found on the outside surfaces of cells. Cystine bonds help stabilize protein structure.

PrP^Sc may have lost its intermolecular cystine bond, thus exposing the sulfur side chains on the cysteine molecules (arrows on right side of figure above). Such exposed cysteine residues are highly reactive. Loss of an intermolecular cystine bond may result from:

• mutations of the molecule such as those found in familial CJD or GSS, the changes in the amino acid sequence may alter the structure of PrP^C possibly making the normal cystine bond less stable
• random misfolding of the PrP^C protein
• reductive 'damage' to the protein in the intercellular mileau (the cells cytoplasm is a reducing environment)
• interaction with PrP^Sc (see below)

Experimental reduction of PrP^C does cause the protein to take on more ß-structure (possibly similar to the increase in ß-structure found in PrP^Sc). A cause-effect relationship between PrP^C reduction and the formation of PrP^Sc has not been established, but the evidence has been reviewed and put forth as a plausible theory for the exponential growth of amyloid fibrils in brains affected by TSEs .

	In brief, the free cysteine residues [cys-SH] react to form linear polymers of PrPSc (see figure to the left). Thus PrPSc polymers would be held together by covelent bonds involving intramolecular cystine bonds (i.e. the cysteine residues are supplied by different protein molecules). Such a structure could explain the stability of amyloid fibrils. The end of an amyloid fibril would have a single cys-SH molecule (white arrow). This type of structure is a disulfide polymer (more than one protein molecule [polymer] held together by a disulfide bonds [cys-S-S-cys]). (note this figure is for illustration, if or which cysteines react with each other is unknown)
	Important to the development of spongiform encephalopathies, the free cys-SH residue at the end of the amyloid fibril may interact with PrPC (see figure to left; for simplicity most of the PrP structure is not shown). During the interaction, PrPC is converted to PrPSc and added to the end of the fibril by a rearrangement of its cystine bond from an intercellular bond to an intracellular bond. The ability to rearrange cystine bonds is called disulfide isomerase activity.
When PrPSc aggregates interact with cells, they are internalized along with PrPC. Within the reductive environment of the cell, the fibrils may dissociate into numerous smaller fibrils exposing additional cys-SH ends to further recruit PrPC leading to an exponential increase in PrPSc within fibrils.

The proof of cystine bond rearrangements being involved in TSE development awaits the 3D structure of PrP^Sc. Despite the importance of attaining this bit of information, the structure of the proteins within amyloid fibrils is not known. In large part, the difficultly in obtaining this information is based on the aggregation qualities of PrP^Sc. PrP^Sc is not soluble as a monomer and readily aggregates making it difficult to obtain structurally intact PrP^Sc molecules for analysis. Treating PrP^Sc aggregates to dissociate the protein likely free monomers by changing the structure of PrP^Sc.

Several lines of evidence support a role of cystine bond rearrangement in TSE formation:

• reduction of PrP^C does change its structure making it similar to that of PrP^Sc
• blocking cysteines destroys ability of PrP^Sc to transmit disease
• mutations removing the cysteines experimentally block PrP^C conversion to PrP^Sc
• in animals the growth of PrP^Sc is thought to occur within cells (i.e. within a reducing environment; intracellular reduction may break or weaken normal cystine bonds possibly allowing their rearrangement when interacting with PrP^Sc)
• many of the mutations that result in TSE are clustered near the cysteine residues; possibly destabilizing normal tertiary protein structure allowing for disulfide polymerization
• redox (reduction-oxidation; in part the breaking and making of cystine bonds) reactions often are accentuated by metal ions; the metal binding octapeptide repeat of PrP binds copper and its presence accentuates development of Scrapie in mice ; also mutations of the PrP gene that result in additional octapeptide repeats are associated with the development of human TSE.

Of particular interest, the PrP^Sc 27-30 found in amyloid plaques are often truncated resulting in the loss of both cysteine residues. The proposed method of prion 'growth' outlined above implies that PrP^Sc 27-30s are not a disease transmitting form of the protein. Transmission of the disease might require exposure to the mature intact PrP^{Sc and not the proteolytically cleaved peptides found in plaques}. Additionally, the intramolecular bonds may stabilize the structure of PrP^Sc, but would not do so to PrP^Sc 27-30. Stability of PrP^Sc 27-30 may be an effect from amyloid plaque formation.

click on the image for more about amyloid plaques and protein structure