Figure 4: Reaction network and consistent mechanisms.
(a) Monomeric PrPC is represented by blue spheres and PrPSc aggregates are represented by blue bars. Aggregates can grow by incorporating more PrPC and can multiply for example by fragmentation. Growth and multiplication couple together in an auto-catalytic manner, the whole cycle is referred to as replication. Finally, the loss of aggregates by degradation and sequestration into plaques both have the effect of preventing the removed aggregates from further participating in the aggregation reaction (b) The scaling exponent, γ, is plotted against the functional form of the increase of aggregate mass with time, from linear to exponential. For the formation of linear aggregates without multiplication, the mass increase is linear in the absence of any spontaneous aggregate formation from monomer alone (II), or low-order polynomial if spontaneous formation of aggregates occurs (III). If aggregates are able to multiply, the mass increase is exponential (I, IV, V). Fragmentation of aggregates results in a scaling of approximately 0.5 (I), direct monomer conversion leads to a scaling of 1 (IV), and secondary nucleation gives a scaling of 1 or above (V).