A Cubic Equation of State for Amyloid Plaque Formation

Protein function depends on both protein structure and amino acid (aa) sequence. Here we show that modular features of both structure and function can be quantified from the aa sequence alone for the amyloid 770 aa precursor protein A4. Both the new second order hydropathicity scale, based on evolutionary optimization (self-organized criticality), and the standard first order scale, based on complete protein (water-air) unfolding, yield thermodynamically significant features of amyloid function related to spinodal decomposition into modules. Spinodal crossings are marked by breaks in slope of algebraic functions of aa sequences. The new scale is associated with a lower effective temperature than the standard scale.