Structure of amyloid
All amyloids share a unique ultrastructure as seen by electron microscopy. Thin, nonbranching protein fibrils constitute approximately 90% of amyloid deposits. Fibrils tend to aggregate laterally to form fibers. X-ray diffraction studies show that the polypeptide chains are oriented perpendicularly to the long axis of the fibril, forming a cross β-pleated sheet conformation. Serum amyloid P component (SAP), a protein composed of two pentagonal subunits forming a doughnut-like structure, makes up 5%. SAP is 50% homologous to C-reactive protein but not an acute-phase reactant. The remainder of amyloid is composed of small amounts of certain glycosaminoglycans (GAGs) including heparan sulfate and dermatan sulfate as well as specific apolipoproteins (E and J).
