Prion is an infectious agent composed of protein in misfolded a form. It is a product of transformation normal PrP protein that is necessary for nervous system working. While transformation the primary structure of protein (amino acid sequence) remains constant, but his form or spatial conformation is changing. Protein structure becomes more dense and stable. Unfortunately it becomes more protected against temperature, proteolysis and chemical influence. Partially it caused by higher than normal beta-sheets contain. Cause protein with hi contain of beta-sheets has more inner hydrogen connections that makes structure tougher. It was first reason. Prions are dissoluble in water. There are no close relations between water particles and dissoluble protein molecule; it means that conduction of kinetic energy from water to this protein is low. Third reason is adhesion between prions molecules. Prions are presented in cell like big crystalloids, which consist of many and many prion particles. They keep together very strong and becomes more stable while number of prions is grow. It means that all prion molecules can’t be destroyed in one moment, or even during short period of time. Usually normal autoclaving continues 15 minutes in 121 °C, or 134 °C for 3 minutes. But during this time outer layers of prion conglomerate will destroy protecting inner layers. More continuous autoclaving need to kill prion. Three reasons: 1. High content of beta-sheet; 2. Dissoluble in water; 3. Adhesion property, tendency to form large conglomerate.
Sources: 1. http://en.wikipedia.org/wiki/Prion#Prion_replication_mechanism 2. Jiali Li et al. Darwinian Evolution of Prions in Cell Culture // Science. 2010. V. 327. P. 869–872.