Hey this is from the abstract of: Sequence optimization and designability of enzyme active sites - PNAS v. 102 (2005) Full text is here: http://www.pnas.org

Hey this is from the abstract of: Sequence optimization and designability of enzyme active sites - PNAS v. 102 (2005) Full text is here: http://www.pnas.org/content/102/34/12035.full http://www.pnas.org/content/102/34/12035.full """ Since it was first observed that all known natural proteins adopt one of only ≈1,000 distinct protein folds (1) [emphasis mine], researchers working in the field of computational protein design have endeavored to understand the basis of this biophysical phenomenon and apply the resultant knowledge to the construction of unnatural proteins. Motivated by the observation that certain folds are associated with more natural sequences than others, the principle of fold designability (the number of unique sequences compatible with the backbone structure of the fold) was introduced (1). To place the problem on a mathematical foundation, an analogy has been drawn between sequence space and phase space in statistical physics, where the backbone structure represents a macroscopic state of the system and distinct sequences are microscopic states whose occupation of the macroscopic state is associated with an energy, the folding free energy of the protein. In this language, a fold's designability is correlated with its sequence entropy (2). """