Determinants of thermostability in the cytochrome P450 fold. 
Cytochromes P450 are found throughout the biosphere in a wide range of environments, serving a multitude of physiological functions.  The ubiquity of the P450 fold suggests that it has been co-opted by evolution many times, and likely presents a useful compromise between structural stability and conformational flexibility.  The diversity of substrates metabolized and reactions catalyzed by P450s makes them attractive starting materials for use as biocatalysts of commercially useful reactions.  However, process conditions impose different requirements on enzymes to those in which they have evolved naturally.  Most natural environments are relatively mild, and therefore most P450s have not been selected in Nature for the ability to withstand temperatures above ~40°C, yet industrial processes frequently require extended incubations at much higher temperatures.  Thus, there has been considerable interest and effort invested in finding or engineering thermostable P450 systems.  Numerous P450s have now been identified in thermophilic organisms and analysis of their structures provides information as to mechanisms by which the P450 fold can be stabilized.  In addition, protein engineering, particularly by directed or artificial evolution, has revealed mutations that serve to stabilize particular mesophilic enzymes of interest.  Here we review the current understanding of thermostability as it applies to the P450 fold, gleaned from the analysis of P450s characterized from thermophilic organisms and the parallel engineering of mesophilic forms for greater thermostability.  We then present a perspective on how this information might be used to design stable P450 enzymes for industrial application.  This article is part of a Special Issue entitled: Cytochrome P450 biodiversity and biotechnology, edited by Erika Plettner, Gianfranco Gilardi, Luet Wong, Vlada Urlacher, Jared Goldstone.