Despite numerous studies, understanding the structural basis of protein stability in thermophilic organisms has remained elusive. One of the main reasons is the limited number of thermostable protein structures available for analysis, but also the difficulty in identifying relevant features to compare. Notably, an intuitive feeling of “compactness” of thermostable proteins has eluded quantification. With the unprecedented opportunity to assemble a data set for comparative analyses due to the recent advances in structural genomics, we can now revisit this issue and focus on experimentally determined structures of proteins from the hyperthermophilic bacterium Thermotoga maritima. We find that 73% of T. maritima proteins have higher contact order than their mesophilic homologs. Thus, contact order, a structural feature that was originally introduced to explain differences in folding rates of different protein families, is a significant parameter that can now be correlated with thermostability.
