Molecular basis of thermostability in the lysozyme from bacteriophage T4

Abstract

MOST proteins are denatured at temperatures above 50–60°C, although some enzymes, especially those from thermophilic organisms, remain active at temperatures up to 80–90 °C. The determination of the three-dimensional structure of the thermostable protease thermolysin showed that heat-stable proteins do not contain unusual structural features absent from less stable proteins^1,2. Furthermore, the amino acid sequences of similar proteins from both mesophilic and thermophilic sources have been shown to be homologous, suggesting that the respective structures are similar^3,4. Nevertheless, such homologous amino acid sequences also include many differences which obscure those amino acid changes actually responsible for differences in thermostability. We report here the structure of a temperature sensitive (ts) mutant of T4 phage lysozyme. This permits the first direct comparison of two protein structures in which all differences are directly related to a change in thermal stability. It is shown that, except for the replacement of a partially exposed arginine by a histidine, the three-dimensional structure of the ts lysozyme is virtually identical with that of native lysozyme.