Hydrophobic interactions determine the optimum temperature of a housekeeping enzyme.

Chordates have adapted to diverse thermal environments, with poikilotherms adjusting to external temperatures and homeotherms maintaining stable body temperatures. While housekeeping enzymes conserve their activities, they function at different body temperatures among species. However, the determinants for the optimum temperatures of housekeeping enzymes largely remain unclear. In this study, we identified the determinants of the optimum temperatures of chordate adenylate kinase 1 (AK1), a key housekeeping enzyme. The optimum temperatures of AK1s were shown to be closely correlated with the normal body temperature of each chordate (11 species). A combination of enzymatic assays, computational analyses of numerous physicochemical interactions, and structural dynamics analyses of intact and mutant AK1s verified that the number of hydrophobic interactions among four amino acids in specific secondary structures (the 4th-17th regions) is a major determinant of the optimum temperatures of chordate AK1s. This allowed us to generate a predictive model for the optimum temperatures of native chordate AK1s: AK1 optimum temperature = 2.3587 × (the number of 4th-17th interactions) + 0.3663 × (the number of 13th-17th interactions) -5.9695, with a maximum error of ±1.70 °C. In contrast, sequence similarity and molecular phylogenetic relationships did not correlate with the optimum temperatures of chordate AK1s. Furthermore, these results suggest an evolutionary scenario for thermal adaptation of chordate AK1. Collectively, our study provides evidence that subtle hydrophobic interactions play a crucial role in determining the temperature preferences of a chordate housekeeping enzyme, offering new insights into the functional evolution and diversification of enzyme homologs.