Ak.1 protease, a thermostable subtilisin isolated originally from Bacillus st. Ak.1, was purified to homogeneity from the Escherichia coli clone PB5517. It is active against substrates containing neutral or hydrophobic branched-chain amino acids at the P₁ site, such as valine, alanine or phenylalanine. The Km and kcat of the enzyme decrease with decreasing temperature, though not to the same degree with all substrates, suggesting that specificity changes with temperature. The protease is markedly stabilized by Ca²⁺ ions. At 70 °C, a 10-fold increase in Ca²⁺ concentration increases the half-life by three orders of magnitude. Ak.1 protease is stabilized by Ca²⁺ to a greater extent than is thermitase. This may be due, in part, to the presence of an extra Ca²⁺-binding site in Ak.1 protease. Other metal ions, such as Sr²⁺, increase the thermostability of the enzyme, but to a significantly lower degree than does Ca²⁺. The structure of the protease showed the presence of a disulphide bond located within the active-site cleft. This bond influences both enzyme activity and thermostability. The disulphide bond appears to have a dual role: maintaining the integrity of the substrate-binding cleft and increasing the thermostability of the protease. The protease was originally investigated to determine its usefulness in the clean-up of DNA at high temperatures. However, it was found that this protease has a limited substrate specificity, so this application was not explored further.