Specificity of the commercially important serine protease, proteinase K, has been investigated by measuring
free energies of association of proteinase K with turkey ovomucoid third domain inhibitor variants at contact positions P2,
P1, P1', P2', and P3'. Correlations of these values were run with similar values that have been obtained for six other serine
proteases. Among the six proteases, subtilisin Carlsberg shows a near perfect correlation (Pearson Product correlation coefficient
= 0.93 to 0.99) with proteinase K at all of these positions. Proteinase K has only 35% sequence identity with subtilisin
Carlsberg, yet, the two enzymes are nearly identical in their specificity at P2 to P3' positions. With other serine proteases
such as bovine chymotrypsin, human leukocyte elastase, porcine pancreatic elastase, Streptomyces griseus protease
A and B, proteinase K showed relatively poor or no correlation.