A clade of trypsins found in cold-adapted fish.

A clade of trypsins, known as group III, is identified by phylogenetic analysis. These trypsins occur in fish that spend all or part of their lives at temperatures near 0 degrees C and may represent extreme psychrophilic enzymes. A principal component analysis of amino acid compositions distinguishes group III from mesophilic trypsins, as do molecular trees and multidimensional scaling of molecular sequence distances. The primary sequences of group III trypsins, in conjunction with the known structures of mesophilic trypsins, permit insight into function and mechanisms of cold adaption. The techniques employed are broadly applicable to phylogenies characterized by a markedly different, or "fast-evolving," clade. An updated lactate dehydrogenase molecular tree illustrates an additional fast-evolving clade.