Serine proteases function in cascade systems such as blood clotting, immunity, and digestive processes. Family and evolutionary relationships among the various enzymes can be inferred from highly conserved markers that can be extended to the enzyme substrates as well, and members are classified from their primary specificities. Several members of the trypsin superfamily have non–trypsin-like primary specificities, but attempts to determine their phylogeny have so far failed. In the August Molecular Cell, Merridee Wouters and colleagues at the Victor Chang Cardiac Research Institute report the construction and functional analysis of a synthetic protein predicted theoretically to be the ancestor of the present-day enzymes trypsin, chymotrypsin, and elastase, and discover that it has a wider tolerance for substrates than both its descendents and its own ancestor (Molecular Cell, 12:343-54, August 2003).

Wouters et al. inferred ancestral sequences from parsimony analysis of a multiple alignment of 56 immune defense protease (IDP)...

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