Serpins: structure, function and molecular evolution.

The superfamily of serine proteinase inhibitors (serpins) are involved in a number of fundamental biological processes such as blood coagulation, complement activation, fibrinolysis, angiogenesis, inflammation and tumor suppression and are expressed in a cell-specific manner. The average protein size of a serpin family member is 350-400 amino acids, but gene structure varies in terms of number and size of exons and introns. Previous studies of all known serpins identified 16 clades and 10 orphan sequences. Vertebrate serpins can be conveniently classified into six sub-groups. We provide additional data that updates the phylogenetic analysis in the context of structural and functional properties of the proteins. From these, we can conclude that the functional classification of serpins relies on their protein structure and not on sequence similarity.