Sequence alignment of Epo and Pla. The β-strands and the extracellular loops are numbered, and the 31 differing amino acids in the extracellular side of the protein are shown in bold face in gray background. The catalytic residues are indicated with asterisks. The sequences were aligned with ClustalW , and the image was rendered with Alscript . The secondary structures are defined by Stride , and they are based on the known Pla structure .