Plasma kallikrein (<db_xref db="EC" dbkey="3.4.21.34"/>) and coagulation factor XI (<db_xref db="EC" dbkey="3.4.21.27"/>) aretwo related plasma serine proteases activated by factor XIIA and which sharethe same domain topology: an N-terminal region that contains four tandemrepeats of about 90 amino acids and a C-terminal catalytic domain.The 90 amino-acid repeated domain contains 6 conserved cysteines. It has beenshown [<cite idref="PUB00000342"/>] that three disulphide bonds link the first and sixth, second andfifth, and third and fourth cysteines. The domain can be drawn in the shape ofan apple (see below) and has been accordingly called the 'apple domain'.<pre> x x x x x x x C---C x x x x x x Cx x x x x | x x x Schematic representation of an x Cx x x x apple domain. x x x x x x x x x x x x x x x x x x x C---C .....x x.....</pre>Apart from the cysteines, there are a number of other conserved positions inthe apple domain.