Serine carboxypeptidase S28 family comprises carboxypeptidase PRCP and the aminopeptidase DPP7 []. The cap domain (SKS) is formed by 11 α-helices and two strands interconnected by loops. It contains four disulphide bonds which are assumed to be involved in stabilising the structure. The SKS domain is a rare fold possibly present only in the S28 serine peptidase family [].