Human dipeptidyl peptidase I (DPPI), also known as cathepsin C, is an enzyme belonging to the family of papain-like cysteine proteases. Its characteristic features are the exclusion domain, which enables its exopeptidase activity as well as tetramerization and its dependence on chloride ions for enzymatic activity. The evolution of DPPI most likely began with the addition of the exclusion domain to form a monomeric exopeptidase.
The aim of this work was to investigate the evolution of DPPI with particular attention to structural features that enabled its oligomerization. By examining the subunit interfaces, we first determined the amino acid residues that form hydrogen bonds between the subunits. We then analyzed 41 sequences of DPPI and its homologs. The amino acid residues that bind the chloride ion are highly conserved in all species, leading us to conclude that chloride ion binding is an evolutionarily conserved feature of DPPI. From the conservation of glycosylation patterns and amino acid residues forming bonds between subunits, we concluded that oligomerization likely evolved with the emergence of vertebrates.
In our analysis, we determined that the lateral interaction between the subunits was substantially weaker than the head-to-tail interaction, so we aimed to stabilize it in the second part of this work. We identified 3 most suitable positions on the interaction surface, that would be able to form disulfide bonds after their replacement by Cys residues and thereby stabilize the interaction. In the last part of the work, we wanted to verify these findings experimentally, but due to the insolubility of the produced mutant, we could not confirm the effect of these mutations.
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