Interplay between tetrameric structure, enzymatic activity and allosteric regulation of human dipeptidyl-peptidase I

Human dipeptidyl-peptidase I (DPPI) is a tetrameric enzyme from the family of papain-like cysteine peptidases. It is ubiquitously expressed and plays important roles in general protein turnover, skin homeostasis and proteolytic processing of effector peptidases in immune cells. In this work we investigate allosteric regulation of DPPI and its relation to the oligomeric structure. First, we investigate the functional significance of the tetrameric state by comparing the kinetic properties of the tetrameric form (DPPI_tet) with a recombinant monomeric form (DPPI_mono). We find that both forms have very similar kinetic properties for the hydrolysis of a commonly used synthetic substrate. In agreement with previous studies, no cooperativity is observed in the tetramer. The only significant difference between both forms is a higher catalytic rate of DPPI_mono. We then characterize three compounds, 3'-nitrophthalanilic acid, chlorogenic acid and caffeic acid that affect DPPI activity via kinetic mechanisms consistent with binding outside of the active site. These compounds are the first known modifiers of DPPI that do not act as specific inhibitors. Chlorogenic acid and caffeic acid act as linear mixed and linear catalytic inhibitors, respectively, and do not discriminate between both forms. In contrast, 3'-nitrophthalanilic acid is a hyperbolic inhibitor that binds DPPI_tet and DPPI_mono with different affinities and inhibits their activities via different kinetic mechanisms. Altogether, these results show that the tetrameric structure of DPPI is not necessary for enzymatic activity, however, oligomerization-related structural features can play a role in its regulation.