V. and Société Française de Biochimie et Biologie Moléculaire (SFBBM) Oligopeptidase B (OpdB; EC 3.4.21.83) is a trypsin-like peptidase belonging to the family of serine prolyl oligopeptidases; two-domain structure of the enzyme includes C-terminal peptidase catalytic domain and N-terminal seven-bladed β-propeller domain. Importance of the interface between these domains and particularly of the 5 salt bridges for enzyme activity was established for protozoan OpdBs. However, these salt bridges are not conserved in γ -proteobacterial OpdBs including the peptidase from Serratia proteamaculans (PSP). In this work, using comparative modelling and protozoan OpdBs' crystal structures we created 3D models of PSP in open and closed forms to elucidate the mechanism underlying inactivation of the truncated form of PSP1-655 obtained earlier. Analysis of the models shows that in the closed form of PSP charged amino acid residues of histidine loop, surrounding the catalytic triad His652, participate in formation of the inter-domain contact interface between catalytic and β-propeller domains, while in the open form of PSP disconnection of the catalytic triad and distortion of these contacts can be observed. Complete destruction of this interface by site-directed mutagenesis causes inactivation of PSP while elimination of the individual contacts leads to differential effects on the enzyme activity and substrate specificity. Thus, we identified structural factors regulating activity of PSP and supposedly of other γ-proteobacterial OpdBs and discovered the possibility of directed modulation of their enzymatic features.