**4. Conclusions**

For study the effect of the marine sponge metabolites with a therapeutic potential, we used two well-characterized α-glycosidases for justifying a possible mechanism of their inhibitor action. Monanchomycalin B, normonanchocidin A, monanchocidin A have been shown to be irreversible slow-binding inhibitors of the GH36 family α-galactosidase α-PsGal from the marine bacterium *Pseudoalteromonas* sp. KMM 701, but have no effect on the activity of the GH109 family α-NaGa from the marine bacterium *Arenibacter latericius* KMM 426T. The inhibitory ability of the alkaloids depends on the chemical structure of the anchor parts of their molecules. The alkaloids can be arranged in the descending order of the binding-affinity: normonanchocidin A > monanchomycalin B > monanchocidin A, and in the decreasing order of the inactivation rate: monanchomycalin B > monanchocidin A > normonanchocidin A. These highly active marine compounds selectively acted on the enzymes from the different structural GH families, binding to the electronegative areas of the protein surfaces formed mainly by carboxylic acid side groups in the active-site-directed manner. The well-characterized α-glycosidases of marine bacteria have been proved to be suitable models for characterizing the novel properties of the alkaloids.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-3397/17/1/22/s1, Figure S1. 3D-superposition of the α-PsGal active site with D-galactose and spermidine residue of monachomycalin B (**a**), tetra-substituted morpholinone derivative of monanchocidine A (**b**) and monosubstituted diaminopropane of normonanchocidine A (**c**). Parts of guanidine alkaloids are shown as "ball and stick" with grey color, galactose shown as "stick" with yellow color. The molecular surface closed to the ligands is shown in pink (H-bonding), green (hydrophobic) and blue (mild polar).

**Author Contributions:** Conceptualization, enzyme investigation, and writing-original draft preparation: I.B.; bioinformatics analysis and computer modeling of protein-inhibitor complex structures: G.L.; construction, expression, and purification of recombinant and mutant enzymes: L.S. and L.B.; purification and identification of compounds: L.Sh. and T.M.; formal analysis: O.S.; resources: L.T.

**Funding:** This research was funded by the framework of the State Assignment of the Pacific Institute of Bioorganic Chemistry, Far Eastern Branch, Russian Academy of Sciences, project no. 0266-2016-0002. The financial support is provided by Ministry of Education and Science of Russia (Agreement 02.G25.31.0172, 01.12.2015). Isolation and identification of alkaloids were performed with a partial support of the Far Eastern Branch of the Russian Academy of Sciences, project no. 18-4-026.

**Conflicts of Interest:** The authors declare no conflict of interest.
