*Article* **Effect of Pentacyclic Guanidine Alkaloids from the Sponge** *Monanchora pulchra* **on Activity of** α**-Glycosidases from Marine Bacteria**

**Irina Bakunina 1,\*, Galina Likhatskaya 1, Lubov Slepchenko 1,2, Larissa Balabanova 1,2, Liudmila Tekutyeva 2, Oksana Son 2, Larisa Shubina 1 and Tatyana Makarieva 1**


Received: 12 November 2018; Accepted: 25 December 2018; Published: 1 January 2019

**Abstract:** The effect of monanchomycalin B, monanhocicidin A, and normonanhocidin A isolated from the Northwest Pacific sample of the sponge *Monanchora pulchra* was investigated on the activity of α-galactosidase from the marine γ-proteobacterium *Pseudoalteromonas* sp. KMM 701 ( α-PsGal), and <sup>α</sup>-*N*-acetylgalactosaminidase from the marine bacterium *Arenibacter latericius* KMM 426<sup>T</sup> (α-NaGa). All compounds are slow-binding irreversible inhibitors of α-PsGal, but have no effect on α-NaGa. A competitive inhibitor D-galactose protects α-PsGal against the inactivation. The inactivation rate (*k*inact) and equilibrium inhibition ( *K*i) constants of monanchomycalin B, monanchocidin A, and normonanchocidin A were 0.166 ± 0.029 min−<sup>1</sup> and 7.70 ± 0.62 μM, 0.08 ± 0.003 min−<sup>1</sup> and 15.08 ± 1.60 μM, 0.026 ± 0.000 min−1, and 4.15 ± 0.01 μM, respectively. The 2D-diagrams of α-PsGal complexes with the guanidine alkaloids were constructed with "vessel" and "anchor" parts of the compounds. Two alkaloid binding sites on the molecule of α-PsGal are shown. Carboxyl groups of the catalytic residues Asp451 and Asp516 of the α-PsGal active site interact with amino groups of "anchor" parts of the guanidine alkaloid molecules.

**Keywords:** sponge *Monanchora pulchra*; pentacyclic guanidine alkaloids; GH36 α-galactosidase; GH109 <sup>α</sup>-*N*-acetylgalactosaminidase; slow-binding irreversible inhibitor; monanchomycalin B; monanhocidin A; normonanhocidin A
