*Article* **Mutagenesis Studies and Structure-function Relationships for GalNAc/Gal-Specific Lectin from the Sea Mussel** *Crenomytilus grayanus*

**Svetlana N. Kovalchuk 1, Nina S. Buinovskaya 1, Galina N. Likhatskaya 2, Valery A. Rasskazov 1,†, Oksana M. Son 3, Liudmila A. Tekutyeva <sup>3</sup> and Larissa A. Balabanova 1,3,\***


Received: 17 October 2018; Accepted: 21 November 2018; Published: 27 November 2018

**Abstract:** The GalNAc/Gal-specific lectin from the sea mussel *Crenomytilus grayanus* (CGL) with anticancer activity represents a novel lectin family with β-trefoil fold. Earlier, the crystal structures of CGL complexes with globotriose, galactose and galactosamine, and mutagenesis studies have revealed that the lectin contained three carbohydrate-binding sites. The ability of CGL to recognize globotriose (Gb3) on the surface of breast cancer cells and bind mucin-type glycoproteins, which are often associated with oncogenic transformation, makes this compound to be perspective as a biosensor for cancer diagnostics. In this study, we describe results on in silico analysis of binding mechanisms of CGL to ligands (galactose, globotriose and mucin) and evaluate the individual contribution of the amino acid residues from carbohydrate-binding sites to CGL activity by site-directed mutagenesis. The alanine substitutions of His37, His129, Glu75, Asp127, His85, Asn27 and Asn119 affect the CGL mucin-binding activity, indicating their importance in the manifestation of lectin activity. It has been found that CGL affinity to ligands depends on their structure, which is determined by the number of hydrogen bonds in the CGL-ligand complexes. The obtained results should be helpful for understanding molecular machinery of CGL functioning and designing a synthetic analog of CGL with enhanced carbohydrate-binding properties.

**Keywords:** galactose-specific lectin; *Crenomytilus grayanus*; carbohydrate-binding site; molecular docking; site-specific mutagenesis; carbohydrate-binding activity
