Lactases

Lactases, enzymes that hydrolyze lactose, are very important enzymes from medical and industrial points of view. Loss of lactase in adulthood, a common trait in humans and other mammals, causes intolerance to lactose, and henceforth to milk and milk-based, lactose-containing food products (cakes, ice cream, etc). Consequently, there is a huge market demand for lactose-free products. Although microbial lactases are widely used in the production of lactose-free milk, there is ample room for the development of lactases with improved properties.

While the normal function of lactases is the hydrolysis of the disaccharide lactose into glucose and galactose monosaccharides, lactases and related β-galactosidases can be engineered to catalyze the reverse reaction yielding galactooligosaccharides (GOS), compounds with outstanding prebiotic properties. This represents another important biotechnological application of these enzymes (Marín-Navarro et al., 2014; Talens-Perales et al., 2016a).

Most lactases and other β-galactosidases of industrial interest are classified in Family 2 of Glycoside Hydrolases (GHF2). A detailed analysis of the topology and phylogenetics of enzymes belonging to this family allows discovering relationships between their structure and function (Talens-Perales et al., 2016b).

Phylogenetic analysis based on the catalytic domain of GHF2 enzymes. Numbers indicate groups of enzymes with different domain architectures.

Thermostable β-galactosidase from the thermophilic bacterium Thermotoga maritima (TmLac) is an interesting enzyme for biotechnological applications such as lactose hydrolysis and GOS biosynthesis. The structural properties of TmLac, to which it owes its resistance to denaturation, facilitate its conservation and manipulation. The three-dimensional structure of TmLac has been obtained by cryo electron microscopy at 2.0 Å resolution (Míguez Amil et al., 2019).

Structure of the Thermotoga maritima β-galactosidase. (A) Overall cryo-EM structure reconstruction showing monomers in different colors. “Top” (upper panel) and “side” (lower panel) views are represented. (B) Overall structural model in cartoon representation. Dimer A/B is highlighted in surface representation. Subunit A shows different colors for the different domains as specified in panel C. Subunits B, C, and H contacting subunit A are labeled. Active site location is indicated. N and C termini are labeled in subunit A. (C) Orthogonal views of the monomer in cartoon representation colored following domain architecture.

Different procedures for lactase immobilization into solid supports have been implemented to produce zymoactive materials that can be used in devices designed for efficient lactose-free milk production (Fabra et al. 2019, Talens-Perales et al., 2020, Fabra et al., 2021).