microorganisms Review Bacterial α-Glucan and Branching Sucrases from GH70 Family: Discovery, Structure–Function Relationship Studies and Engineering Manon Molina, Gianluca Cioci, Claire Moulis, Etienne Séverac and Magali Remaud-Siméon * Toulouse Biotechnology Institute (TBI), Université de Toulouse, CNRS, INRAE, INSA, 135, Avenue de Rangueil, CEDEX 04, F-31077 Toulouse, France; [email protected] (M.M.); [email protected] (G.C.); [email protected] (C.M.); [email protected] (E.S.) * Correspondence: [email protected]; Tel.: +33-561-559-446 Abstract: Glucansucrases and branching sucrases are classified in the family 70 of glycoside hydro- lases. They are produced by lactic acid bacteria occupying very diverse ecological niches (soil, buccal cavity, sourdough, intestine, dairy products, etc.). Usually secreted by their producer organisms, they are involved in the synthesis of α-glucans from sucrose substrate. They contribute to cell protection while promoting adhesion and colonization of different biotopes. Dextran, an α-1,6 linked linear α-glucan, was the first microbial polysaccharide commercialized for medical applications. Advances in the discovery and characterization of these enzymes have remarkably enriched the available diversity with new catalysts. Research into their molecular mechanisms has highlighted important features governing their peculiarities thus opening up many opportunities for engineering these Citation: Molina, M.; Cioci, G.; catalysts to provide new routes for the transformation of sucrose into value-added molecules. This Moulis, C.; Séverac, E.; article reviews these different aspects with the ambition to show how they constitute the basis for Remaud-Siméon, M. Bacterial promising future developments. α-Glucan and Branching Sucrases from GH70 Family: Discovery, Keywords: glucansucrase; branching sucrase; GH70; lactic acid bacteria; sucrose-active enzymes Structure–Function Relationship Studies and Engineering. Microorganisms 2021, 9, 1607. https://doi.org/10.3390/ microorganisms9081607 1. Introduction Polymers are used as basic materials for the manufacture of innumerable daily life Academic Editor: Dietmar Haltrich products, as well as more sophisticated material in medicine, diagnostics and fine chem- istry. Most of them are derived from petroleum-based chemistry. Environmental and Received: 6 July 2021 sustainability concerns push forward the research on renewable and recyclable bio-derived Accepted: 25 July 2021 structures showing equivalent, new or improved physico-chemical properties. Among Published: 28 July 2021 bio-sourced polymers, polysaccharides from plant, algae, fungi or microbial organisms are considered nowadays as promising options to replace petroleum-based products with Publisher’s Note: MDPI stays neutral new architectures. Many of them, especially plant and algal polysaccharides, are already with regard to jurisdictional claims in important feedstocks for well-established bio-based industries [1]. The market for microbial published maps and institutional affil- polymers is more modest but continuously growing. Their production is independent iations. of environmental and climatic fluctuations and downstream processing is generally easy, which is a significant advantage. However, despite a great diversity offering a strong potential for innovation, only a few of them—namely xanthan, gellan, hyaluronic acid, welan, clavan, fucopol, pullulan and dextran—are currently marketed [2–4]. Given the Copyright: © 2021 by the authors. knowledge on polysaccharide synthetic pathways as well as the huge amount of available Licensee MDPI, Basel, Switzerland. enzyme sequences and the progress made in protein and metabolic engineering, the supply This article is an open access article of enzymatically or microbiologically produced polysaccharides, shaped on demand, is distributed under the terms and expected to rise significantly [5,6]. conditions of the Creative Commons In this field, α-glucans produced by lactic acid bacteria (LAB) from different genera Attribution (CC BY) license (https:// (mainly Streptococcus, Leuconostoc, Lactobacillus, Weissela) when grown on sucrose are partic- creativecommons.org/licenses/by/ ularly appealing targets [7–9]. These polymers participate in biofilm formation and can 4.0/). Microorganisms 2021, 9, 1607. https://doi.org/10.3390/microorganisms9081607 https://www.mdpi.com/journal/microorganisms Microorganisms 2021, 9, x FOR PEER REVIEW 2 of 28 Microorganisms 2021, 9, 1607 In this field, α-glucans produced by lactic acid bacteria (LAB) from different genera2 of 27 (mainly Streptococcus, Leuconostoc, Lactobacillus, Weissela) when grown on sucrose are par- ticularly appealing targets [7–9]. These polymers participate in biofilm formation and can protectprotect the the bacteria bacteria against against environmental environmental stre stressessses (e.g., (e.g., dessication, dessication, biocides, biocides, antibiotics, antibiotics, phagocytephagocyte attack). attack). They They can can also also mediate mediate the theadhesion adhesion to surfaces to surfaces [10]. [ 10In]. particular, In particular, α- glucansα-glucans formed formed by the by theStreptococcusStreptococcus genusgenus were were well-s well-studiedtudied due due to totheir their contribution contribution to to thethe dental dental plaque plaque formation, formation, the the tooth tooth surfac surfacee colonization colonization and and the the development development of of dental dental cariescaries [11]. [11 ].The The panel panel of of polymer polymer structures structures characterized characterized to to date date and and obtained obtained from from this this cheapcheap and and abundant abundant agro-resource agro-resource is isbr broad.oad. Dextran Dextran is ismainly mainly composed composed of of α-1,6α-1,6 osidic osidic linkageslinkages in in the the main main chain, chain, but but can can also also contain contain αα-1,2/-1,2/α-1,3α-1,3 or or α-1,4α-1,4 in in main main chain chain or or at at branchingbranching points, points, mutan mutan contains contains a amajority majority of of α-1,3α-1,3 osidic osidic bonds, bonds, reuteran reuteran mixes mixes α-1,4α-1,4 andand α-1,6α-1,6 osidic osidic linkages linkages and and alternan alternan is isquasi-strictly quasi-strictly composed composed of of alternating alternating α-1,3α-1,3 and and α-1,6α-1,6 osidic osidic bonds bonds (Figure (Figure 1).1). The The sizes, sizes, types types and and arrangements arrangements of of α-osidicα-osidic linkages linkages in in thethe polymers polymers as as well well as as the the degree degree of of branching branching are are highly highly dependent dependent on on the the producing producing bacterialbacterial strains strains and and their their enzymatic enzymatic arsenal. arsenal. They They are are the the primary primary determinants determinants of of the the physicochemical,physicochemical, biological, biological, and and mech mechanicalanical properties properties of individual αα-glucan,-glucan, andand govern gov- erntheir their range range of potentialof potential applications. applications. The The dextran dextran produced produced by Leuconostoc by Leuconostoc mesenteroides mesen- teroidesNRRL NRRL B512-F B512-F (an α-1,6 (an linkedα-1,6 linkedα-glucan α-glucan with very with few veryα -1,3few branching) α-1,3 branching) was the was first the mi- firstcrobial microbial biopolymer biopolymer commercialized commercialized since since 1948 1948 to be to used be used as plasma as plasma substitute. substitute. For For this thismedical medical application, application, only only low low molar molar mass mass fractions fractions obtained obtained after after partial partial hydrolysis hydrolysis and andsolvent solvent fractionation fractionation of theof the native native high high molar molar mass mass polymer polymer are used are [used7,8,12 [7,8,12–14].–14]. Nowa- Nowadays,days, the marketthe market still still mainly mainly concerns concerns low low molar molar mass massα-1,6 α-1,6 linked linked linear linear dextrans dextrans for forbiomedical biomedical or or analytical analytical applications applications [15 [15]]. Other. Other usages usages in in food, food, cosmetics, cosmetics, nanotechnolo- nanotech- nologiesgies and/or and/or mineral mineral extractions extractions have have also al beenso been reported reported [14 ,[14,16].16]. Moreover, Moreover, the insolubilitythe insol- ubilityand mechanical and mechanical properties properties of α-1,3 of linked α-1,3 linked glucans glucans that originated that originated from Streptococcus from Streptococ-species cus(produced species (produced with recombinant with recombinant enzymes) enzymes) have also have recently also recently raised attention raised attention to develop to α developversatile versatile-glucan-based α-glucan-based biomaterial biomaterial [17,18]. [17,18]. Figure Figure1. Overview 1. Overview of α-glucan of α-glucan architectures architectures produced produced by glucansucrases by glucansucrases (dextransucrases, (dextransucrases, mutansucrases, mutansucrases, reuteransucrases, reuteransu- alter- nansucrases)crases, and alternansucrases) branching sucrases and branching (α-1,2 BRS sucrases and α-1,3 (α-1,2 BRS) BRS from and sucroseα-1,3 BRS). from sucrose. TheThe bacterial bacterial enzymes enzymes involved involved in inα-glucanα-glucan production production from from sucrose sucrose are are called called glu- glu- cansucrasescansucrases (GSs) (GSs) and and they they can can be be assisted byby branchingbranching