Bacterial Sucrose Isomerases: Properties and Structural Studies
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Biologia 63/6: 1020—1027, 2008 Section Cellular and Molecular Biology DOI: 10.2478/s11756-008-0166-0 Review Bacterial sucrose isomerases: properties and structural studies Moez Rhimi,RichardHaser & Nushin Aghajari* Laboratoire de BioCristallographie, Institut de Biologie et Chimie des Protéines, UMR 5086–CNRS/Université de Lyon, IFR128 “BioSciences Gerland – Lyon Sud”, 7 Passage du Vercors, F-69367 Lyon cedex 07, France; e-mail: n.aghajari@ibcp.fr Abstract: Due to their significant role in food industry, sucrose isomerases are good candidates for rational protein engineering. Hence, specific modifications in order to modify substrate affinity and selectivity, product specificity but also to adapt their catalytic properties to particular industrial process conditions, is interesting. Our work on the structural studies of the sucrose isomerase, MutB, which presents the first structural data available on a trehalulose synthase and the first experimental data on complexed forms of sucrose isomerases represents a significant advance in the understanding of these enzymes. In this review we give an overview of what is known on biochemical properties and structural aspects of different sucrose isomerases in particular those reported from bacteria. Key words: sucrose isomerases; trehalulose; isomaltulose; immobilized cells; structural studies. Introduction (Fig. 1) are structural isomers of sucrose and are natu- rally present in honey in very low quantities (Low & Sucrose, α-d-glucopyranosyl-(1,2)-β-d-fructofuranosi- Sporns 1988). These natural sugars display a sweet- de, is the most widespread disaccharide translocated ening power, bulk, organoleptic and physical proper- by plants (Bieniawska et al. 2007). High amounts of ties similar to those of sucrose (Takazoe 1989; Hamada this sugar are stored in the edible parts of several 2002). Furthermore, several characteristics make these plants, making it the mainly abundant natural sweet- two isomers advantageous over sucrose for some ap- ener (Salvucci 2003). Currently, sucrose is widely used plications in food industry. In fact, isomaltulose (also in many agro-alimentary products. However, this dis- known as palatinose) and trehalulose are non-cariogenic accharide is efficiently metabolized, which highly influ- and have low glycaemic indexes by decreasing the rate ences the glycaemia of the body and promotes den- with which monosaccharides and insulin are released tal caries (Sissons et al. 2007). Such effects are un- in blood, promoting their use for diabetics (Yamada et desirable for health and especially for those suffering al. 1985). These sugars were reported to induce a se- from obesity and diabetes (Saris 2003; Schulze et al. lective promotion of growth of beneficial bifidobacteria 2004). Nowadays we assist in an increasing consumer in human intestinal microflora and to be highly stable demand for low caloric food products especially in the in foods and beverages (Birch & Wu 2007). The safety case of diabetes and low caloric diets, the actual chal- of isomaltulose and trehalulose was also checked, and lenge is to set out sweeteners having a low caloric con- these natural sugars were without side effects and ap- tent combined with intense taste. Artificial sweeteners proved as being safe for use in human food (Goda et as, e.g., aspartame, cyclamate and saccharine were re- al. 1988). Thus, these disaccharides can be used as a ported to be efficient sucrose substitutes (Renwick 2006; sucrose substitute in foods, soft drinks and medicines Cardoso & Bolini 2007). While these artificial com- (Takazoe 1989). Due to its reducing properties, iso- pounds have a high sweetening power, they are among maltulose is attractive as an industrial precursor for the most controversial food additives due to allegations the manufacturing of biosurfactants and biocompatible of adverse health effects including dermatological prob- polymers (Lichtenthaler & Peters 2004). lems, headaches, mood variations, cardiopulmonary dif- In nature, trehalulose and isomaltulose, which act ficulties, seizures, and cancer (Hertelendy et al. 1993; as a reserve material during periods of low carbon avail- Ravaud et al. 2007). Thus, industrials focused on the ability in many bacteria, are present in low quantities use of natural and hypo-caloric sweeteners having a and their chemical synthesis is not easy (Ravaud et al. high sweetness value. 2007; and references therein). Fortunately these disac- Isomaltulose (6-o-α-d-glucopyranosyl-d-fructose) charides can be produced at an industrial scale through and trehalulose (1-o-α-d-glucopyranosyl-d-fructose) isomerization of sucrose using sucrose isomerases which * Corresponding author c 2008 Institute of Molecular Biology, Slovak Academy of Sciences Bacterial sucrose isomerases: properties and structural studies 1021 Fig. 1. Isomerization and hydrolysis of sucrose as catalyzed by sucrose isomerases. bio-convert sucrose into both isomaltulose and trehalu- less rapidly cleaved in the small intestine than sucrose. lose. Numerous microorganisms have been recognized This causes an attenuated blood glucose and insulin for their ability to produce sucrose isomerase with dif- response, a property which is very attractive for dia- ferent levels of isomaltulose and trehalulose production. betics (Kawai et al. 1985; Yamada et al. 1985). It is Sucrose isomerases, also called sucrose mutases or α- worth noting that pH-telemetry studies, which are the glucosyltransferases (EC 5.4.99.11), belong to the gly- internationally recognized standard for tooth-friendly coside hydrolase (GH) family 13 of retaining enzymes claims, proved the non-cariogenicity of palatinose (iso- (Henrissat 1991). These family 13 proteins display a maltulose) (Lina et al. 2002); trehalulose being also re- number of conserved active site residues (described ported to be “tooth friendly” (Hamada 2002). More- later in the text), and act by a retaining double dis- over, also prebiotic (bifidogenic) properties of isomal- placement mechanism via a covalent glycosyl-enzyme tulose on bifidobacteria were reported (Mattes et al. intermediate, and yielding the α-anomeric products. 1998). For these reasons palatinose and trehalulose ap- Structural studies of sucrose isomerases have been pear as ideal substitutes for sucrose in several agro- reported for the isomaltulose synthase PalI from Kleb- alimentary and pharmaceutical applications. siella sp. LX3 (Zhang et al. 2003a,b), for the Pseu- Sucrose isomerases from Agrobacterium radiobac- domonas mesoacidophila MX-45 trehalulose synthase, ter MX-232 and Pseudomonas mesoacidophila MX- MutB (Ravaud et al. 2005, 2007) and for the isomal- 45 produce primarily trehalulose with yields evalu- tulose synthase SmuA from Protaminobacter rubrum ated to 91% (Miyata et al. 1992; Nagai-Miyata et al. (Ravaud et al. 2006). In order to gain detailed in- 1993), whereas enzymes from Pantoea dispersa, Er- sights into the catalytic mechanism, crystal structures winia rhapontici, Serratia plymuthica, Klebsiella sp. of MutB (native and mutant enzymes) in complex with LX3 and Enterobacter sp. FMB1 produced essen- substrates, substrate analogues and inhibitors have tially isomaltulose with percentages around 75–85% been solved and reported (Ravaud et al. 2007). (Cheetham 1984; Veronese & Perlot 1999; Zhang et al. 2002; Wu & Birch 2005). Due to difficulties in the chem- Properties and production of isomaltulose and ical synthesis of these sugars, their microbial production trehalulose is of commercial interest. Many bacterial strains can convert sucrose to isomaltulose and/or trehalulose in Isomaltulose and trehalulose are occurring naturally in different ratios by using their sucrose isomerases. The honey and sugar cane juice. The taste and appearance enzymes from Protaminobacter rubrum (Yamada et al. of these disaccharides are similar to sucrose, while their 1985), Serratia plymuthica (Krastanov & Yoshida 2003; sweetening power is around 30% and 70%, when com- Krastanov et al. 2006), Erwinia rhapontici (Krastanov pared to that of sucrose, for palatinose and trehalu- & Yoshida 2003) and Enterobacter sp. FMB1 (Cho et al. lose, respectively (Cheetham et al. 1982; Salvucci 2003). 2007) produce essentially isomaltulose (75–90%), minor Viscosities of aqueous solutions and organoleptic prop- quantities of trehalulose and trace amounts of fructose erties of these sugars are quite similar to that of su- and glucose. Also several Klebsiella species are known crose (Mattes et al. 1998). In addition hereto, toxic- to produce 60–70% palatinose (Krastanov & Yoshida ity analysis revealed that isomaltulose and trehalulose, 2003), while Pseudomonas mesoacidophila MX-45 and as sucrose, are non-toxic (Kawai et al. 1985; Jonker Agrobacterium radiobacter MX-332 strains convert su- et al. 2002). As concerns their physicochemical prop- crose mainly to trehalulose (∼90%), small amounts of erties, these sucrose isomers appear adequate for sub- isomaltulose and trace amounts of the monosaccharides stitution of sucrose in mainly sweet foods. Interestingly, (Cheetham et al. 1982). isomaltulose and trehalulose are hardly fermented and Production of palatinose from sucrose by using im- 1022 M. Rhimi et al. Table 1. Biochemical properties of sucrose isomerase reported in the literature. Organisms Topt pHopt pI Km Iso- Tre- Specific activity Reference ( ◦C) (mM) maltulosea halulosea (U/mg) Klebsiella pneumoniae NK33-98-8 30 6.0 NR 42.7 77 33 2362 Aroonnual et al. (2007) Pantoea dispersa UQ68J 30–35 5.0 7.0 40 91 3 562 Wu & Birch (2005) Klebsiella planticola UQ14S