Crystal Structure and Substrate Specificity of D-Galactose-6-Phosphate Isomerase Complexed with Substrates

D-Galactose-6-phosphate isomerase from Lactobacillus rhamnosus (LacAB; EC 5.3.1.26), which is encoded by the tagatose-6-phosphate pathway gene cluster (lacABCD), catalyzes the isomerization of D-galactose-6-phosphate to D-tagatose-6-phosphate during lactose catabolism and is used to produce rare sugars as low-calorie natural sweeteners. The crystal structures of LacAB and its complex with D-tagatose-6-phosphate revealed that LacAB is a homotetramer of LacA and LacB subunits, with a structure similar to that of ribose-5-phosphate isomerase (Rpi). Structurally, LacAB belongs to the RpiB/LacAB superfamily, having a Rossmann-like αβα sandwich fold as has been identified in pentose phosphate isomerase and hexose phosphate isomerase. In contrast to other family members, the LacB subunit also has a unique α7 helix in its C-terminus. One active site is distinctly located at the interface between LacA and LacB, whereas two active sites are present in RpiB. In the structure of the product complex, the phosphate group of D-tagatose-6-phosphate is bound to three arginine residues, including Arg-39, producing a different substrate orientation than that in RpiB, where the substrate binds at Asp-43. Due to the proximity of the Arg-134 residue and backbone Cα of the α6 helix in LacA to the last Asp-172 residue of LacB with a hydrogen bond, a six-carbon sugar-phosphate can bind in the larger pocket of LacAB, compared with RpiB. His-96 in the active site is important for ring opening and substrate orientation, and Cys-65 is essential for the isomerization activity of the enzyme. Two rare sugar substrates, D-psicose and D-ribulose, show optimal binding in the LacAB-substrate complex. These findings were supported by the results of LacA activity assays.

Several sugar phosphate isomerases, including ribose-5phosphate isomerase [18,19], galactose-5-phosphate isomerase [20], mannose-6-phosphate isomerase [21], and glucose-6-phosphate isomerase [18], have been studied for their application in the production of rare sugars by the interconversion between aldoses and ketoses. Similar studies have examined the efficiency and substrate specificity of LacAB for the production of rare sugars such as D-psicose and D-ribulose from D-allose and D-ribose, respectively [20].
In the present study, to understand the structural features and substrate specificity of multimeric LacAB, we determined the crystal structure of LacAB from Lactobacillus rhamnosus in its native form and in a complex with tagatose-6-phosphate as a product or with D-allose and D-ribose as substrates. We analyzed the active site residues using site-directed mutagenesis and simple enzyme activity assays.

Overall structure of the LacAB
For co-expression of the LacA and LacB genes, each gene was obtained from the lactose operon and subcloned into pQE-80L vector containing two Shine-Dalgarno sequences as ribosomal binding sites. For structural and kinetic studies, LacA and LacB proteins were overexpressed as His-tagged proteins in E. coli. The purified LacA and LacB proteins gave bands of 15 kDa and 19 kDa, respectively, on sodium dodecyl sulfatepolyacrylamide gels.
Initially, the structure of LacAB was determined at a resolution of 1.96 Å using a crystal obtained from microseeding using the hanging drop vapor diffusion method. To obtain the LacAB-product complex, the crystals were soaked with tagatose-6-phosphate; X-ray data were collected at 1.65 Å resolution. The structure of the complex was resolved by molecular replacement with a refined substrate-free LacAB structure as the search model.
In the crystal structure, the two LacABs in asymmetric unit (subunit AB and CD) is related by non-crystallographic symmetry, and the whole LacAB homotetramer is arranged with crystallographic 2-fold symmetry of each two asymmetric units, which is shown in Figure 2A, has dimensions of 108 × 79 × 54 Å (Figure 2A and B). This interface is predicted by PISA [22], leading to stable homotetramer formation of LacAB in solution with a ΔG of dissociation of 7.8 kcal/mol. To confirm the solution state of LacAB, the molar mass was Figure 1. Schematic of the reaction catalyzed by LacAB and alignment of LacA, LacB, and RpiB sequences. A, Galactose-6-phosphate (Gal6P) is converted to tagatose-6-phosphate (Tag6P) by LacAB during lactose catabolism. B, Multiple sequence alignment of LacA and LacB from Lactobacillus rhamnosus (GenBank accession numbers ZP03210387 and ZP03210388, respectively), EcRpiB from Escherichia coli (PDB ID 1NN4; NP418514), MtRpiB from Mycobacterium tuberculosis (2VVP; YP006515902), and CtRpiB from Clostridium thermocellum (3PH4; YP001038990). The sequences are for precursors, and the numbering is based on LacA. Highly conserved residues are shown in red type and boxed in blue; strictly conserved residues are shown on a red background. Secondary structure elements are indicated in pink for LacA and in green for LacB. Residues interacting directly with bound Taga6P are indicated with a triangle in pink (LacA) and green (LacB). The figure was prepared using ESPript [41]. analyzed using an online size exclusion chromatography-fast protein liquid chromatography (SEC-FPLC) coupled with multiangle light scattering (MALS) and UV detector. Figure S1 shows the UV280nm-LS overlay of LacAB. The column retention time in SEC was 13.62 min, and the MALS estimated molecular weight (Mw) of the major peak was about 135.5 kDa with a polydispersity (Pd) of 1.026.
Each subunit of LacA and LacB forms a Rossmann-like αβα sandwich fold, which was initially identified in pentose phosphate and hexose phosphate isomerases, including ribose-5-phosphate isomerases (RpiA and B), from many species [17,23,24]. As shown in Figure 2C and D, the structures of LacA and LacB subunits, like that of RpiB, contain five α-helices. Two α-helices (α2 and α3) are located on one side of a five-stranded parallel β-sheet with strands in the order β2, β1, β3, β5, and β4, and the other three (α1, α4, and α5) are on the other side. Another α-helix (α6) extends from the main αβα sandwich fold and is situated horizontally to the other helices. The LacB also contains a seventh α-helix (α7) in the Cterminus, which is 30 amino acids longer than the LacA Cterminus. A structure-based alignment of the αβα sandwich fold sequences, including ribose-5-phosphate isomerase sequences from various species, is presented in Figure 1B.

Interface between LacA and LacB
The LacAB monomer involves face-to-face contacts between the respective flat interfaces of the two proteins and interactions between a helix of LacA and two helices of LacB located near the C-terminus. The LacAB interface has a total buried surface area of 2069 Å 2 in LacA and 1895 Å 2 in LacB, representing 21% of the total surface area of LacA and 28% of the total surface area of LacB, respectively. The third αβα folds (α3/β4/α4) of LacA and LacB are interlocked perpendicularly to each other ( Figure S2). The two C-terminal helices of LacB are also perpendicular. The surfaces of the two proteins interact by direct polar contacts, 12 hydrogen bonds, and hydrophobic interactions among Val-67, Met-71, Val-80, Val-83, Thr-89, Met-92, and Thr-93 in LacA and Thr-67, Ile-69, Thr-73, Met-85, Thr-90, Leu-99, Val-110, and Ile-141 in LacB. In addition, the hydrophilic residues Asn-74, Glu-85, and Glu-86 in LacA and Ser-72, Asp-76, Asp-87, Asp-88, and Ser-91 in LacB are indirectly connected by seven intervening water molecules. The interactions between the C-terminal α-helix (α7) of LacB and the antiparallel α-helix (α6) of LacA create a stronger dimer than that established by the RpiB structure. The four residues Ile-133, Met-137, Leu-138, and Met 141 of α6 in LacA form hydrophobic interactions with Tyr-170, Trp-165, Leu-162, Phe-158, and Phe-157 of α7 and Tyr-42, Pro-43, and Ile-44 of α2 in LacB. Water molecules are not present at these interactions.

Active site pocket between LacA and LacB
A structural comparison between LacAB and RpiB by CCP4MG analysis suggested that the active site of LacAB is at the interface of each LacA and LacB subunit [25]. To verify this, the structure of LacAB in a complex with D-tagatose-6-  Figure 1B. C, A ribbon diagram of a LacA subunit showing the five parallel β-sheets in the center surrounded by five α-helices, with α1, α4, and α5 on the left, and α2 and α3 on the right. The α6 helix is located perpendicular to the α3β4α4 motif. D, A ribbon diagram of the LacB subunit, as described for LacA in Figure 2C. The extra C-terminal α7 helix is located nearly parallel to the α2 helix. doi: 10.1371/journal.pone.0072902.g002 phosphate (Tag6P) as a product was determined in a soaking experiment ( Figure 3A and B). The active site pocket was confirmed by the electron density of Tag6P in a wide-mouthed trapezoid-shaped cavity, 10 Å deep, between LacA and LacB ( Figure 3C). In contrast to the two substrate-binding sites of the RpiB homodimer ( Figure S3A), only one active site was verified in the LacAB monomer based on the Tag6P electron density map ( Figure 4A and B). The other vacant and shallow site corresponding to the RpiB was consisted of entirely different residues from the active site, and occupied by hydrophobic residues Phe-40 (LacA), Leu-99 and Leu-137 (LacB) plus bulky residues Tyr-65 (LacA) and Tyr-94 (LacB). The active site is located at the interface surrounding two helices (α4 and α6) in LacA and the β1/α1 loop, α2, and β3/α3 loop in LacB. The tunnel in a narrow side of the dent, which was disclosed in the structure of the active site of the MtRpiB homodimer, is sealed off by His-114 of α5 in LacB of another LacAB ( Figure 3C). The pocket is composed of residues Met-92, His-96, Asn-97, Arg-130, His-131, and Arg-134 in LacA plus Asp-8, His-9, Ile-10, Arg-39, Tyr-42, Cys-65, Thr-67, Ile-69, and Thr-73 in LacB ( Figure 3C).
The Superpose program [26] of the CCP4 suite [27] did not demonstrate any structural differences in conformation between substrate-free LacAB and its complex with Tag6P; the root mean square deviation was 0.12 Å for all Cα atoms. Figure   4C shows the details of the active site in the LacAB-Tag6P complex. The binding of Tag6P in the complex is stabilized by many hydrogen bonds with hydrophilic residues in the active site pocket. The phosphate group of Tag6P is bound by bidentate or tridentate hydrogen bonds to the guanidinium group of three arginine residues (Arg-130 and Arg-134 in LacA and Arg-39 in LacB) at average distances of 2.9, 2.8, and 3.0 Å, respectively. Additionally, the phosphate group of Tag6P, which is O6-linked to C6 of D-tagatose, forms a 3-Å polar interaction with the side chain of His-9 (LacB). The O3 hydroxyl group of Tag6P interacts with the carboxyl group of Asp-8 (LacB) and the main chain of Gly-66 (LacB), while the O4 hydroxyl group interacts with the imidazole group of His-96 (LacA). The terminal O1 hydroxyl group interacts with the side chain of Cys-65 (LacB) and the amide nitrogen of Gly-70 (LacB) at distances of 3.3 and 2.8 Å, respectively. The O2 carbonyl interacts with the amide nitrogen of Thr-67 (LacB) at 2.9 Å. The carboxamide group of Asn-97 (LacA) and the hydroxyl group of Thr-67 (LacB) are directly connected to O1 and O2 of Tag6P and simultaneously related by hydrogen bonds mediated by a water molecule.

Substrate specificity of LacAB
The structures of the LacAB complex with D-allose and Dribose revealed that the substrates are converted to D-psicose and D-ribulose, respectively, which bind in the same orientation as Tag6P in the active site pocket ( Figure 5A and D). The electron density maps of D-psicose and D-ribulose were unambiguously observed ( Figure 5B and E). To identify and confirm the bound substances in LacAB complex, D-psicose and D-ribulose as the products were substituted with D-allose and D-ribose as the substrates, as well as D-allose and Dribose as the products by ketose conversion, respectively. However, these substitutions were revealed higher B-factors for ligand after refinement. The interactions between LacAB and the bound sugars are shown schematically in Figure 5C and F. The O1, O3, and O4 hydroxyl groups as well as the O2 carbonyl group of the substrates interact with the same LacAB residues as seen in the LacAB-Tag6P complex. Additionally, the imidazole group of His-96 (LacA) directly binds to the O4 hydroxyl group of D-ribulose via hydrogen bonding at a distance of 3.0 Å. In the LacAB-psicose complex, the O6 hydroxyl group is 2.7 Å from the guanidinium group of Arg-134 (LacA).

Functional studies using mutants of LacAB
Site-directed mutagenesis was performed to study the functional roles of LacAB residues that interact directly with Tag6P. The enzymatic activities of the mutant proteins were determined by a colorimetric method using cysteine-carbazolesulfuric acid to measure aldose conversion from D-ribose to Dribulose (Table S1). The catalytic activity of the T67A (LacB) mutant, which interacted with the O1 hydroxyl and O2 carbonyl of Tag6P, was approximately 20-fold lower than that of the wild-type enzyme. The H96A (LacA) mutant, which bound to the O4 and O5 hydroxyl groups by polar ineteraction at distances of 2.9 Å and 3.5 Å, respectively, exhibited 25-fold lower activity compared with the wild-type enzyme activity. The N97A (LacA) mutant bound to the terminal O1 hydroxyl group of Tag6P and had an activity level of approximately 30% that of the wild-type enzyme. Three mutants showed no enzymatic activity: the C65A (LacB) mutant, which interacted with the terminal O1 hydroxyl group; the D8N (LacB) mutant, which interacted with the O3 hydroxyl group; and the H9A (LacB) mutant, which interacted with the O6 hydroxyl group.

Comparison between the LacAB and RpiB active sites
Structurally, LacA and LacB belong to the RpiB/LacAB superfamily and have approximately 26% sequence identity with RpiB ( Figure 1B), which converts ribose-5-phosphate to ribulose-5-phosphate in the pentose phosphate pathway. Overall, the structure of LacAB is analogous to that of RpiB, except that the C-terminal α7 helix of LacB, which binds the phosphate group, is not present in RpiB and LacAB has only one active site, which is located in the same position as one of the two RpiB active sites. A comparison between the LacAB active site identified by substrate binding and one of the active sites of MtRpiB from M. tuberculosis (PDB ID 2VVP) showed that the only structural difference between the two sites is the substitution of the phosphate-binding residue Arg-113 in β5/α5 of MtRpiB by Arg-39 in β2/α2 of LacB ( Figure S3B). Another putative pocket region of LacAB was identified by the program PocketPicker [28] and by structural similarity with the substratebinding site of MtRpiB. However, this putative pocket did not show the electron density of Tag6P and has no sequence similarity with the MtRpiB site.

Discussion
Galactose-6-phosphate isomerase (LacAB) is encoded by the tagatose-6-phosphate pathway gene cluster and belongs to the same family as RpiB. A structural alignment produced using the DALI server [29] revealed that LacAB is structurally similar to RpiB, with a Z-score higher than 20. Superimposition of the LacAB structure onto that of RpiB from E. coli (PDB ID 2VVR) gave a the root-mean-square deviation of 1.2 Å for 564 Cα atoms of a tetramer. The crystal structures of RpiB with and without substrates have been reported for various species. Simultaneous expression of LacA and LacB proteins resulted in the formation of heterodimeric complex, and no homodimeric complex, despite structural similarities between the two proteins. The only structural difference between LacA and LacB is the additional C-terminal α-helix in LacB ( Figure S3C), making LacB 30 amino acids longer than LacA and RpiB proteins. LacA and LacB expressed separately as well as their combined extracts were found to have no galactose-6phosphate isomerase activity [5]. Therefore, we considered that the heterodimerization of LacA and LacB is more favorable than homodimeric formation because of the interaction between the C-terminal α7 helix of LacB and the α6 helix of LacA. Although LacA and LacB have similar motifs and structures, they share only 17% sequence identity, which is lower than their identity with RpiB. In particular, the active site residues differ between LacA and LacB, excluding Gly-66 and Gly-70 in LacB, which are predicted to be involved in structural conformation.
The active site of LacAB has two structural features responsible for a substrate orientation distinct from that in RpiB. The active site of LacAB is coterminous with that of RpiB, except that Arg-39 (LacB) of LacAB replaces Asp-42 of MtRpiB and interacts with the phosphate group of hexose phosphate, instead of Arg-113 in MtRpiB [30,31]. LacAB and RpiB also differ with respect to the shape and size of the substratebinding pocket. At approximately 15 Å in length, the pocket in LacAB is 3 Å longer than the pocket in RpiB and is sufficient to accommodate hexose phosphate. The primary factor that determines the pocket size is the location of Arg-130 (LacA), Arg-134 (LacA), and Arg-39 (LacB) to form the outside frame of the pocket. These three long and flexible arginine side chains interact with the phosphate moiety of the substrate and can change its direction away from the catalytic residue in order to accommodate the six-carbon substrate [31]. Superimposition of the LacAB and MtRpiB structures showed that Gly-129 (LacA) and Arg-130 (LacA) of the α6 helix are shifted toward the Cterminal α7 helix of LacB, with the movement of Cα atoms by 2.43 Å and 2.22 Å, respectively, for an overall Cα root-meansquare deviation of 1.2 Å. Regarding pocket shape, the substrate-binding site of LacAB is wide and open, whereas that of MtRpiB is tunnel shaped and covered partially by Arg-113.
Functional studies on active-site mutants of these enzymes have indicated the identities and roles of key residues. For example, His-102 and His-99 are known to be intrinsic residues for furanose ring opening of ribose-5-phosphate in MtRpiB and EcRpiB, respectively [30,31]. Therefore, the binding of the conserved His-96 (LacA) to O4 and O5 of Tag6P was predicted to play a role in ring opening and isomerization of the substrate. Consistent with this proposed role of optimally orienting the substrate for catalysis, the H96A (LacA) mutant of LacAB had approximately 25-fold lower catalytic activity compared with the wild-type enzyme. Comparable mutations in EcRpiB (H99N) and TcRpiB (H102A) from Trypanosoma cruzi resulted in wild-type K m values, but 26-fold and 10-fold lower k cat values, respectively [31,32]. The D8A and H9A mutations of Asp-8 (LacB) and His-9 (LacB), which are conserved in the RpiB/LacAB superfamily, essentially inactivated the enzyme, suggesting that these two residues are necessary for catalytic activity. His-9 (LacB) is a conserved residue in RpiB/LacAB superfamily proteins. However, the homologous histidine residue in RpiB is not involved in binding the pentose oxygen atom that is comparable to the O6 of Tag6P. In a previous study regarding the mechanism of RpiB action, Cys-65 (LacB), which interacts with the terminal O1 hydroxyl group, was proposed to be the catalytic base to accept a proton from C2 and return a proton to C1 in the final step [30]. As an exception to all other known RpiBs, the non-conserved Glu-75 of MtRpiB acts as a general base. Thr-67 (LacB) has been suggested to similarly participate in proton transfer between O1 and O2 of the enediolate intermediate [31]. That study also proposed that Asn-100 of EcRpiB, consistent with Asn-97 (LacA) of LacAB, may help bind and stabilize the high-energy intermediate. In addition, Tyr-42 (LacB), which presumably helps to bind the ring-form of the substrate via stacking interactions, may have an additional role in fixing the position of Asn-97 (LacA) with a hydrogen bond [31]. Thus, LacAB uses a six-carbon sugarphosphate as a substrate, whereas the structurally similar RpiB uses a five-carbon sugar-phosphate substrate.
Phosphoglucose isomerase (PGI, EC 5.3.1.9) and phosphomannose isomerase (PMI, EC 5.3.1.8) from Pyrobaculum aerophilum, which both belong to the PGI superfamily, have been analyzed structurally to elucidate their substrate interactions and mechanism of action. In contrast to the structures of RpiB/LacAB superfamily proteins, the overall structures of PGI and PMI comprise two globular domains and an arm-like C-terminal tail. The active site of these enzymes is similar to that of LacAB with regard to the histidine that is involved in ring-opening residue, but it is distinctly different from the LacAB active site in that the substrate phosphate group is held by three serines with short side chains [33,34].
In the present study, the crystal structure of LacAB was determined and was shown to be similar to that of D-ribose-5phosphate isomerase. We adduced evidence to support the proposed functions of specific amino acid residues in the mechanism of LacAB activity based on the relative activities of site-directed mutants. Detailed kinetic experiments using mutants with various active site residue mutations may expedite the clarification of the enzyme mechanism and allow enzyme modifications for the effective production of functional rare sugars.

Construction of LacAB
The two genes encoding galactose-6-phosphate isomerase, LacA and LacB, were isolated from Lactobacillus rhamnosus by polymerase chain reaction (PCR) using specific primes. The two PCR products were subcloned into BamHI and SalI restriction sites of the expression vector pQE-80L (Qiagen, Hilden, Germany), which contains six N-terminal His tags. To create LacAB mutants for studies of the mechanism of action, site-directed mutagenesis of substrate-binding residues of the active site was performed using the PCR primers shown in Table S2.

Expression, purification, and crystallization of the enzyme
The recombinant plasmids containing native and mutated lacAB genes were transformed into E. coli strain BL21 (DE3). The transformed cells were grown at 37°C in Luria-Bertani (LB) medium containing ampicillin (50 mg/L) until the optical density at 600 nm reached 0.8. Then, 0.25 mM isopropyl β-D-1thiogalactopyranoside (IPTG) was added to induce protein expression at 30°C for 6 h. The induced cells were harvested and sonicated in lysis buffer (50 mM Tris-HCl, pH 7.5 and 200 mM NaCl). After the lysates were clarified by centrifugation, the His-tagged LacAB was purified from the crude extract by immobilized metal affinity chromatography on a HisTrap column (GE Healthcare, Waukesha, WI, USA) and elution with a buffer containing 250 mM imidazole, followed by size exclusion chromatography on a Superdex 200 prep-grade column (GE Healthcare) equilibrated with 50 mM Tris-HCl, pH 7.5 and 200 mM NaCl. The purified enzyme was concentrated to 13 mg/mL using a Vivaspin 20-mL centrifugal concentrator (Sartorius).
The initial screening for crystallization was performed at 22°C with the sitting-drop vapor diffusion method using Crystal Screen (Hampton Research) and Wizard kits (Emerald BioSystems, Bedford, MA, USA). Needle-shaped crystals were obtained from 0.2 M potassium formate containing 20% PEG 3350. High-resolution-quality and size-improved crystals were obtained from the same solution using microseeding with the hanging-drop method.

Data collection and structure determination
Crystals of native LacAB were transferred to the crystallization solution containing 20% (v/v) glycerol as a cryoprotectant. Data of soaked native LacAB crystals were obtained at a single wavelength to 1.96 Å resolution at 100K in liquid nitrogen on beamline 7A at Pohang Accelerator Laboratory, Pohang, Korea. To obtain crystals of the LacABtagatose-6-phosphate complex, a LacAB crystal was soaked in crystallization solution containing 10 mM D-tagatose-6phosphate for 30 min. Crystals of LacAB with substrates were obtained by soaking LacAB crystals in cryoprotectant solution containing 45 mM D-ribose and 40 mM D-allose for 2 hr, respectively. Single-wavelength data of the crystal complexes were collected at 1.65 Å resolution. All crystals had the space group P2 1 2 1 2 with unit cell parameters of a = 108.3 Å, b = 116.0 Å, c = 54.7 Å, and α = β = γ = 90°. The X-ray diffraction data were integrated and scaled with the program HKL2000 [35] (Table S3).
The crystal structures of native LacAB and the LacABproduct complex were determined by molecular replacement based on ligand-free ribose-5-phosphate isomerase (RpiB; PDB ID 1NN4) originating from E. coli as a search model; this protein shares 26% amino acid sequence identity with LacA and LacB according to a BLAST search, respectively. The structures of the two monomers were solved by rotation and translation searches using the program Phaser in the Phenix software suite [36], and an electron density map of the whole LacAB molecule was obtained. Owing to its uniqueness, the Cterminal 30-residue segment of LacB could not be structurally described through molecular replacement, and further model building was performed manually using the program Coot [37]. Refinement by the CNS program [38] produced the final model, with R and R free values of 19.8% and 22.8%, respectively. Tagatose-6-phosphate in the LacAB-product complex was located based on the Fo-Fc electron density map calculated by molecular replacement using CNS. The stereochemistry of the refined structure was evaluated using ProCheck software [39]. The figures were prepared using PyMOL (PyMOL Molecular Graphics System by W.L. Delano), and the structures were analyzed using the CCP4 software suite [27]. Details of the data collection and structural refinement are shown in Table  S3. The atomic coordinates and structure factors (codes 4LFK for the free form of LacAB, 4LFL for its complex with Dtagatose-6-phosphate, 4LFM for its complex with D-psicose, and 4LFN for its complex with D-ribulose) have been deposited in the Protein Data Bank (http://www.rcsb.org).

Online size exclusion chromatography (SEC) coupled with multi-angle light scattering (MALS)
Online size exclusion chromatography (SEC) coupled with multi-angle light scattering (MALS) with embedded LS (light scattering) and UV detectors were used in the determination of molar mass in the purified LacAB. Chromatographic separation was achieved under isocratic conditions using an analytical Superdex 200 10/300 GL column connected to fast protein liquid chromatography (FPLC) with 50 mM Tris-HCl, pH 7.5 and 200 mM NaCl buffer at a flow rate of 1 mL/min. UV detector in FPLC was equipped sequentially with a MALS detector (Wyatt Technology Corporation, Santa Barbara, CA) applied a laser light at 658 nm and 18 detectors. The signals from UV and MALS were imported into Unicorn 5.11 and Astra V 5.3.4 software that was used for processing and analyzing the MALS data.

Activity assay of LacAB and mutants
Substrate isomerization and product accumulation in the reaction mixture were determined using a colorimetric method [40]. Relative activities of LacAB and various active-site mutants were measured using 25-200 mM D-ribose as the substrate. Cysteine hydrochloride, sulfuric acid, and an alcoholic solution of carbazole were added in serial order to the reaction mixture, which contained the generated D-ribulose. After incubation at room temperature, the absorbance of the mixture was measured at the wavelength of 560 nm. LacB contains an extra helix, α7, at its C-terminus, and the β2/ α2 loop of LacA is oriented toward the inside of the molecule. This figure was prepared using the Superpose program [25]. (TIF)