FEBS Letters 581 (2007) 3127–3130

Substrate spectrum of L-rhamnulose related to models derived from two ternary complex structures

Dirk Grueninger, Georg E. Schulz* Institut fu¨r Organische Chemie und Biochemie, Albert-Ludwigs-Universita¨t, Albertstrasse 21, 79104 Freiburg im Breisgau, Germany

Received 18 April 2007; revised 29 May 2007; accepted 29 May 2007

Available online 6 June 2007

Edited by Hans Eklund

2. Materials and methods Abstract The L-rhamnulose kinase from Escherichia coli participates in the degradation pathway of L-rhamnose, a common natural deoxy-hexose. The structure of the enzyme in The L-rhamnulose-1-phosphate was produced by an aldol condensation of dihydroxyacetone phosphate and lactaldehyde using a ternary complex with its substrates ADP and L-rhamnulose ˚ L-rhamnulose-1-phosphate aldolase [11,12]. The reaction mixture con- has been determined at 1.55 A resolution and refined to tained 2.4 mmol dihydroxyacetone phosphate and 6 mmol D/L-lactal- Rcryst/Rfree values of 0.179/0.209. The result was compared with dehyde in 180 ml buffer of 20 mM Tris–HCl, pH 7.2. The reaction the lower resolution structure of a corresponding complex con- was started by adding 1.7 lmol (50 mg) of the aldolase and ran at taining L- instead of L-rhamnulose. In light of the two 37 C for 2 h. The protein was removed by denaturation (95 C, established sugar positions and conformations, a number of rare 5 min) and centrifugation. The solution was concentrated and run sugars have been modeled into the active center of L-rhamnulose through a silicagel column (Silicagel 60, Merck Darmstadt) using a kinase and the model structures have been compared with the 2:1 (v/v) mixture of iso-propanol and saturated NH3 in water. The known enzymatic phosphorylation rates. Rare sugars are of ris- L-rhamnulose-1-phosphate was identified by thin-layer chro- matography, confirmed by 1H and 31P NMR, separated and dried ing interest for the synthesis of bioactive compounds. using a rotary evaporator. 2007 Federation of European Biochemical Societies. Published Since the charge removal at three surface residues in RhuK mutant by Elsevier B.V. All rights reserved. E69A-E70A-R73A gave rise to well-ordered crystals [6], we kept these mutations in the reported analysis. The triple mutant was overexpres- Keywords: Epimer tolerance; Ligand fitting; Phosphoryl sed in E. coli JM105 and purified as described [6]. The protein was dia- transfer; Rare 2-ketoses; X-ray diffraction lyzed against 10 mM b-mercaptoethanol and concentrated to 20 mg/ ml. Co-crystallization was carried out at 20 C using the hanging drop method. The drops consisted of 2 ll of a solution of 12 mg/ml protein, 20 mM ADP and 20 mM L-rhamnulose-1-phosphate (unbuffered) and 1. Introduction 2 ll of the reservoir containing 17% (w/v) PEG 8000 and 120 mM LiCl (unbuffered). Crystals grew to maximum sizes of about 300 · 150 · 100 lm3 within three days. For X-ray analysis, the crystals In numerous plants the sugar L-rhamnose is a frequent cell wall were transferred to 34% (w/v) PEG 8000 with 120 mM LiCl and shock- constituent [1] that is metabolized by various bacteria, among frozen in a gas stream at 100 K. them Escherichia coli [2,3].InE. coli therespectiveoperonis X-ray diffraction data were collected at the Swiss Light Source (SLS, Villigen, Switzerland) and processed with XDS [13]. Using REFMAC5 composed of L-rhamnose permease, L-rhamnose , L- [14] an initial model was obtained by rigid body refinement of the rhamnulose kinase (RhuK, EC 2.7.1.5), L-rhamnulose-1-phos- established RhuK structure (PDB code 2CGL). The substrate L- phate aldolase and two regulatory proteins [4]. Here, we are con- rhamnulose was generated with PRODRG2 [15] and introduced into cerned with the kinase that uses ATP to phosphorylate the the model. Water molecules were added with ARP/wARP [16]. The sugar. The reaction results in L-rhamnulose-1-phosphate, which model was improved with COOT [17] and refined with REFMAC5. Models of the rare sugars were produced with PRODRG2 [15] starting is then split by the aldolase into L-lactaldehyde and dihydroxyac- from the furanose conformation observed for L-fructose [6] and L- etone phosphate [5]. rhamnulose bound to RhuK. They were manually placed using COOT RhuK belongs to the -actin-hsp70 family exhibit- [17]. The figures were prepared using POVscript [18] and POVRAY ing the characteristic b-sheet in an N-terminal as well as a C-ter- (http://www.povray.org). The coordinates and structure factors are minal domain and the ATP in a cleft between the deposited in the Protein Data Bank accession code 2UYT. two domains [6]. The similarity is significant indicating a com- mon evolutionary origin [7]. A comparison between holo and 3. Results and discussion apo structures of RhuK showed a large induced-fit during sub- strate binding involving a relative rotation of about 10 between RhuK is a monomer with a mass of 54110 Da comprising two the N- and C-terminal domains [6]. require such a con- approximately equal sized domains, each containing a characteri- formational change to exclude water, which would otherwise stic five-stranded b-sheet covered on both sides by helices [6].The compete for the phosphoryl group. An induced-fit was initially known structure of residues 2–480 and ADP was placed into the observed for hexokinase [8]. For the ATP actin and crystal unit cell. After adding b-L-rhamnulose and water mole- hsp70, a corresponding rotation on phosphoryl transfer serves cules, the model was refined to 1.55 A˚ resolution yielding good as a conformational signal [9] or as a power stroke [10]. quality indices (Table 1). Given the high resolution, 14 residues were observed in double conformations. None of them was in *Corresponding author. Fax: +49 761 203 6161. E-mail address: [email protected] (G.E. Schulz). theactivecenter.ADPandthenaturalsubstrateL-rhamnulose were well outlined by the electron density. Since the average B-fac- Abbreviations: RhuK, L-rhamnulose kinase; E. coli, Escherichia coli tors of both substrates matched that of the surrounding protein,

0014-5793/$32.00 2007 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.febslet.2007.05.075 3128 D. Grueninger, G.E. Schulz / FEBS Letters 581 (2007) 3127–3130

Table 1 The adenine moiety of ADP binds in a non-polar crevice made a Structure determination up of Leu315, Pro316 and Ile319 on one side, and Gly402, Data collection Gly403 and Gln405 on the other. The ribose binds with its 30-hy- Resolution range (A˚ ) 79–1.55 (1.62–1.55) droxyl group to Gln304-Ne. The conformation of the diphos- Unique reflections 60401 (6646) phate moiety is stabilized by hydrogen bonds to backbone Multiplicity 6.9 (5.5) Completeness 98.6 (89.4) amides and side-chains from the two b-hairpins around posi- RsymI (%) 5.1 (37) tions 14 and 259, as well as from Gly402-N. The bound ADP Average I/r 23.2 (4.7) corresponds closely to that of the L-fructose complex [6] except for the b-phosphoryl, which is rotated about 25 around the Pb– Refinement Oab bond. As a consequence the bc-bridge oxygen is at a 4.5 A˚ Protein atoms 3783 Ligand atoms 38 distance from the 1-hydroxyl nucleophile of the bound L- Solvent atoms 322 rhamnulose, which is 0.2 A˚ shorter than observed in the L-fruc- Rcryst/Rfree (5% test set) 0.179/0.209 tose complex, corroborating the associative type of phosphoryl ˚ 2 B-factor average total/water (A ) 14.5/25.6 transfer [20]. As shown in Fig. 1, the c-phosphoryl group can be Rmsd bond lengths (A˚ )/bond angles () 0.010/1.34 Ramachandran: favored/allowed/disallowed (%) 93.9/5.9/0.2 modeled in a staggered conformation between nucleophile and leaving group in an ideal geometry for a direct in-line transfer. aValues in parentheses are for the highest shell. The data were collected at a wavelength of 0.91841 A˚ at beamline PX-I of the Swiss Light The sugar L-rhamnulose was bound in the cleft at the same L Source. The crystal belongs to spacegroup P212121 with 1 molecule per position as -fructose (Fig. 1, Table 2). Its 6-methyl group fitted asymmetric unit, 36% solvent, and unit cell parameters a = 51.0 A˚ , well into a non-polar pocket formed by Gly83, Val84, Tyr102 b = 51.1 A˚ , c = 158.9 A˚ . and Phe134 from the N-domain (Fig. 2). The hydroxyl groups form hydrogen bonds to His236, Asp237, Thr238, Asn296, and the sites were considered fully occupied. The substrate binding via two water molecules to Glu284, all from the C-domain. The structure is shown in Fig. 1. The co-crystallization experiment with 1-hydroxyl group of L-rhamnulose is hydrogen-bonded to ADP and L-rhamnulose-1-phosphate but without the divalent Asp237 functioning as the catalytic base. The binding site is vir- cation required for [12,19] were designed to show RhuK tually the same as observed with the L-fructose complex [6]. in complex with the reactants. Unfortunately, the transferred Whereas its natural phosphoryl acceptor is L-rhamnulose, phosphoryl group was missing in the crystal. A disordered phos- activity data suggested that RhuK plays a role in the metabo- phate was ruled out because the corresponding space was filled lism of quite a number of ‘rare sugars’ [21], phosphorylating a by well-ordered water molecules. We conclude that the phos- broad spectrum of 2-ketoses [12,19,22].InTable 2, we list the phoryl group was hydrolyzed during crystallization. 16 ketoses tested for RhuK activity and state their steric rela-

Fig. 1. Stereoview of the active center displayed in a ribbon plot of RhuK. The two domains have different colors. The (Fo Fc)-difference electron density map of ADP and b-L-rhamnulose is shown at the 3.5 r contour level. Some residues and their interactions are shown. Hydrogen bonds are indicated as broken lines. The c-phosphoryl group before the transfer (pink) is modeled into the trigonal bipyramid describing the transfer reaction. The presence of the modeled Mg2+ ion (pink) at the expected position between the b- and c-phosphoryl groups most likely causes a rearrangement of the water molecules. D. Grueninger, G.E. Schulz / FEBS Letters 581 (2007) 3127–3130 3129

Table 2 Activity of RhuK from E. coli for 2-keto-furanoses Substrate Model Fig. 2 C atom configurationa Residue at atom C5 Activity (%) Reference 34 5

L-rhamnulose A RS S CH3 100 – b 6-deoxy-D-sorbose A RS R CH3 87 [22] L-xylulose – RS - H 11, 87 [12,19] c L-fructose B RS S CH2OH 77 [12] d L-fuculose A RR S CH3 30, 56 [12,19] d D-ribulose – RR –H 24 [12] b,c D-psicose – RR R CH2OH 0, 23 [12,19] d c L-tagatose B RR S CH2OH 19 [12] b D-sorbose B RS R CH2OH 16 [12] e d D-xylulose – S R –H 2 [19] b D-fructose – SRR CH2OH 1 [19] e L-ribulose B S S –H 1 [19] e 6-deoxy-L-psicose A S SS CH3 0 [22] e b D-tagatose – S SR CH2OH 0 [19] e d c L-sorbose – S R S CH2OH 0 [19] e d b D-rhamnulose – S R R CH3 0 [22] aWe observed the 2S-configuration in both structurally established complexes with RhuK and used it throughout for modeling. bThe 5-residue makes tight contacts in the non-polar pocket formed by Gly83, Val84, Tyr102 and Phe134. cThe 5-hydroxyl forms a hydrogen bond to Gly83-O. dThe 4-hydroxyl loses its hydrogen bonds to Asn296 but finds a reasonably large low-polarity pocket. eThe 3-hydroxyl protrudes into a narrow pocket and loses its hydrogen bonds to His236, Thr238 and water molecule II. tion to the prime substrate L-rhamnulose. Since L-rhamnulose family of ‘rare sugars’ that have been found in a number of and L-fructose assume the S-configuration at the C2 atom (b- organisms [23–25]. Recently, the interest in rare sugar produc- anomer) when bound to RhuK and since RhuK obviously re- tion increased appreciably because they are useful ingredients quires the R-configuration at the C3 atom (Table 2), we sug- of various bioactive compounds [11,12,23–25]. gest that all 2-ketoses bind as 2S-anomers, which are the a- and b-anomers in the D- and L-series, respectively. The 2R- Acknowledgements: We thank C. Beierlein for the gift of lactaldehyde anomer would not fit the binding site (Fig. 2). Accordingly, and for discussions as well as the team of the Swiss Light Source (Vil- ligen, Switzerland) for their help in data collection. we modeled all sugars as 2S-anomers manually into the active center so that the catalytic base Asp237 forms hydrogen bonds References to the 1- as well as to the 2-hydroxyls. Given this fit, the remainder of the sugars is spatially rather restricted so that [1] McNeil, M., Darvill, A.G., Fry, S.C. and Albersheim, P. (1984) we refrained from using docking programs, but positioned Structure and function of the primary cell walls of plants. Annu. the sugars by mere rotation/translation. None of the modeled Rev. Biochem. 53, 625–663. sugars forms a hydrogen bond shorter than 2.5 A˚ or a van-der- [2] Chiu, T.-H. and Feingold, D.S. (1969) L-rhamnulose-1-phosphate ˚ aldolase from Escherichia coli. Crystallization and properties. Waals contact closer than 3.5 A, indicating that the models are Biochemistry 8, 98–108. consistent with binding to a slightly relaxed protein. [3] Graninger, M., Kneidinger, B., Bruno, K., Scheberl, A. and The fits explain the observed activities [12,19,22] rather well. In Messner, P. (2002) Homologs of the Rml from Salmonella particular, we show in Fig. 2 multiple hydrogen bonds around the enterica are responsible for dTDP-b-L-rhamnose biosynthesis in the Gram-positive thermophile Aneurinibacillus thermoaerophilus 3R-hydroxyl, which are lost in the 3S-epimer. The surrounding DSM 10155. Appl. Environ. Microbiol. 68, 3708–3715. water molecule I is suspended from Tyr102 and polarizes the [4] Moralejo, P., Egan, S.M., Hidalgo, E. and Aguilar, J. (1993) nucleophile. Water molecules II and III are held by Glu284 (form- Sequencing and characterization of a gene cluster encoding the ing a buried saltbridge to Arg292) and promote binding of the 3- enzymes for L-rhamnose metabolism in Escherichia coli.J. and 4-hydroxyls. All sugar models are compatible with the three Bacteriol. 175, 5585–5594. [5] Kroemer, M., Merkel, I. and Schulz, G.E. (2003) Structure and bound water molecules although in some cases their 3- and 4- catalytic mechanism of L-rhamnulose-1-phosphate aldolase. Bio- hydroxyls make close contacts to them. However, a contact with chemistry 42, 10560–10568. bound water is rather easily relaxed, in particular when it is (like [6] Grueninger, D. and Schulz, G.E. (2006) Structure and reaction molecule III) backed up by other water. The residue at the C5 mechanism of L-rhamnulose kinase from Escherichia coli. J. Mol. Biol. 359, 787–797. atom of the sugar seems to be of lesser importance as it faces a [7] Buss, K.A., Cooper, D.R., Ingram-Smith, C., Ferry, J.G., non-polar pocket formed by Gly83, Val84, Tyr102 and Phe134. Sanders, D.A. and Hasson, M.S. (2001) Urkinase: structure of In summary, the data of Table 2 show rather clearly that the acetate kinase, a member of the ASKHA superfamily of configuration at the C3 atom is most important for activity. . J. Bacteriol. 183, 680–686. 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(1991) Use of dihydroxyacetone 3130 D. Grueninger, G.E. Schulz / FEBS Letters 581 (2007) 3127–3130

Fig. 2. Stereoview of the sugar binding site of RhuK showing the established bound substrates (black) and models of half of the enzymatically tested 2- ketoses (Table 2). Hydrogen bonds are black dashed lines. Chain cuts are marked by halos. The residues are given in their domain colors (Fig. 1). (A) Bound b-L-rhamnulose (black) with hydrogen bonds. The red dotted line runs to the c-phosphoryl-donating oxygen of the b-phosphoryl group at a 4.5 A˚ distance from the 1-hydroxyl nucleophile (Fig. 1). The modeled rare sugars 6-deoxy-a-D-sorbose (5-epimer, blue), b-L-fuculose (4-epimer, green) and 6- deoxy-b-L-psicose (3-epimer, magenta) are also shown. (B) Same as panel (A) but bound b-L-fructose (black) with hydrogen bonds [6]. The modeled rare sugars are a-D-sorbose (5-epimer, blue), b-L-tagatose (4-epimer, green) and b-L-ribulose (3-epimer without the 6-CH2OH residue, magenta).

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