Rhamnose isomerase

Knowledge of the three-dimensional structure of an enzyme is the basis for understanding its substrate specificity. Thus, the E. coli Rhal was crystallized and its X-ray crystal structure was solved in collaboration with I. Korndorfer [22]. The enzyme is a tight tetramer of four (P/a)g barrels, with structural similarity to xylose isomerase (Eig. 2.2.5.3). The structures of complexes of rhamnose isomerase with the inhibitor L-rhamnitol and the natural substrate L-rhamnose were 

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L-Rhamnose isomerase catalyzes the interconversion of L-rhamnose and L-rhamnulose. L-Rhamnose, a deoxy sugar, is found in bacteria and plants and it plays an essential role in many pathogenic bacteria. The pathway for the metabolism of this sugar does not exist in humans, and this makes enzymes of this pathway attractive targets for therapeutic intervention. " Rhamnose isomerase from E. coli is a tetramer of (/ /a)8-barrels similar to xylose... 

RHAMNOSE ISOMERASE RHAMNOSE ISOMERASE a-L-RHAMNOSIDASE a-L-Rhamnosides, a-L-RHAMNOSIDASE t-Rhamnulose,... 

While DHAP aldolases produce ketose derivatives, access to biologically important and structurally more diverse aldose isomers is achievable by use of enzymatic ketol isomerase interconversions. For this purpose, we had previously shown that L-rhamnose isomerase (Rhal E.C. 5.3.1.14) and L-fucose isomerase (Fuel E.C. 5.3.1.3) from E. coli display a relaxed substrate tolerance. Both enzymes convert sugars and their derivatives with distinct stereopreference at C2 and common (3R)-OH configuration, but tolerate alterations in configuration or substitution pattern at subsequent positions of the chain (Scheme 2.2.5.5) [11, 12]. 

Fig. 2.2.S.3 Structure of rhamnose isomerase, and proposed hydride-shift mechanism for the ketol isomerization.

Class V Isomerase E. coli Rhamnose Isomerase 1DE5 Glu 32 Asp 26 His 39 Asp ... 

Rhamnose isomerase of Escherichia coli appears from the structure of its ligand complexes to have a similar mechanism to glucose isomerase. ... 

Fuc 1-P and Rha 1-P aldolases have also been utilized in whole cell systems with DHA and catalytic inorganic aresenate1971. With L-lactaldehyde as the substrate in the Rha 1-P aldolase reaction, the aldol product L-rhamnulose was subsequently iso-merized to L-rhamnose, catalyzed by rhamnose isomerase. No such isomerization was observed with L-xylulose, the corresponding aldol product using glycolaldehyde as the substrate. Recent studies have since shown that both rhamnose and fucose isomerase require fixed stereochemistry only up to C3 for aldohexose substrates ... 

R,3R) for rhamnose isomerase and (2S,3R) for fucose isomerase1861. A sequential application of Fuc 1-P aldolase and fucose isomerase was employed for the preparation of L-fucose analogs [98L... 

We describe here the enzymological characteristics and application of isomerases, especially D-xylose (glucose) isomerase, phosphoglucose isomerase, triose phosphate isomerase, L-rhamnose isomerase, L-fucose isomerase, maleate cis-trans isomerase, and unsaturated fatty acid cis-trans isomerase. N-Acetyl-D-glucosamine 2-epimerase is not an isomerase, but for convenience we will also describe the characteristics and use of the enzyme because this section deals with sugar-metabolizing enzymes. 

L-Rhamnose is an important component of bacterial cell walls, and is metabolized in coli through a pathway similar to that of glucose 6-phosphate in glycolysis. Rhamnose isomerase catalyzes the first reaction in the pathway to produce l-rhamnulose from L-rhamnose (Fig. 17-30). The enzyme gene was cloned from coli and overexpressed12521, and the enzyme was purified and characterized12521. 

Rhamnose isomerase is composed of four identical subunits with a molecular mass of about 47 kDa. It has the maximum activity around 7.6, and requires Mn2 to provide the highest activity. The enzyme shows no significant sequence similarity to any other ketol isomerases including xylose isomerase. However, rhamnose isomerase was found, by X-ray crystallography, to be most similar to xylose isomerase12521. The monomer of rhamnose isomerase is composed of (P/aJg-barrels, and the structure and arrangement of the barrel are very similar to those of xylose isomerase. However, each of them has an additional a-helical domain, which is involved in subunit assembly and differs from each other only in its structure. The... 

Figure 17-30. Reaction catalyzed by rhamnose isomerase. Since both substrate and product occur in cyclic forms, L-rhamnose isomerase catalyzes ring opening before isomerization. Reprinted from Korndorfer et al. 252).

Figure 17-31. Superposition of the metal binding sites of rhamnose isomerase (residues named and drawn with thick bonds) and zylose isomerase (thin bonds). Reprinted from Korndorfer et al. 252. ...

Bhuiyan et al.12531 immobilized i-rhamnose isomerase from Pseudomonas sp. LL172 on chitopearl beads, and used it to produce L-mannose from L-fructose. The immobilized enzyme was found to be stable it retained about 90% of the initial activity after five repeated batch reactions. The concentration of L-mannose relative to L-fructose was about 3 7 at equilibrium. D-Allose was also produced from d-psicose with the immobilized L-rhamnose isomerase. Since D-psicose is readily produced from D-fructose with D-tagatose 3-epimerase, D-allose can be produced from D-fructose by combination of the two enzymes immobilized on chitopearl beads. Bhuiyan et al. 254] found that the reaction progresses steadily until 40% of the D-psicose is converted into D-allose. The immobilized D-tagatose 3-epimerase was also stable even after repeated uses, and D-allose was produced efficiently in the system. 

Very interesting applications of the preparative potential of microbial ketol isomerases were reported by Fessner and co-workers, who gained access to preparative amounts of L-rhamnose isomerase (EC 5.3.1.14) as well as L-fucose isomerase (EC 5.3.1.3) from E. colihy cloning and over-expression [95]. In this way they were able to convert (Scheme 34) a variety of d- and L-pentoses and -hex-oses into the corresponding 2-ketoses, such as L-mannose (111) (to L-fructose, 113), D-gulose (114) (to D-sorbose, 8) and D-allose (22) (to o-psicose, 10), to mention just a few. 

Keywords Aldose ketol isomerases. Enzymatic isomerisation, L-Fucose isomerase, D-Glucose (D-Xylose) isomerase. Non-natural substrates, L-Rhamnose isomerase... 

Y.S. Kim, K.C. Shin, Y.R. Lim, D.K. Oh, Characterization of a recombinant 1-rhamnose isomerase from Dictyoglomus turgidum and its application for 1-rhamnulose production, Biothec-... 

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