L-Ribulose

Arabinose isomerase [EC 5.3.1.3] catalyzes the interconversion of D-arabinose and D-ribulose. The enzyme will also utilize L-fucose and more slowly utilize L-galactose and D-altrose as substrates. L-Arabinose isomerase [EC 5.3.1.4] catalyzes the interconversion of L-arabinose and L-ribulose. 

This enzyme [EC 5.1.3.4] catalyzes the interconversion of L-ribulose 5-phosphate and D-xylulose 5-phosphate. 

These enzymes vary widely in secondary and tertiary structure.1273 Mannose-6-phosphate isomerase is a 45 kDa Zn2+-containing monomer. The larger 65 kDa L-fucose isomerase, which also acts on D-arabinose, is a hexameric Mn2+-dependent enzyme.1273 L-Arabinose isomerase of E. coli, which interconverts arabinose and L-ribulose, is a hexamer of 60-kDa subunits128 while the D-xylose isomerase of Streptomyces is a tetramer of 43-kDa subunits.129 The nonenzymatic counterpart of the isomerization catalyzed by the enzyme is the base-catalyzed Lobry deBruyn-Alberda van Ekenstein transformation (Eq. 13-25).130... 

L-arabinose isomerase, which interconverts L-arabinose and L-ribulose araB, L-ribulokinase, which uses ATP to phosphorylate L-ribulose at C-5 araD, L-ribulose 5-phosphate epimerase, which interconverts L-ribulose 5-phosphate and L-xylulose 5-phosphate talB, transaldolase and tktA, transketolase. 

L-Ribulose-5-phosphate 4-epimerase catalyzes epimerization at the C4 position of L-ribulose-5-phosphate to form D-xylulose-5-phosphate, allowing bacteria to utilize arabi-nose as an energy source in the pentose phosphate pathway ". The enzyme from E. coli is comprised of four equal 25.5 kDa subunits and shows very close resemblance to the... 

Vanhessche, K, Gonzales Bello, C, Vandewalle, M, Total synthesis of (—)-neplanocin A from L-ribulose, Synlett, 921-922, 1991. 

The L-alanine catalyzed reaction of 25 and BnOCH2CHO gives 5-0-benzyl-1,3-di-O-isopro-pylidene-L-ribulose [121]. The direct asymmetric intermolecular aldol reactions are also catalyzed by small peptides. For instance, in the presence of 30mol% of L-Ala-L-Ala in DMSO containing 10 equivalents of H2O, 25 reacted with 4-cyanobenzaldehyde giving the corresponding aldols with an antilsyn ratio of 13 1 and ee of 99% for the anti aldol (65% yield) [145]. 

L-Arabinose isomerase has been purified from Escherichia coli. The enzyme catalyzes the isomerization of L-arabinose to L-ribulose by a proton transfer mechanism in contrast to xylose isomerase which uses a hydride transfer mechanism to perform a similar isomerization [20], This enzyme has high substrate specificity for L-arabinose, and its catalytic activity is dependent on manganese(II). It has been shown, however, that the enzyme could catalyze the exchange of the proton attached to C2 of arabinose with the solvent even in the absence of metal ion [20],... 

In Escherichia coli, L-arabinose isomerase (ECAI) catalyzes the isomerization of L-arabinose to L-ribulose. This enzyme is also of commercial interest as it catalyzes the conversion of D-galactose to D-tagatose in vitro. The crystal stmcture of ECAI was solved and refined at 2.6 angstrom resolution [152]. 

A wide range of bacterial species utilizes D-xylose and L-arabinose as carbon and energy sources. In most cases, the direct isomerization of aldopentoses to their corresponding ketoses is the first step in pentose metabolism. For example, D-xylose is converted into D-xylulose and L-arabinose is converted into L-ribulose L-ribulose can be converted into D-xylulose by epimerase. D-Xylulose is the key intermediate for further metabolism. For this reason, most bacterial species can also readily utilize L-arabinose. 

L-ribulose 5-phosphate 4-epimerase divalent metal (Mn2+ > Ni + > Ca + > Zn +) glycoaldehyde phosphate + metal-bound enolate retro-aldol C—C bond cleavage ... 

Another group of sugar epimerases, which uses a metal cofactor instead of NADH/NAD+, takes an entirely different approach to epimerization. L-ribulose 5-phosphate 4-epimerase, which is involved in the bacterial metabolism of arabinose, performs a retro-aldol cleavage of a C-C bond to yield a metal-stabilized enolate of dihydroxyacetone and glycoaldehyde phosphate, similar to the reaction catalyzed by class II aldolases [77-79]. The glycoaldehyde phosphate is thought to rotate, such that addition of the enolate generates the isomeric product. 

N. Lotong, and K. Matsushita, Membrane-bound sugar alcohol dehydrogenase in acetic add bacteria catalyzes L-ribulose formation and NAD-dependent ribitol dehydrogenase is independent of the oxidative fermentation, Biosci. Biotechnol. Biochem., 65 (2001) 115-125. 

Class V Isomerase E. coli L-ribulose-5-phosphate-4- IJDI His,e 1 HisL 73 Hisb/i H2O ... 

S ATP -I- L-xylulose <1, 2> (<1> specific for L-isomer, no phosphorylation of D-xylulose, D-/L-ribulose, D-/L-xylose, D-/L-arabinose, D-lyxose, D-ri-bose, D-/L-glucose, o-fructose, L-sorbose, D-mannose, D-galactose, D-al-trose, D-/L-fucose, L-rhamnose, L-erythrulose, D-/L-arabitol, xylitol, ribi-tol or erythritol [1]) (Reversibility <1,2> [1,2]) [1, 2]... 

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