L-Ribulose 5-phosphate 4-epimerase

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... 

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. 

Ribulose phosphate epimerase, EC 5.1.3.4. Its official name is L-ribulose-5-phosphate-4-epimerase. This is a key enzyme in the pentose phosphate pathway. An epimer is a stereoisomer variant of a sugar differing in the configuration at only one carbon atom (see Chap. 11). 

Fig. 1) [1-4]. Genes araA, araB, and araD are the structural genes for L-arabinose isomerase [5], L-ribulokinase [5], and L-ribulose 5-phosphate 4-epimerase, respectively, the first three enzymes involved in L-arabinose metabolism. These genes, together with their controlling sites,... 

Polarity is in the direction araB, ara A, araP. Nonsense mutations in gene araB lead to reduced levels of L-arabinose isomerase and L-ribulose 5-phosphate 4-epimerase [5,6]. Nonsense mutations in gene araA result in redueed levels of L-ribulose 5-phosphate 4-epimerase, but do not effect a reduction of kinase levels [40]. Thus the controlling elements for this operon must be the proximal end of the araB gene—that is, the end farthest from gene araA. 

The effect of mutations in one structural gene on the activity of an adjacent structural gene in an operon was discovered in the L-arabinose system [2,73]. Certain mutations in gene araB were found to produce a coordinate decrease or increase in inducible levels of L-arabinose isomerase and L-ribulose 5-phosphate 4-epimerase as compared to corresponding levels found in the wild type [2,5,37]. In addition, mutations in the araA gene were found to produce increased inducible levels of L-ribulokinase [2], while some mutations effected a decrease in inducible levels of the epimerase [40]. Moreover, the increases or... 

Perhaps the key reaction in this chapter was the aldol condensation in Section 22.2. Although it is not mentioned in the discussion, the aldol reaction is reversible under certain circumstances. Enzymes known as aldolases can catalyze both the forward and reverse aldol reactions. An example is the retro-aldol/ aldol mechanism employed by the enzyme l-ribulose-5-phosphate-4-epimerase, found in both prokaryotes and eukaryotes, to invert the stereochemistry of the hydroxyl-bearing carbon in 150 to that in 153. In other words, 1-ribulose-5-phosphate is epimerized to give d-xylulose-5-phosphate (see Chapter 28, Section 28.1, for a discussion of these carbohydrates). 

The fungal oxidoreductive catabolism of these two pentoses is unique for fungi and produces ultimately D-xylulose 5-phosphate, which is an intermediate of the canonical pentose phosphate pathway (Fig. 18.3). In contrast, prokaryotes use an isomerase step to convert D-xylose to D-xylulose, and L-arabinose to L-ribulose. o-xylulose 5-phosphate is then either formed by the action of xylulokinase (in the case of D-xylose) or a sequence of L-ribulokinase and L-ribulose-5-phosphate 4-epimerase (in the case of L-arabinose) (Mishra and Singh 1993). 

RIBULOSE-5-PHOSPHATE 4-EPIMERASE a-XYLOSIDASE j8-XYLOSIDASE d-XYLULOKINASE l-XYLULOKINASE X-Pro dipeptidase... 

Different pathways are available in nature for metabolism of arabinose and xylose which are converted to xylulose 5-phosphate (intermediate com-poimd) to enter the pentose phosphate pathway as shown in Figure 10.5. In yeasts, xylose is first reduced by xylose reductase to xylitol, which in turn is oxidized to xylulose by xylitol dehydrogenase. In bacteria and some anaerobic fungi, xylose isomerase is responsible for direct conversion of xylose to xylulose. Xylulose is finally phosphorylated to xylulose-5-phos-phate by xylulokinase. In fungi, L-arabinose is reduced to L-arabitol (by arabinose reductase), L-xylulose (by arabitol dehydrogenase), xylitol (by L-xylulose reductase). Xylitol is finally converted to xylulose (by xylitol dehydrogenase), whose activity is also part of xylose utilization pathways. In bacteria, L-arabinose is converted to L-ribulose (by L-arabinose isomerase), L-ribulose-5-P (by L-ribulokinase) and finally D-xylulose-5-P (by L-ribulose-5-P 4-epimerase) (Bettiga et al., 2008). 

Other reactions which are used for the preparation of some esters are those catal) ed by isomerases. Thus fructose 6-phosphate may be converted to glucose 6-phosphate, ribose 5-phosphate to ribulose 5-phosphate. Specific epimerases which lead to inversion at positions 3 or 4 have been employed. For instance D-xylulose 5-phosphate can be converted to L-ribulose 5-phosphate by a 4-epimerase, or to D-ribulose 5-phosphate by a 3-epimerase. 

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