Ribulose-5-phosphate cleavage

Some lactic acid bacteria of the genus Lactobacillus, as well as Leuconostoc mesenteroides and Zymomonas mobilis, carry out the heterolactic fermentation (Eq. 17-33) which is based on the reactions of the pentose phosphate pathway. These organisms lack aldolase, the key enzyme necessary for cleavage of fructose 1,6-bisphosphate to the triose phosphates. Glucose is converted to ribulose 5-P using the oxidative reactions of the pentose phosphate pathway. The ribulose-phosphate is cleaved by phosphoketolase (Eq. 14-23) to acetyl-phosphate and glyceraldehyde 3-phosphate, which are converted to ethanol and lactate, respectively. The overall yield is only one ATP per glucose fermented. 

The origin of ribose-5-phosphate from ribulose- phosphate has been discussed, as well as the degradation of these pentose phosphates to smaller fragments. The problem of forming ribose phosphate arises in at least two other conditions (1) the utilization of ribose added as a nutrient for microbial growth, and (2) the generation of ribose from nucleosides, either directly by the hydrolytic cleavage of a nucleoside, as in... 

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. 

A regioselective dephosphorylation was used in the synthesis of 2 -carboxy-D-arabinitol 1-phosphate (Table 13-6, entry 2), a natural inhibitor of ribulose 1,5-bisphosphate carboxylase. Either acid or alkaline phosphatases can be used for the selective hydrolysis of the 1-phosphoryl group of 2 -carboxyl-D-arabinitol 1,5-bisphosphate. With acid phosphatase, the conversion was essentially quantitative yielding exclusively the 1-phosphate derivative (cleavage of the 5-phosphoryl group). On the other hand, hydrolysis with alkaline phosphatase gave a 4 1 mixture of the 1-and 5-phosphate derivatives. 

The biosynthetic pathway of riboflavin (and FMN/FAD), for bacteria/archaea/ fungi, requires one molecule of GTP and two molecules of ribulose 5-phosphate as substrates (Pigure 6.68) [295, 296]. The first biosynthetic step is catalyzed by the bifunctional GTP cyclohydrolase II/3,4-dihydroxy-2-butanone-4-phosphate synthase, rib A, and involves the hydrolytic release of formate and pyrophosphate from GTP. The product 2,5-diamino-6-ribosylamino-4(3//)-pyrimidinone 5 -phosphate 48 is converted to 5-amino-6-ribitylamino-2,4(l/7,3//)-pyrimidinedione 5 -phosphate 50 by two reaction steps, catalyzed by the bifunctional riboflavin-specific deam-inase/reductase, ribG, involving the hydrolytic cleavage of the 2-amino group of... 

The transketolase enzyme of Racker el obtained in crystalline form from baker s yeast, catalyzes the cleavage of ribulose-5-phosphate, with the formation of D-glyceraldehyde-3-phosphate upon the addition of an acceptor aldehyde, such as ribose-5-phosphate or glycolaldehyde. The reaction of hydroxypyruvate with D-glyceraldehyde-3-phosphate as acceptor aldehyde leads to the decarboxylation of the hydroxypyruvate with the formation of ribulose-5-phosphate. The transketolase enzyme was demonstrated to have a requirement for thiamine pyrophosphate. ... 

The discovery of ribulose-5-phosphate has posed the problem of whether this compound or the aldopentose phosphate is the immediate substrate for cleavage to triose phosphate. An enzyme preparation has been isolated from yeast by de la Haba and Racker, which will convert ribulose-5-phosphate to triose phosphate but which will produce this compound from ribose-5-phosphate only after a marked lag. The could be eliminated by another protein fraction from yeast presumably containing the pentose phosphate isomerase. However, it was observed that ribulose-5-phosphate alone is less readily cleaved to triose phosphate than is the reaction mixture of ribose-fi-phosphate and pentose isomerase. 

Most recently the enzyme of yeast catalyzing the cleavage of ribulose-5-phosphate has been crystallized and designated transketolase. " The pentose phosphate isomerase has been removed from the crystalline transketolase, which maintains its activity on ribulose-5-phosphate. The purified enzyme also catalyzes the formation of the ketopentose phosphate from glyceraldehyde-3-phosphate and a 2-carbon donor such as hy-droxypyruvate. Transketolase contains thiamine pyrophosphate as a coenzyme. 

COz-acceptor. The inability to isolate glycolaldehyde or indeed any 2-carbon fragments as such in the experiments on cleavage of ribulose-5-phosphate, described in the previous section, suggests the formation of a 2-carbon enzyme complex. Indeed one may conceive that this complex at the oxidation-reduction level of glycolaldehyde is condensed with CO2 to form 3-phosphoglyceric acid after steps involving phosphorylation and reduction. 

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