Ribulose synthesis

Write a balanced equation for the synthesis of a glucose molecule from ribulose-l,5-bisphosphate and COg that involves the first three reactions of the Calvin cycle and subsequent conversion of the two glyceraldehyde-3-P molecules into glucose. 

Vierling, E. Key, J.L. (1985). Ribulose 1,5-bisphosphate carboxylase synthesis during heat shock. Plant Physiology, 78,155-62. 

Ribulose 5-phosphate is the substrate for two enzymes. Ribulose 5-phosphate 3-epimerase alters the configuration about carbon 3, forming another ketopentose, xylulose 5-phosphate. Ribose 5-phosphate ketoisom-erase converts ribulose 5-phosphate to the corresponding aldopentose, ribose 5-phosphate, which is the precursor of the ribose required for nucleotide and nucleic acid synthesis. Transketolase transfers the two-carbon... 

The last enzyme in the oxidative part is phosphogluconate dehydrogenase [3], which releases the carboxylate group of 6-phosphogluconate as CO2 and at the same time oxidizes the hydroxyl group at C3 to an 0x0 group. In addition to a second NADPH+H", this also produces the ketopentose ribulose 5-phosphate. This is converted by an isomer-ase to ribose 5-phosphate, the initial compound for nucleotide synthesis (top). 

The oxidative portion of the pentose phosphate pathway consists of three reactions that lead to the formation of ribulose 5-phosphate, C02, and two molecules of NADPH for each molecule of glucose 6-phosphate oxidized (Figure 13.2). This portion of the pathway is particularly important in the liver and lactating mammary glands, which are active in the biosynthesis of fatty acids, in the adrenal cortex, which is active in the NADPH-dependent synthesis of steroids, and in erythrocytes, which require NADPH to keep glutathione reduced. 

The six-carbon chain of ManNAc 6-P can be extended by three carbon atoms using an aldol-type condensation with a three-carbon fragment from PEP (Eq. 20-7, step c) to give N-acetylneuraminic acid (sialic acid).48 Tire nine-carbon chain of this molecule can cyclize to form a pair of anomers with 6-membered rings as shown in Eq. 20-7. In a similar manner, arabi-nose 5-P is converted to the 8-carbon 3-deoxy-D-manno-octulosonic acid (KDO) (Fig. 4-15), a component of the lipopolysaccharide of gram-negative bacteria (Fig. 8-30), and D-Erythrose 4-P is converted to 3-deoxy-D-arafrmo-heptulosonate 7-P, the first metabolite in the shikimate pathway of aromatic synthesis (Fig. 25-1).48a The arabinose-P used for KDO synthesis is formed by isomerization of D-ribulose 5-P from the pentose phosphate pathway, and erythrose 4-P arises from the same pathway. 

Enzymes of KDO Synthesis and Metabolism and Their Inhibition. The KDO pathway can be thought of as a minor branched pathway in carbohydrate metabolism initiating with the key intermediate in the hexose-monophosphate shunt, D-ribulose-5-phosphate. As shown in Figure 2 the biosynthesis and utilization are known to involve at least five sequential reactions ... 

Figure 2. Expanded pathway for the synthesis and use of KDO. The pathway initiates with D-giucose-6-phosphate, shows the branch point whereby D-ribulose-5-phosphate can be used to synthesize either D-arabinose-5-phosphate or D-ribose-5-phosphate and terminates with the transfer of KDO to the lipid A precursor. The numbers in parenthesis below the various enzymes correspond to their specific activities (nmoles per minute per...

D-Arabinose-5-phosphate isomerase, the first key enzyme in the synthesis of KDO, catalyzes the interconversion of D-ribulose-5-phosphate and D-arabinose-5-phosphate. This enzyme was briefly studied by Volk (18) and later by Lim and Cohen (19). Due to the instability of the enzyme, we have only purified this enzyme 100-fold. The reversible reaction is readily monitored by measuring the formation of the keto-sugar from the aldo-sugar by the method of Dische and Borenfreund (20). The K values for D-ribulose-5-phojsphate and D-arabinose-5-phosphate are 0.9 to 1.5 and 1 to 3 x 10 M, respectively. 

In vivo measurements of lipopolysaccharide synthesis in E. coli B have indicated that two nanomoles of KDO must be synthesized per minute per mg of protein in order to meet the cellular requirement for LPS synthesis under the normal conditions of growth on glucose-minimal medium (27). We have measured the specific activities of the enzymes involved in KDO synthesis in crude extracts of E. coli B including those enzymes responsible for the synthesis of D-ribulose-5-phosphate, the precursor of D-arabinose-5-phosphate. D-Ribulose-5-phosphate is a key intermediate in carbohydrate metabolism as shown in Figure 2, since it is the direct precursor of both D-ribose-5-phosphate and D-arabinose-5-phosphate... 

The rubisco reaction forms part of a cycle of reactions, called the Calvin cycle, that leads to the regeneration of ribulose 1,5-bisphosphate (ready to fix another C02) and the net production of glyceraldehyde 3-phosphate for the synthesis... 

This requires the generation of six separate molecules of ribulose bisphosphate from ribulose phosphate, at the cost of one ATP each. Furthermore, two molecules of 1,3-bisphosphoglycerate must be made from the two 3-phosphoglycerates that are the initial product of each C02 fixation reaction. Conversion of each 1,3-bisphospho-glycerate requires an NADPH as well therefore, two NADPH equivalents are consumed for each C02 fixed. Another way of saying this is that carbon is reduced from an oxidation number of -4 in C02 to an oxidation number of zero in carbohydrate (CH20). Therefore, synthesis of one mole of glucose requires the input of 18 ATPs and 12 NADPHs. 

Answer The reductive pentose phosphate pathway regenerates ribulose 1,5-bisphosphate from triose phosphates produced during photosynthesis, in a series of reactions involving sugars of three, four, five, six, and seven carbons and the enzymes transaldolase and transketo-lase. The oxidative pentose phosphate pathway plays a different metabolic role it provides NADPH for reductive biosynthesis and pentose phosphates for nucleotide synthesis. 

When glucose is converted to ribulose 5-phosphate. NADPH is produced in the redox reactions of the oxidative part of the pentose phosphate pathway. For every molecule of glucose entering the pathway, two NADP+ molecules are reduced. Some of this NADPH is utilized in the synthesis of palmitic acid. To calculate the amount consider the following ... 

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