Reaction Ribulose

The dark reactions of photosynthesis are actually stimulated by the light reactions. The central enzyme in the dark reactions, ribulose-l,5-bisphosphate carboxylase, is stimulated by high pH and C02 and... 

A transketolase has been isolated from plant tissues and from animal tissues, it catalyses the following reaction ribulose-5-P + ribose-5-P sedoheptulose -7-P + glyceraldehyde-3-P... 

As indicated, ribulose bisphosphate carboxylase/oxygenase catalyzes an alternative reaction in which Og replaces COg as the substrate added to RuBP (Figure 22.29a). The ribulose-l,5-bisphosphate oxygenase rezLCtion diminishes plant... 

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. 

This enzyme interconverts ribulose-5-P and ribose-5-P via an enediol intermediate (Figure 23.30). The reaction (and mechanism) is quite similar to the phosphoglucoisomerase reaction of glycolysis, which interconverts glucose-6-P and fructose-6-P. The ribose-5-P produced in this reaction is utilized in the biosynthesis of coenzymes (including N/ DH, N/ DPH, F/ D, and Big), nucleotides, and nucleic acids (DNA and RNA). The net reaction for the first four steps of the pentose phosphate pathway is... 

This reaction converts ribulose-5-P to another ketose, namely, xylulose-5-P. This reaction also proceeds by an enediol intermediate, but involves an inversion at C-3 (Figure 23.31). In the reaction, an acidic proton located a- to a carbonyl carbon is removed to generate the enediolate, but the proton is added back to the same carbon from the opposite side. Note the distinction in nomenclature here. Interchange of groups on a single carbon is an epimerization, and interchange of groups between carbons is referred to as an isomerization. 

FIGURE 23.31 The phosphopentose epimerase reaction interconverts ribulose-5-P and xylulose-5-phosphate. The mechanism involves an enediol intermediate and occurs with inversion at C-3. 

Dehydrogenation of 6-phosphogluconate to ribulose 5-phosphate. This reaction is catalyzed by 6-phosphogluconate dehydrogenase according to the scheme ... 

Ribulose 5-phosphate is capable of a reversible isomerization to other pentose phosphates-xylulose 5-phosphate and ribose 5-phosphate. These reactions are catalyzed by two respective enzymes, viz., pentose-phosphate epimerase and pentose-phosphate isomerase, according to the scheme below ... 

Two other pentose phosphates (ribose 5-phosphate and xylulose 5-phosphate), which are derived from ribulose 5-phosphate, are important for the subsequent reaction of the cycle. Two molecules of... 

Tapia, O., Andres, J. and Safont, V. S. Enzyme catalysis and transition structures in vacuo. Transition structures for the enolization, carboxylation and oxygenation reactions in ribulose- 1,5-bisphosphate carboxylase/oxygenase enzyme (Rubisco), J.Chem.Soc.Faraday Trans., 90 (1994), 2365-2374... 

Binding of C02 takes place in aqueous medium by the carboxylation reaction of ribulose-diphosphate (RuDP) with the formation of 3-phospho-glycerine acid (PGA) - table 5. Water molecule and radical C=0 at the distances of molecular interaction have quite similar values of PE-parameters for forming the general structural grouping of dimeric composite type. Total PE-parameter of water molecule and radical C=0 hearly equals PE-parameter of C02 and therefore the molecules of C02 and H20 join RuBP with the formation of two radicals COOH b PGA (table 5). In ferment RuDP- carboxylase, Mg atoms and 0" ions (5.4867 eV and 4.755 eV) play an active role, their PE-parameters similar to PE-parameter of radical COOH. 

FUNCTION RUBISCO CATALYSES TWO REACTIONS THE CARBOXYLATION OF D-RIBULOSE 1,5-BISPHOSPHATE, THE PRIMARY EVENT IN PHOTOSYNTHETIC CARBON DIOXIDE FIXATION, AS WELL AS THE OXIDATIVE FRAGMENTATION OF THE PENTOSE SUBSTRATE IN THE PHOTORESPIRATION PROCESS. BOTH REACTIONS OCCUR SIMULTANEOUSLY AND IN COMPETITION AT THE SAME ACTIVE SITE. 

C3 photosynthetic pathway Also known as the Calvin cycle. A series of enzymatically mediated photosynthetic reactions during which C02 is reduced to 3-phosphoglyceraldehyde and the C02 acceptor, ribulose 1,5-bisphosphosphate, is regenerated. 

The second part consists of a series of reactions in which ribulose 5-phosphate is reconverted back to glucose 6-phosphate (i.e. 5C 6C ) (Figure 6.20) (Appendix 6.8). Some key processes that depend on NADPH, and therefore the pentose phosphate pathway, are identified in Table 6.3 and presented in Figure 6.21. 

In this reaction, one molecule of ribulose-1,5-bisphosphate (metabolite 1) and one molecule of CO2 (metabolite 2) give rise to two molecules of 3-phosphoglycerate (metabolite 3). The enzyme responsible has the EC number 4.1.1.39. The annotated enzyme list shows that this refers to ribulose bisphosphate carboxylase ( rubisco for short). Rubisco belongs to enzyme class 4 (the lyases) and, within that group, to subclass 4.1 (the car-boxy-lyases). It contains copper as a cofactor ([Cu]). 

This enzyme [EC 1.1.1.137] catalyzes the reaction of d-ribitol 5-phosphate with NAD(P)+ to produce D-ribulose 5-phosphate and NAD(P)H. 

Probably the most abundant naturally occurring catalyst, this enzyme [EC 4.1.1.39], also known as rubisco , catalyzes the reaction of D-ribulose 1,5-bisphosphate with carbon dioxide to produce two 3-phospho-D-glycerate. The enzyme can also use dioxygen as a substrate instead of carbon dioxide, producing 3-phospho-D-glycerate and 2-phosphoglycolate. 

The light independent reactions take place in the stroma with the help of ATP and NADPH. In a process called the Calvin-Benson cycle, or carbon fixation, carbon dioxide from the atmosphere is captured and converted into carbohydrates [135]. The reaction is catalyzed by the enzyme RuBisCO (ribulose-1,5-biphosphate... 

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