Pentose phosphate cycle functions

The biological function of the pentose phosphate cycle involves the production of two compounds NADP H2, which is a reductive force in the synthesis of various materials, and the metabolite ribose... 

In summary, current evidence [39-41] is thus consistent with the view that the ferredoxin/thioredoxin system functions in photosynthetically diverse types of plants as a master switch to restrict the activity of degradatory enzymes and activate biosynthetic enzymes in the light. It is significant that enzymes controlled by the ferredoxin/thioredoxin system (FBPase, SBPase, NADP-G3PDH, and PRK) function in the regenerative phase of the reductive pentose phosphate cycle that is needed to sustain its continued operation - i.e, to regenerate the carbon dioxide acceptor, Rbu-1,5-P2, from newly formed 3-PGA. It seems likely that one of these thioredoxin-linked enzymes limits the regeneration of Rbu-1,5-P2. 

The Calvin cycle or reductive pentose phosphate cycle occurs in all green plants and many microorganisms. Carboxylation is catalyzed by ribulose-bis-phosphate carboxylase-oxygenase (Rubisco). Rubisco also functions as an oxygenase during photorespiration, but its affinity for O2 is 20-80 times lower than for CO2. As with most enzymes, Rubisco has a preference for lighter stable isotopes and CO2 fixation results in the depletion of C (5 C) ranging from —10%o to —2Q%c. 

This pathway is variously known as the pentose phosphate, hexose monophosphate or phosphogluconate pathway, cycle or shunt. Although the pentose phosphate pathway achieves oxidation of glucose, this is not its function, as indicated by the distribution of the pathway in different tissues. Only one of the carbons is released as CO2, the key products are NADPH and ribose 5-phosphate, both of which are important for nucleotide phosphate formation and hence for synthesis of nucleic acids (Chapter 20). The... 

In these tissues the cycle may operate as indicated in Fig. 17-8A with the C3 product also being used in biosynthesis. Furthermore, any of the products from C4 to C7 may be withdrawn in any desired amounts without disrupting the smooth operation of the cycle. For example, the C4 intermediate erythrose 4-P is required in synthesis of aromatic amino acids by bacteria and plants (but not in animals). Ribose 5-P is needed for formation of several amino acids and of nucleic acids by all organisms. In some circumstances the formation of ribose 5-P may be the only essential function for the pentose phosphate pathway.120... 

Niacin is a water-soluble vitamin. The RDA of niacin for the adult man is 19 mg. Niacin is converted in the bi>dy to the cofactor nicotinamide adenine dinucleotide (NAD). NAD also exists in a phosphorylated form, NADP The phosphate group occurs on the 2-hydrr>xyl group of the AMP half of the coenzyme, NAD and NADP are used in the catalysis of oxidation and reduction reactions. These reactions are called redox reactions. NAD cycles between the oxidized form, NAD, and the reduced form, NADH + H. The coenzyme functions to accept and donate electrons. NADP behaves in a similar fashion. It occurs as NADP and NADPH + HT The utilization of NAD is illustrated in the sections on glycolysis, the malatc-aspartate shuttle, ketone body metabolism, and fatty acid oxidation. The utilization of NADP is illustrated in the sectirrns concerning fatty acid synthesis and the pentose phosphate pathway. 

The pathway borrows heavily from the nonoxidative branch of the pentose phosphate pathway and from gluconeogenesis. Without doubt, the pathways yield sugars as well as NADPH for reductive biosynthesis. Thus, only a few new enzymes would have to evolve through mutations to enable the complete Calvin cycle to function. 

Explain the functions of the Calvin cycle and the pentose phosphate pathway. 

Respiration is the reduction of O2 to H2O during the oxidation of carbohydrate to CO2. There are two types of respiration in photosynthetic organisms a dark respiration and a photorespiration [3]. Dark respiration includes O2 reduction and the oxidation of NADH and FADH2 in mitochondrial membranes, glycolysis, the Krebs cycle, and the oxidative pentose phosphate pathway. Respiration is commonly subdivided into two functional components growth respiration, supplying energy for new biomass production, and... 

Nicotinamide functions in the animal body as the active group of two important coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP).These coenzymes are involved in the mechanism of hydrogen transfer in living cells (see Chapter 9) NAD is involved in the oxidative phosphorylation system, the tricyclic acid (TCA) cycle and the metabolism of many molecules, including pyruvate, acetate, (3-hydroxy-butyrate, glycerol, fatty acids and glutamate NADPH is the hydrogen acceptor in the pentose phosphate pathway. 

Glyceraldehyde 3-phosphate, it will be recalled (Scheme 11.13), is a progenitor of glucose-6-phosphate, which, via the pentose phosphate pathway, is also (i.e., in addition to the Calvin cycle) on the path to ribulose-5-phosphate. Now, via the common enol, ribulose-5-phosphate is readily converted to ribose-5-phosphate (EC 5.3.1.6), which is then bisphosphorylated (EC 2.7.6.1) to the a-diphosphate so that a leaving group is in place that will allow replacement by the terminal amido function of glutamine (Gin, Q) on its way to glutamate (Glu, E) with the formation of 2-aminoribose-5-phosphate. 

Thiamine was the first vitamin to have its precise biochemical functions determined. In the form of its pyrophosphate, thiamine participates in several very important enzyme systems namely (1) pyruvate dehydrogenase (page 232) which converts pyruvate to acetyl-CoA and carbon dioxide in the course of carbohydrate breakdown (2) the reaction of the citrate cycle in which oxoglutarate is oxidatively decarboxylated to succinyl-CoA (page 242) (3) the transketolase reaction of the pentose phosphate pathway of glucose breakdown (page 233). 

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