Pentose phosphate pathway ribose produced

The pentose phosphate pathway would produce the necessary NADPH, and the ribose 5-phosphate would be converted into the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate. 

Ribose-5-phosphate is a pentose phosphate pathway intermediate produced by phosphopentose isomerase-catalyzed rearrangement of ribulose-5-phosphate. -i-j ... 

D-Ribulose, which has the following structure, is an intermediate in the pentose phosphate pathway that produces ribose, a precursor for nucleic acid biosynthesis. Classify D-ribulose by chain length and its carbonyl group. 

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... 

BOTH RIBOSE-5-P AND NADPH ARE NEEDED BY THE CELL In this case, the first four reactions of the pentose phosphate pathway predominate (Figure 23.37). N/VDPH is produced by the oxidative reactions of the pathway, and ribose-5-P is the principal product of carbon metabolism. As stated earlier, the net reaction for these processes is... 

MORE NADPH THAN RmOSE-5-P IS NEEDED BY THE CELL Large amounts of N/VDPH can be supplied for biosynthesis without concomitant production of ribose-5-P, if ribose-5-P produced in the pentose phosphate pathway is recycled to produce glycolytic intermediates. As shown in Figure 23.39, this alternative involves a complex interplay between the transketolase and transaldolase reac-... 

The requirement for NADPH far exceeds an equal requirement for ribose 5-phosphate (necessary for the production of nucleic acids and nucleotides), and so the second phase of the pentose phosphate pathway converts the C5 sugar, by a series of reversible reactions, into the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate. This interconversion is shown in Fig. 11-27. Not only does the second phase of the pathway conserve all the carbon atoms of the C5 sugar, but it produces erythrose 4-phosphate (C4), xylulose 5-phosphate (C5), and sedoheptulose 7-phosphate (C7), which are available to other metabolic processes. 

Thus, the equivalent of glucose 6-phosphate can be completely oxidized to CO 2 with the concomitant generation of NADPH. In essence, ribose 5-phosphate produced by the pentose phosphate pathway is recycled into glucose 6-phosphate by transketolase, transaldolase, and some of the enzymes of the gluconeogenic pathway. 

While the main function of glycolysis is to produce ATP, there are minor catabolic pathways that produce specialized products for cells. One, the pentose phosphate pathway, produces NADPH and the sugar ribose 5-phosphate. NADPH is used to reduce substrates in the synthesis of fatty acids, and ribose 5-phosphate is used in the synthesis of nucleic acids. 

E. In the first three reactions of the pentose phosphate pathway, glucose is converted to ribulose 5-phosphate and C02, with the production of NADPH. These reactions are not reversible. Ribose 5-phosphate and xylulose 5-phosphate may be formed from ribulose 5-phos-phate. A series of reactions catalyzed by transketolase and transaldolase produce the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate. 

TPP is also the coenzyme in the transketo-lase reaction (Fig. 8.27) found in the pentose phosphate pathway that interconverts hex-oses, pentoses, tetroses, and trioses. This reaction removes carbons 1 and 2 of a ketose and transfers them to an acceptor aldose. Examples include TPP transferring carbons 1 and 2 of xylulose-5-P to ribose-5-P, producing glyc-eraldehyde-3-P (5 carbons minus 2 carbons) and sedoheptulose-7-P (5 carbons plus 2 carbons). This reaction is reversible. A second reversible reaction has TPP transferring carbons 1 and 2 of xylulose-5-P to erythrose-4-P, producing fructose-6-P (4 carbons plus 2 carbons) and glyceraldehyde-3-P (5 carbons minus 2 carbons). 

Glycolysis. Ribose 5-phosphate can also be produced from intermediates of glycolysis (Figure 27-6). The enzymes involved are those of the nonoxidative phase of the pentose phosphate pathway, that occur in many tissues. 

Thiamine pyrophosphate is also an important cofactor for the transketolase reactions in the pentose phosphate pathway of carbohydrate metabolism (Fignre 15-3). These reactions are important in the reversible transformation of pentoses into the glycolytic intermediates fructose 6-phosphate and glyc-eraldehyde 3-phosphate. Again, it is the reactive carbon on the thiazole ring of TPP that reacts with a ketose phosphate (xylnlose 5-phosphate) to canse the release of an aldose phosphate with two fewer carbons (glyceraldehyde 3-phosphate). The TPP-bonnd glycoaldehyde unit is then transferred to a different aldose phosphate (ribose 5-phosphate or erythrose 4-phosphate) to produce a ketose phosphate that has two carbons more (sedoheptulose 7-phosphate or fructose 6-phosphate). 

The pentose phosphate pathway produces NADPH, ribose-5-phosphate, and several glycolytic intermediates. 

The pentose phosphate pathway, in which glucose-6-phos-phate is oxidized, occurs in two phases. In the oxidative phase, two molecules of NADPH are produced as glucose-6-phosphate is converted to ribulose-5-phosphate. In the nonoxidative phase, ribose-5-phosphate and other sugars are synthesized. If cells need more NADPH than ribose-5-phos-phate, a component of nucleotides and the nucleic acids, then metabolites of the nonoxidative phase are converted into glycolytic intermediates. 

TK is one of the enzymes involved in the oxidative pentose phosphate pathway, and requires the cofactors thiamine pyrophosphate (TPP)12191 and Mg2+[218). It reversibly transfers the C1-C2 ketol unit from D-xylulose 5-phosphate to D-ribose 5-phosphate, and generates D-sedoheptulose 7-phosphate and D-Gly 3-P. D-Erythrose 4-phosphate also functions as an acceptor of the ketol unit from D-xylulose 5-phosphate, to produce Fru 6-P and D-Gly 3-P (Fig. 14.2-1). TK from baker s yeast is commercially available, and the enzyme can also be isolated from spinach[220, 2211 TK from E. coli has been overexpressed and prepared on a large scale12221. In ketol transfer reactions,... 

The pentose-phosphate pathway. The early steps of the pentose-phosphate pathway are shown in detail as for a pentose-phosphate formation. This pathway produces two NADPHs and ribose-5-phosphate. The reformation of hexose phosphate involves C3,... 

In addition to providing reducing power (NADPH), the pentose phosphate pathway provides sugar phosphates that are required for biosynthesis. For instance, ribose-5-phosphate is used for the s5mthesis of nucleotides such as ATP. The four-carbon sugar phosphate, erythrose-4-phosphate, produced in the third stage of the pentose phosphate pathway is a precursor of the amino acids phenylalanine, tyrosine, and tryptophan. 

Sedoheptulose-7-phosphate is an intermediate in the pentose phosphate pathway produced by transketolase-catalyzed rearrangements of xylulose-5-phosphate and ribose-5-phosphate to form glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate. 

Of what value are the ribose-5-phosphate and erythrose-4-phosphate that are produced in the pentose phosphate pathway ... 

As purines are built on a ribose base (see Fig. 41.2), an activated form of ribose is used to initiate the purine biosynthetic pathway. 5-Phosphoribosyl-l-pyrophosphate (PRPP) is the activated source of the ribose moiety. It is synthesized from ATP and ribose 5 -phosphate (Fig. 41.3), which is produced from glucose through the pentose phosphate pathway (see Chapter 29). The enzyme that catalyzes this reaction, PRPP synthetase, is a regulated enzyme (see section 1I.A.5) however, this step is not the committed step of purine biosynthesis. PRPP has many other uses, which are described as the chapter progresses. 

What are the oxidative reactions of the pentose phosphate pathway The pentose phosphate pathway is an alternative pathway for glucose metabolism. In this pathway five-carbon sugars, including ribose, are produced from glucose. In the oxidative reactions of the pathway, NADPH is also produced. 

What are the nonoxidative reactions of the pentose phosphate pathway, and why are they important The nonoxidative reactions of the pentose phosphate pathway produce five-carbon sugars, particularly ribose. They are important when an organism has less need for NADPH but needs the sugars. 

The authors next turn their attention to the pentose phosphate pathway, which is common to all organisms. The role of the pentose phosphate pathway is to produce NADPH, which is the currency of reducing power utilized for most reductive biosyntheses. In addition, this pathway generates ribose 5-phosphate needed for DNA synthesis and can produce various three-, four-, five-, six-, and seven-carbon sugars. 

The major pathway of catabolism of monosaccharides, such as glucose and fructose, is glycolysis. Monosaccharides are converted into pyruvate with the generation of ATP [6]. Pyruvate is an intermediate in several metabolic pathways, but the majority of it is converted to acetyl coenzyme A, which enters the citric acid cycle. Although more ATP is generated in the citric acid cycle, the most important product is NADH, which is derived from NAD+ as the acetyl coenzyme A is oxidized. This oxidation releases carbon dioxide as a waste product. An alternative route for glucose catabolism is the pentose phosphate pathway, in which pentose sugars such as ribose is produced. 

---