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Chapter 20 Biochemistry6e

Chapter 20 Biochemistry6e - Chapter 20 Biochemistry(The...

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Chapter 20 Biochemistry (The Calvin Cycle and) the Pentose Phosphate Pathway by Berg, Tymoczko and Stryer [Note-Due to time limitations, only the Pentose Phosphate Part of this chapter, and that in part, will be covered.] Generation of NADPH and the Synthesis of 5-C Sugars by the Pentose Phosphate Pathway The Pentose Phosphate Pathway (also known as the hexose monophosphate shunt, the phosphogluconate pathway, the pentose shunt) is present in all organisms. NADPH generation from glucose is the hallmark of the pathway. The NADPH so generated can be used in the biosynthesis of fats and other high-energy reductant-requiring pathways (Table 20.2) . The pentose phosphate pathway may also be used to catabolize pentose sugars, to either metabolize or generate 4-7 carbon sugars and to generate pentose sugars to be consumed in nucleotide biosynthesis. The pentose phosphate pathway is a cytosolic process which consists of two phases: the oxidative generation of NADPH and the interconversion of sugars, which is not an oxidative process ( Figure 20.19 ). In the oxidative phase, NADPH is made when glucose-6-phosphate is oxidized to ribulose-5- phosphate and CO 2 . The reaction is: Glucose-6-P + 2 NADP + + H 2 O Ribose-5-P + 2 NADPH + 2 H + + CO 2 The non-oxidative phase results in the interconversion of the 3-7 carbon sugars such that pentoses for nucleotide (RNA, DNA, FAD, NAD + , Coenzyme A) synthesis may occur and the generation of glycolytic intermediates may also be accomplished. All of these reactions occur in the cytoplasm. Glucose-6-phosphate dehydrogenase begins the oxidative phase of the pentose phosphate pathway. The enzyme is highly specific for NADP + , with its Km being so 1000 times lower for NADP + than for NAD + . The product of this reaction is 6-phosphoglucono-δ-lactone, an intramolecular ester between the C-1 carboxyl and C-5 hydroxyl groups. 6-phosphoglucono-δ-lactone is hydrolyzed by 1
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lactonase to yield 6-phosphogluconate. It is the 6-phosphogluconate that is oxidatively decarboxylated to yield ribulose-5-P with NADP + being reduced to NADPH + H + being made as well ( Figures 20.20, 20.11 ). Ribulose-5-P can be converted to ribose-5-P via the enzyme phosphopentose isomerase, and hence ribose, for use in DNA and RNA synthesis, can be generated. The pentose phosphate pathway yields 2 NADPH and one ribulose-5-P for every glucose-6-P metabolized. The problem that arises is that the cell often needs NADPH far more than it requires ribulose-5-P or its isomer, ribose-5-P. The carbons of the ribulose-5-P are “salvaged” by the enzymes transketolase and transaldolase which function to convert the C 5 molecules into a variety of C
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Chapter 20 Biochemistry6e - Chapter 20 Biochemistry(The...

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