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Questions and Answers
What type of units does transaldolase transfer from a ketose to an aldose?
The oxidative phase of the pentose phosphate pathway is highly active in which type of cells?
What is the result of the nonoxidative phase of the pentose phosphate pathway?
What regulates the activity of G-6-PD in the pentose phosphate pathway?
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Diets high in what substance are correlated with an increase in the synthesis of G-6-PD?
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Which product is primarily generated from the pentose phosphate pathway along with ribose-5-phosphate?
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Where does the pentose phosphate pathway primarily take place in the cell?
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What is the primary enzyme involved in the first reaction of the oxidative phase of the pentose phosphate pathway?
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During the oxidative decarboxylation, what compound is produced alongside the second molecule of NADPH?
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How many NADPH molecules are produced during the oxidative phase when glucose-6-phosphate is converted to ribulose-5-phosphate?
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What type of reactions govern the oxidative phase of the pentose phosphate pathway?
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Which component is a result of the hydrolysis of 6-phosphogluconolactone?
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What cellular processes is the oxidative phase of the pentose phosphate pathway most active in supporting?
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Study Notes
Transketolase and Transaldolase
- Transketolase is an enzyme requiring TPP (thiamine pyrophosphate) that transfers two-carbon units from ketose to aldose.
- It catalyzes a reaction where a two-carbon unit is transferred from xylulose-5-phosphate to ribose-5-phosphate.
- Transaldolase transfers three-carbon units from a ketose to an aldose.
Nonoxidative Phase of the Pathway
- The nonoxidative phase results in the synthesis of ribose-5-phosphate along with glycolytic intermediates glyceraldehyde-3-phosphate and fructose-6-phosphate.
- This phase is crucial for producing ribose-5-phosphate, a key building block for nucleotide synthesis.
Pentose Phosphate Pathway Overview
- Also known as the hexose monophosphate shunt, the pentose phosphate pathway is an alternative glucose oxidation pathway that does not generate ATP.
- Prominent products include NADPH and ribose-5-phosphate.
- Occurs in the cytoplasm and comprises two distinct phases: oxidative and nonoxidative.
Importance in Plants and Cells
- In plants, the pathway synthesizes glucose during the dark reactions of photosynthesis.
- The oxidative phase is highly active in red blood cells due to their high need for NADPH.
- G-6-PD regulates the pentose phosphate pathway, with activity inhibited by NADPH.
- Diets rich in carbohydrates enhance G-6-PD and phosphogluconate dehydrogenase synthesis.
Oxidative Phase Details
- The oxidative phase consists of three key reactions, converting glucose-6-phosphate to ribulose-5-phosphate and producing two molecules of NADPH.
- Glucose-6-phosphate is oxidized by glucose-6-phosphate dehydrogenase (G-6PD).
- First reaction products include 6-phosphogluconolactone and NADPH; 6-phosphogluconolactone is hydrolyzed to produce 6-phospho-D-gluconate.
- A second NADPH is produced during the oxidative decarboxylation of 6-phosphogluconate.
- This phase primarily supports lipid synthesis and helps protect against oxidative damage due to high levels of NADPH, which act as powerful antioxidants.
Non-Oxidative Phase Conversions
- Begins with the conversion of ribulose-5-phosphate to ribose-5-phosphate through isomerization catalyzed by ribulose-5-phosphate isomerase.
- Useful intermediates produced in this phase include ribose-5-phosphate, fructose-6-phosphate, and glyceraldehyde-3-phosphate.
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Description
This quiz focuses on the enzyme transketolase and its role in the transfer of carbon units between ketoses and aldoses. It covers key reactions including those catalyzed by transaldolase and the synthesis of important metabolites in the nonoxidative phase of the pentose phosphate pathway.
