Pentose Phosphate Pathway

• The pentose phosphate pathway (PPP) is also known as the hexose monophosphate pathway
• Reading assignment: Chapter Section 14.6, which covers the Pentose Phosphate Pathway of Glucose Oxidation

Learning Objectives of PPP

• The two phases of the pentose phosphate pathway (PPP) should be described
• Describe the enzymes and key compounds involved in the pathway
• Discuss the functions of the PPP in cellular metabolism
• Describe NADPH regulation of partitioning into glycolysis
• List the uses of NADPH, including its antioxidant mechanisms
• Understand the biochemical basis of G6PD deficiencies and its connection to hemolytic anemias

Two Phases of the Pentose Phosphate Pathway

• The oxidative phase generates NADPH
• NADPH forms at two steps during the oxidative phase, with decarboxylation occurring in the latter reaction
• Ribulose 5-phosphate may convert to ribose 5-phosphate which is used for synthesizing nucleotides and coenzymes for DNA and RNA synthesis
• Ribulose 5-phosphate converts back to glucose 6-phosphate
• NADPH can be used in reductive reactions with glutathione reductase
• NADPH also assists reductive biosynthetic reactions for fatty acids and steroids

Pentose Phosphate Pathway Main aspects

• The primary products are NADPH and ribose 5-phosphate
• NADPH serves as an electron donor in reductive biosynthesis of fatty acids and steroids and repairs oxidative damage
• In red blood cells, ribulose 5-phosphate is converted back to glucose 6-phosphate to generate more NADPH
• NADPH is vital for protecting against and repairing oxidative damage to the red blood cell membrane and hemoglobin
• Ribose 5-phosphate is used in nucleotide synthesis and coenzymes

NADPH and Pentose Generation

• Glucose 6-phosphate dehydrogenase converts glucose 6-phosphate to 6-phospho-glucono-δ-lactone, producing NADPH
• Lactonase converts 6-phospho-glucono-δ-lactone to 6-phosphogluconate
• 6-phosphogluconate dehydrogenase decarboxylates 6-phosphogluconate, producing D-ribulose 5-phosphate and another NADPH
• Phosphopentose isomerase converts D-ribulose 5-phosphate to D-ribose 5-phosphate
• D-ribose 5-phosphate is essential for RNA, DNA, and other crucial compounds
• Overall, the oxidative phase produces two NADPH molecules, one carbon dioxide molecule, and one D-ribose 5-phosphate molecule

D-Ribulose 5-Phosphate Conversion

• D-ribulose 5-phosphate can be converted to D-ribose 5-phosphate for DNA and RNA synthesis
• D-ribulose 5-phosphate can be converted to xylulose 5-phosphate via ribose 5-phosphate epimerase
• Xylulose 5-phosphate is involved in nonoxidative phase of PPP called the pentose phosphate shunt

Nonoxidative Phase of the Pentose Phosphate Pathway

• The nonoxidative phase regenerates glucose 6-phosphate (G-6-P) from ribose 5-phosphate (R-5-P)
• Present in tissues requiring more NADPH than R-5-P such as red blood cells, liver, and adipose tissue
• G-6-P can then be utilized in glycolysis
• R-5-P converts to xylulose 5-phosphate via the action of isomerase and epimerase
• Transketolase converts R-5-P and xylulose 5-phosphate to sedoheptulose 7-phosphate (seven-carbon sugar) and glyceraldehyde 3-phosphate (three carbons).
• Throughout the process, the total number of carbons remains constant
• Transaldolase converts sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate to fructose 6-phosphate and erythrose 4-phosphate
• There is maintenance of the total of ten carbons, and generates two glyceraldehyde 3-phosphate molecules
• Fructose 6-phosphate converts to G-6-P by phosphohexose isomerase (PHI)
• Erythrose 4-phosphate reacts with xylulose 5-phosphate via transketolase to produce fructose 6-phosphate and glyceraldehyde 3-phosphate, and then fructose 6-phosphate can be converted to G-6-P by PHI
• Two iterations of this pathway produces two glyceraldehyde 3-phosphate molecules that are converted to fructose 6-phosphate and G-6-P
• The combination of oxidative and nonoxidative phases allows recovery of G-6-P if R-5-P is not needed
• Complete conversion of ribose-5-phosphate to glucose-6-phosphate requires 6 R5P to make 5 G6P
• There is no ATP generated in the nonoxidative phase

Transketolase and Transaldolase Reactions

• Transketolase utilizes thiamine pyrophosphate (TPP) as a coenzyme, which carries a two-carbon moiety
• Facilitates the transfer of the moiety from a ketose donor to an aldose acceptor
• Generating a new aldehyde sugar moiety and a new ketone sugar moiety
• Xylulose 5-phosphate (ketose) is converted to glyceraldehyde 3-phosphate(aldose)
• Ribose 5-phosphate (aldose) is converted to sedoheptulose 7-phosphate.(ketose).
• Transaldolase removes a three-carbon fragment from sedoheptulose 7-phosphate to generate erythrose 4-phosphate
• Transaldolase conjugates the fragment to glyceraldehyde 3-phosphate to form fructose 6-phosphate.

NADPH Regulation

• NADPH and NADP+ regulate the partitioning between glycolysis and the pentose phosphate pathway
• High NADP+ levels stimulate glucose 6-phosphate dehydrogenase activity (first enzyme in the PPP)
• Elevated NADPH inhibits glucose 6-phosphate dehydrogenase activity

Roles of NADPH

• Aids in hydrogen peroxide reduction via glutathione reductase
• Serves in reductive biosynthetic reactions
• Provides reducing equivalents for monooxygenase-catalyzed reactions
• Assists in phagocytosis by white blood cells
• Involved in the synthesis of nitric oxide (NO)
• Regulates partitioning of the glycolytic and PPP pathways

G-6-P Dehydrogenase Deficiency

• G6PD deficiency can be fatal in cases of high oxidative stress, triggered by certain drugs, herbicides, and foods
• Drugs: antibiotics and antimalarials
• Food: fava beans
• G6PD deficiency can lead to resistance to malaria, due to increased oxidative stress that is less hospitable for malarial parasites
• It is an X-linked recessive disorder
• Males exhibit the condition if their single X chromosome has the disease variant
• Females need both copies of their G6PD genes to have the variants

Glucose-6-Phosphate Dehydrogenase (G6PD) Pathway

• Offers illustration of the protective effects, mitochondrial respiration, ionizing radiation, sulfa drugs, antimalarials, and divicine, all can lead to high oxidative stress in cells
• Divicine generates oxidative stress and can be found in fava beans
• Fava beans can lead to favism, where the oxidative stress damages their red blood cells, causing them to lyse(break open)
• Not all individuals with G6PD deficiency will develop favism because all individuals with favism have G6PD deficiency
• Molecular oxygen can form a superoxide radical which can be reduced to generate hydrogen peroxide
• Hydrogen peroxide is converted to water with reduced glutathione and glutathione peroxidase, thus detoxifying hydrogen peroxide
• The process depletes NADPH and accumulates NADP+.
• Replenishing NADPH decreases oxidative stress in the cell
• Hydroxyl free radicals can cause oxidative stress
• Presence of hydroxyl free radicals causes damage to lipids, proteins and DNA
• In red blood cells, hydroxyl free radicals damage hemoglobin, causing it to form strands that adhere to the membrane, thus causing lysing
• The lysing of red blood cells is known as hemolysis
• Hemolysis results in anemia