The Calvin–Benson Cycle and the Pentose Phosphate Pathway

Chapter 20: The Calvin–Benson Cycle and the Pentose Phosphate Pathway (The Pentose Phosphate Pathway, NADPH, and Glutathione)

Learning Goals

By the end of this chapter, you should be able to:

1. Identify the steps of the pentose phosphate pathway that regenerate NADPH.

2. Distinguish the antioxidant functions of GSH (Glutathione), the enzymes that contribute to GSH pools, the xenobiotic-inactivating functions of GSH, and some enzymes important for xenobiotic conjugation.

Outline

1. The Pentose Phosphate Pathway Generates NADPH and Synthesizes Pentoses

2. Glucose 6-Phosphate Dehydrogenase Plays a Key Role in Protection Against Reactive Oxygen Species

3. Extra Material: Glutathione

   • Free radical scavenger

   • Essential role in redox balance (sulfhydryl buffer)

   • Detoxification of xenobiotics by conjugation (Phase II metabolism)

Section 20.3: The Pentose Phosphate Pathway Generates NADPH and Synthesizes Pentoses

The pentose phosphate pathway is crucial for generating NADPH for use in reductive biosynthesis and for protection against oxidative stress. This pathway occurs in the cytoplasm of all organisms.

TABLE 20.2: Pathways Requiring NADPH

• Reductive biosynthesis:

  • Fatty acid biosynthesis

  • Cholesterol biosynthesis

  • Neurotransmitter biosynthesis

  • Nucleotide biosynthesis

• Protection from oxidative stress:

  • Reduction of oxidized glutathione

  • Cytochrome P450 monooxygenases

The Pentose Phosphate Pathway Consists of Two Phases

1. Oxidative phosphorylation:

   • Generates NADPH

   • Occurs when glucose 6-phosphate is oxidized to ribulose 5-phosphate through three enzyme reactions:
     $ext{Glucose 6-phosphate} + 2 ext{NADP}^+ + ext{H}<em>2 ext{O} ightarrow ext{ribulose 5-phosphate} + 2 ext{NADPH} + 2 ext{H}^+ + ext{CO}</em>2$

2. Nonoxidative interconversion:

   • Involves the interconversion of three-, four-, five-, six-, and seven-carbon sugars.

Overview of the Pentose Phosphate Pathway

• The pathway is essential for producing NADPH and interconverting phosphorylated sugars.

• Two molecules of NADPH are generated in the conversion of Glucose 6-Phosphate into Ribulose 5-Phosphate.

• The key enzymes involved:

  1. Glucose 6-Phosphate Dehydrogenase: Initiates the oxidative phase by converting glucose 6-phosphate into 6-phosphoglucono-δ-lactone, reducing NADP+ to NADPH.

  2. Lactonase: Hydrolyzes 6-phosphoglucono-δ-lactone to 6-phosphogluconate.

  3. 6-Phosphogluconate Dehydrogenase: Oxidatively decarboxylates 6-phosphogluconate to ribulose 5-phosphate, releasing CO2 and converting another NADP+ to NADPH.

Tissues with Active Pentose Phosphate Pathways

TABLE 20.4: Tissues Requiring NADPH

• Adrenal gland: Steroid synthesis

• Liver: Fatty acid and cholesterol synthesis

• Testes: Steroid synthesis

• Adipose tissue: Fatty acid synthesis

• Ovary: Steroid synthesis

• Mammary gland: Fatty acid synthesis

• Red blood cells: Maintenance of reduced glutathione

Glutathione (GSH)

• GSH plays a key role in maintaining redox balance through its high cellular concentration of reduced to oxidized ratios (500:1).

• Synthesis of Glutathione:

1. First step:

   • Involves the synthesis of γ-glutamylcysteine from glutamate and cysteine, requiring ATP:
     $ext{glutamate} + ext{cysteine} + ext{ATP} <br>ightarrow ext{γ-glutamylcysteine} + ext{ADP} + ext{Pi}$

2. Second step:

   • Involves adding glycine to γ-glutamylcysteine to form glutathione, also requiring ATP:
     $ext{γ-glutamylcysteine} + ext{glycine} + ext{ATP} <br>ightarrow ext{glutathione} + ext{ADP} + ext{Pi}$

   • This process is the rate-limiting step for glutathione synthesis.

Antioxidant Function of Glutathione

• Mechanism: Free radical scavenger, reacts with hydrogen peroxide (H2O2) and other peroxides through Glutathione peroxidase (GPx):
  $2 ext{GSH} + ext{ROOH} <br>ightarrow ext{GSSG} + ext{H}_2 ext{O} + ext{ROH}$
  (where ROOH = lipid peroxides)

• Glutathione Reductase (GR): Converts oxidized glutathione (GSSG) back to reduced glutathione (GSH), with NADPH as a coenzyme:
  $ext{GSSG} + ext{NADPH} + ext{H}^+ <br>ightarrow 2 ext{GSH} + ext{NADP}^+$

Maintenance of GSH Pools

• GSH pools within the cell are maintained by glucose 6-phosphate dehydrogenase (G-6-PD).

• The first three steps of the pentose pathway collectively regenerate 2 NADPH, beginning with the action of G-6-PD.

Genetic Factors Affecting G-6-PD Activity

• Polymorphisms leading to G-6-PD deficiencies are prevalent, especially among people of African ancestry.

• These deficiencies can confer some resistance to malaria, as the malaria parasite relies on NADPH for growth, making deficient cells more susceptible to oxidative stress.

• G-6-PD deficiency is often benign but can lead to hemolysis when exposed to substances causing oxidative stress (e.g., Vicine from fava beans).

Fava Beans and G-6-PD Deficiency

• Fava beans produce a pyrimidine glycoside called Vicine.

• Individuals with G-6-PD deficiency may experience hemolysis (destruction of red blood cells) after consuming fava beans.

Glutathione's Role in Hemoglobin Structure

• GSH acts as a sulfhydryl buffer, maintaining hemoglobin's reduced state.

• Inadequate GSH leads to the formation of aggregates, termed Heinz bodies, which damage cell membranes and contribute to red blood cell lysis.

Pharmacogenetic Considerations

• Primaquine: An antimalarial drug that generates reactive oxygen species (ROS), leading to hemolysis in G-6-PD deficient individuals, as RBCs lack alternative pathways for detoxification.

• The first known case of pharmacogenetics arose from studies showing that individuals with African ancestry had a higher risk of hemolytic anemia from primaquine treatment.

Clinical Implications of G-6-PD Deficiency

• Acetaminophen overdose can deplete GSH, causing oxidative liver damage.

• N-Acetylcysteine (NAC) serves as an antidote, restoring GSH pools and providing hepatoprotection if administered within 8 hours of ingestion.

• A potent carcinogen, benzo[a]pyrene (B[a]P), metabolizes to a reactive form which may induce mutations by forming DNA adducts during xenobiotic detoxification.

γ-Glutamyl Transferase (GGT)

• GGT is highly concentrated in the liver and serves as a biomarker for hepatobiliary disease.

• Elevated levels of GGT can arise from cholestasis, alcoholic liver disease, or as a reaction to medications such as phenytoin or phenobarbital.

Summary

• Understanding enzymes contributing to GSH pools, the antioxidant functions of GSH, and enzymatic roles in xenobiotic conjugation is vital for comprehending oxidative stress and cellular detoxification processes.

• Genetic background impacts responses to substances like primaquine and even dietary factors such as fava beans, illustrating the intersection of nutrition, pharmacology, and genetics in medical science.

Learning Goals

By the end of this chapter, you should be able to:

1. Identify the steps of the pentose phosphate pathway that regenerate NADPH.

2. Distinguish the antioxidant functions of GSH (Glutathione), the enzymes that contribute to GSH pools, the xenobiotic-inactivating functions of GSH, and some enzymes important for xenobiotic conjugation.

Outline

1. The Pentose Phosphate Pathway Generates NADPH and Synthesizes Pentoses

2. Glucose 6-Phosphate Dehydrogenase Plays a Key Role in Protection Against Reactive Oxygen Species

3. Extra Material: Glutathione- Free radical scavenger

   • Essential role in redox balance (sulfhydryl buffer)

   • Detoxification of xenobiotics by conjugation (Phase II metabolism)

Section 20.3: The Pentose Phosphate Pathway Generates NADPH and Synthesizes Pentoses

The pentose phosphate pathway (PPP), also known as the hexose monophosphate shunt, is a pivotal metabolic pathway that operates in the cytoplasm of nearly all organisms. Its primary functions are two-fold: generating NADPH for crucial reductive biosynthetic reactions and for protecting cells against oxidative stress, and synthesizing pentose sugars (like ribose 5-phosphate) which are essential precursors for nucleotide and nucleic acid biosynthesis. This pathway is particularly active in tissues with high demands for these products.

TABLE 20.2: Pathways Requiring NADPH

NADPH is a critical coenzyme in numerous anabolic and detoxification pathways, serving as the primary electron donor for reductive processes.

• Reductive biosynthesis: NADPH provides the reducing power for synthesizing complex molecules from simpler precursors.

  • Fatty acid biosynthesis: In adipose tissue, liver, and mammary glands.

  • Cholesterol biosynthesis: Primarily in the liver.

  • Neurotransmitter biosynthesis: Crucial for brain function.

  • Nucleotide biosynthesis: Required for the reduction of ribonucleotides to deoxyribonucleotides by ribonucleotide reductase.

• Protection from oxidative stress: NADPH is indispensable for maintaining the cellular redox balance.

  • Reduction of oxidized glutathione (GSSG): Glutathione reductase utilizes NADPH to regenerate reduced glutathione (GSH), a key antioxidant.

  • Cytochrome P450 monooxygenases: Involved in detoxification of various compounds, including drugs and environmental toxins, requiring NADPH.

The Pentose Phosphate Pathway Consists of Two Phases

The PPP is divided into two distinct but interconnected phases, allowing the cell to produce NADPH, pentoses, or both, depending on metabolic needs.

1. Oxidative Phase:

   • This irreversible phase is the primary source of NADPH production.

   • It begins with glucose 6-phosphate and involves three enzyme-catalyzed reactions that result in the oxidation of glucose 6-phosphate to ribulose 5-phosphate, generating two molecules of NADPH.

   • The overall reaction is:
     $\text{Glucose 6-phosphate} + 2 \text{NADP}^+ + \text{H}<em>2 \text{O} \longrightarrow \text{ribulose 5-phosphate} + 2 \text{NADPH} + 2 \text{H}^+ + \text{CO}</em>2$

   • Key enzymes involved in this phase are essential for the initiation of the pathway and robust NADPH supply.

2. Nonoxidative Phase:

   • This reversible phase allows for the interconversion of various phosphorylated sugars, including three-, four-, five-, six-, and seven-carbon sugars.

   • Its main roles are to:

     • Produce ribose 5-phosphate for nucleotide and nucleic acid (DNA, RNA) synthesis when NADPH is not immediately needed.

     • Convert excess pentose phosphates back into fructose 6-phosphate and glyceraldehyde 3-phosphate, which can re-enter glycolysis or gluconeogenesis.

   • This flexibility ensures that intermediates from the initial oxidative steps can be redirected according to the cell's demand for either NADPH or pentose sugars. Key enzymes here include transketolase (transferring $C<em>2$ units) and transaldolase (transferring $C</em>3$ units).

Overview of the Pentose Phosphate Pathway Enzymes

The pathway is intricately regulated to meet cellular demands.

• Two molecules of NADPH are specifically generated during the oxidative conversion of Glucose 6-Phosphate into Ribulose 5-Phosphate.

• The key enzymes involved in the oxidative phase are:

  1. Glucose 6-Phosphate Dehydrogenase (G-6-PD): This enzyme initiates the oxidative phase and is the rate-limiting step of the entire pentose phosphate pathway. It catalyzes the conversion of glucose 6-phosphate into 6-phosphoglucono-δ-lactone, reducing NADP+ to NADPH. This step is a control point for regulating the flux through the PPP.

  2. Lactonase: This enzyme rapidly hydrolyzes 6-phosphoglucono-δ-lactone to 6-phosphogluconate. Its rapid action prevents the accumulation of the reactive lactone intermediate.

  3. 6-Phosphogluconate Dehydrogenase: This enzyme catalyzes the oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate, releasing $CO_2$ and producing the second molecule of NADPH by converting another NADP+ to NADPH.

Tissues with Active Pentose Phosphate Pathways

Tissues with high metabolic demands for reductive biosynthesis or protection against oxidative stress exhibit elevated activity of the PPP.

TABLE 20.4: Tissues Requiring NADPH

• Adrenal gland: High NADPH demand for the synthesis of steroid hormones (e.g., cortisol, aldosterone) from cholesterol.

• Liver: Crucial for extensive fatty acid and cholesterol synthesis, as well as detoxification reactions catalyzed by cytochrome P450 enzymes.

• Testes: Requires NADPH for steroid hormone (e.g., testosterone) synthesis.

• Adipose tissue: Active in fatty acid synthesis for triglyceride storage.

• Ovary: Engaged in steroid hormone (e.g., estrogen, progesterone) synthesis.

• Mammary gland: Highly active during lactation for fatty acid synthesis, which is a major component of milk.

• Red blood cells (RBCs): Critically rely on NADPH to maintain reduced glutathione (GSH) levels, protecting against oxidative damage from oxygen transport. RBCs cannot regenerate NADPH through other pathways, making G-6-PD activity vital.

Glutathione (GSH)

Glutathione (GSH) is a tripeptide composed of glutamate, cysteine, and glycine. It is a highly abundant intracellular antioxidant and plays a central role in maintaining cellular redox homeostasis across all body cells. The remarkably high cellular concentration of reduced to oxidized glutathione (GSH:GSSG) ratios, typically around 500:1, is a critical indicator of cellular health and reducing power.

• Synthesis of Glutathione:
  GSH is synthesized intracellularly in two ATP-dependent enzymatic steps:

  1. First step: Catalyzed by γ-glutamylcysteine synthetase (or ligase), this step involves the synthesis of γ-glutamylcysteine from glutamate and cysteine. This reaction requires ATP for energy input:
     $\text{glutamate} + \text{cysteine} + \text{ATP} \longrightarrow \text{γ-glutamylcysteine} + \text{ADP} + \text{Pi}$

  2. Second step: Catalyzed by glutathione synthetase, this involves adding glycine to γ-glutamylcysteine to form glutathione. This step also requires ATP:
     $\text{γ-glutamylcysteine} + \text{glycine} + \text{ATP} \longrightarrow \text{glutathione} + \text{ADP} + \text{Pi}$
     This second reaction is generally considered the rate-limiting step for overall glutathione synthesis, making its regulation crucial for maintaining GSH levels.

Antioxidant Function of Glutathione

GSH is a potent antioxidant, directly scavenging harmful reactive oxygen species (ROS) and reactive nitrogen species (RNS), and acting as a substrate for antioxidant enzymes.

• Mechanism as a Free Radical Scavenger: GSH directly reacts with and neutralizes various oxidants, most notably hydrogen peroxide ($H<em>2O</em>2$) and organic hydroperoxides (ROOH), in reactions catalyzed by glutathione peroxidase (GPx) enzymes. These enzymes convert the toxic peroxides into less harmful water and alcohols, respectively, while oxidizing two molecules of GSH to one molecule of oxidized glutathione (GSSG):
  $2 \text{GSH} + \text{ROOH} \longrightarrow \text{GSSG} + \text{H}_2\text{O} + \text{ROH}$
  (where ROOH = lipid peroxides, hydroperoxides from fatty acids, etc.)

• Maintenance of Reduced Glutathione: To replenish the active GSH pool, the oxidized glutathione (GSSG) must be reduced back to GSH. This crucial regeneration is catalyzed by the enzyme glutathione reductase (GR), which uses NADPH as an essential coenzyme:
  $\text{GSSG} + \text{NADPH} + \text{H}^+ \longrightarrow 2 \text{GSH} + \text{NADP}^+$
  This direct link highlights why the Pentose Phosphate Pathway's generation of NADPH is absolutely vital for the continuous functioning of the glutathione antioxidant system.

Maintenance of GSH Pools

The steady-state levels of GSH are meticulously maintained through a balance of synthesis, utilization, and regeneration. A cornerstone of GSH pool maintenance is the constant supply of NADPH, which is primarily provided by glucose 6-phosphate dehydrogenase (G-6-PD), the rate-limiting enzyme of the oxidative phase of the pentose phosphate pathway. The first three enzyme-catalyzed steps of the pentose pathway collectively regenerate 2 molecules of NADPH, initiating with the action of G-6-PD.

Genetic Factors Affecting G-6-PD Activity

Deficiencies in G-6-PD activity are the most common human enzyme deficiency, reflecting significant genetic diversity.

• Polymorphisms leading to G-6-PD deficiencies are notably prevalent, particularly among populations of African, Mediterranean, and Asian ancestry.

• These deficiencies, while potentially detrimental under oxidative stress, paradoxically confer some degree of resistance to malaria (specifically Plasmodium falciparum). The malaria parasite relies heavily on NADPH for its rapid growth and survival within red blood cells. In G-6-PD deficient cells, reduced NADPH levels make the host cells more susceptible to oxidative stress, creating an unfavorable environment for parasite proliferation and leading to premature destruction of infected red blood cells.

• While often benign, G-6-PD deficiency can lead to severe hemolysis (destruction of red blood cells) when individuals are exposed to certain drugs (e.g., antimalarials, sulfa drugs), infections, or specific dietary substances that induce oxidative stress (e.g., Vicine from fava beans).

Fava Beans and G-6-PD Deficiency (Favism)

Consumption of fava beans (broad beans) can trigger acute hemolytic anemia in individuals with G-6-PD deficiency, a condition known as favism.

• Fava beans contain pyrimidine glycosides, primarily Vicine and Convicine.

• Upon digestion, Vicine is metabolized into highly oxidative compounds (e.g., divicine, isouramil) that generate reactive oxygen species.

• In individuals with G-6-PD deficiency, the insufficient production of NADPH compromises the ability of red blood cells to reduce GSSG back to GSH, leading to a rapid depletion of GSH.

• This lack of GSH leaves hemoglobin and other cellular components vulnerable to oxidative damage, resulting in the formation of Heinz bodies and ultimately hemolysis.

Glutathione's Role in Hemoglobin Structure

GSH plays a vital role in maintaining the structural integrity and function of hemoglobin within red blood cells.

• GSH acts as a sulfhydryl buffer, protecting the cysteine residues of hemoglobin and other proteins from oxidation and maintaining them in their reduced, functional state.

• Inadequate levels of GSH, such as those caused by G-6-PD deficiency, lead to increased oxidative stress. This stress causes the oxidation of hemoglobin's sulfhydryl groups, leading to denaturation and the formation of insoluble aggregates of oxidized hemoglobin, descriptively termed Heinz bodies.

• These Heinz bodies attach to the inner surface of the red blood cell membrane, reducing its flexibility, making the cells rigid, and signaling for their early destruction by the spleen (extravascular hemolysis). This contributes significantly to the hemolytic anemia observed in G-6-PD deficient individuals.

Pharmacogenetic Considerations

Understanding an individual's genetic makeup, particularly concerning enzymes like G-6-PD, is crucial for predicting drug responses and preventing adverse drug reactions. This field is known as pharmacogenetics.

• Primaquine: This antimalarial drug, commonly used in the past, generates significant reactive oxygen species (ROS) as part of its mechanism of action. In individuals with G-6-PD deficiency, the inability of their red blood cells to generate sufficient NADPH leads to a severe compromise in their antioxidant defenses. Consequently, primaquine administration can trigger acute and severe hemolytic anemia. Red blood cells are particularly vulnerable because they lack mitochondria, and thus depend almost solely on the PPP for NADPH generation.

• The critical observation of these adverse reactions, especially among individuals of African ancestry during mass antimalarial treatment programs, directly led to the first major discovery in pharmacogenetics. This highlighted how genetic variations could profoundly influence drug metabolism and toxicity.

• Acetaminophen (Paracetamol) overdose: A common cause of acute liver failure, acetaminophen overdose leads to the production of a highly reactive and toxic metabolite, N-acetyl-p-benzoquinone imine (NAPQI). Under normal circumstances, NAPQI is rapidly detoxified by conjugation with GSH. However, in an overdose situation, the cellular GSH stores become rapidly depleted, leaving NAPQI free to bind covalently to hepatocyte proteins, causing widespread oxidative damage and necrosis.

• N-Acetylcysteine (NAC): This compound serves as a life-saving antidote for acetaminophen overdose. NAC acts as a precursor for cysteine, thereby promoting the de novo synthesis of GSH. By restoring cellular GSH pools, NAC replenishes the substrate for NAPQI detoxification and also provides direct antioxidant properties, thereby providing significant hepatoprotection if administered intravenously within 8-10 hours of ingestion.

• Xenobiotic Detoxification and Carcinogenesis: Glutathione plays a crucial role in Phase II metabolism, where it conjugates with various exogenous compounds (xenobiotics) to facilitate their excretion from the body. Some environmental carcinogens, such as benzo[a]pyrene (B[a]P), a polycyclic aromatic hydrocarbon found in cigarette smoke and industrial pollution, undergo metabolic activation to highly reactive epoxy diol intermediates. These reactive metabolites can be detoxified by conjugation with GSH (catalyzed by glutathione S-transferases). However, if GSH levels are insufficient or detoxification pathways are overwhelmed, these reactive intermediates can form DNA adducts, leading to mutations, genomic instability, and ultimately cancer.

γ-Glutamyl Transferase (GGT)

γ-Glutamyl Transferase (GGT), also known as γ-glutamyl transpeptidase, is an enzyme involved in the extracellular degradation of glutathione, initiating the gamma-glutamyl cycle for amino acid transport and cellular GSH regulation.

• GGT is highly concentrated in the epithelial cells of the biliary tree, kidney tubules, and to a lesser extent, in the liver, pancreas, and intestine.

• Elevated levels of GGT in the blood are a sensitive, though non-specific, biomarker for hepatobiliary disease. It is often elevated in conditions such as cholestasis (bile duct obstruction), various forms of alcoholic liver disease, and can also be induced by exposure to certain medications like phenytoin (an anticonvulsant) or phenobarbital (a barbiturate), reflecting increased oxidative stress or altered enzyme induction.

Summary

A comprehensive understanding of the Pentose Phosphate Pathway, particularly the function of glucose 6-phosphate dehydrogenase, is foundational to comprehending the generation of NADPH, a critical coenzyme. NADPH, in turn, is indispensable for reductive biosynthetic pathways and, crucially, for maintaining the reduced state of glutathione (GSH). Enzymes like glucose 6-phosphate dehydrogenase, glutathione reductase, and the enzymes involved in GSH synthesis are vital for sustaining GSH pools. GSH's antioxidant functions, including its role as a free radical scavenger through glutathione peroxidase and its broad contributions to xenobiotic conjugation (Phase II metabolism), are central to cellular detoxification processes and protection against oxidative stress. Genetic variations, such as G-6-PD deficiency, underscore how individual genetic background profoundly impacts responses to dietary factors like fava beans and pharmacological agents like primaquine or acetaminophen, vividly illustrating the intricate intersection of nutrition, pharmacology, genetics, and clinical medicine.