Module 3 Lecture 6 - Cellular Respiration and the Pentose Phosphate Pathway

Overview of Cellular Respiration and Glucose Metabolism

Cellular respiration is the multifaceted metabolic process that breaks down glucose and other organic molecules to produce energy in the form of $ATP$ .
Respiration can occur under two conditions:
- Aerobic Condition: Utilizes oxygen through the Tricarboxylic Acid ( $TCA$ ) cycle (also known as the Krebs cycle) and the Electron Transport Chain ( $ETC$ ).
- Anaerobic Condition: Occurs in the absence of oxygen. In humans, this produces lactate. In microorganisms and some plants, it results in ethanol fermentation. This process serves primarily to recycle $NAD^+$ from $NADH$ to allow glycolysis to continue.
Energy Efficiency:
- Complete oxidation of glucose to $CO_2$ and $H_2O$ releases $2,840\,kJ/mol$ of glucose.
- The cell captures approximately $2,762\,kJ/mol$ ( $97\%$ ) as stored energy ( $ATP$ , $NADH$ , pyruvate).
- About $3\%$ of the energy is lost as heat during these catabolic reactions.

The Three Stages of Cellular Respiration

Stage 1: Glycolysis
- Occurs in the cytosol.
- Does not require oxygen ( $O_2$ ).
- Conversion of one $6$ -carbon glucose molecule into two $3$ -carbon pyruvate molecules.
- Preparatory Phase (Energy Investment): Utilizes $2\,ATP$ to phosphorylate glucose into glucose-6-phosphate and then fructose-1,6-bisphosphate.
- Payoff Phase: Produces $4\,ATP$ and $2\,NADH$ .
- Net Outcome: $2\,ATP$ and $2\,NADH$ per molecule of glucose.
- Key regulatory enzyme: Pyruvate Kinase.
Stage 2: Citric Acid Cycle (TCA Cycle)
- Occurs in the mitochondrial matrix.
- Pyruvate ( $3$ carbons) is converted to Acetyl-CoA ( $2$ carbons) by the Pyruvate Dehydrogenase Complex ( $PDC$ ).
- Pyruvate Dehydrogenase Complex: A multi-enzyme complex consisting of three enzymes: $E1$ , $E2$ , and $E3$ .
- Acetyl-CoA combines with Oxaloacetate ( $4$ carbons) to form Citrate ( $6$ carbons). Oxaloacetate is a rate-limiting factor for this cycle.
- Net Outcome (per Acetyl-CoA): $3\,NADH$ , $1\,FADH_2$ , $1\,GTP$ (equivalent to $ATP$ ), and $2\,CO_2$ .
- Because one glucose yields two pyruvates, the totals are doubled per glucose molecule.
Stage 3: Electron Transport Chain (ETC) & Oxidative Phosphorylation
- Occurs in the inner mitochondrial membrane.
- High-energy electrons from $NADH$ and $FADH_2$ are donated to a series of complexes to create a proton gradient.
- Oxygen is the final electron acceptor, forming water ( $H_2O$ ).

Detail of the Electron Transport Chain Complexes

Complex I (NADH Dehydrogenase): Accepts electrons from $NADH$ . Pumps $4$ protons ( $H^+$ ) into the intermembrane space.
Complex II (Succinate Dehydrogenase): Part of both the TCA cycle and ETC. Converts succinate to fumarate. Does not pump protons. Transfers electrons to coenzyme Q via $FMN$ and iron-sulfur channels.
Complex III (Cytochrome $bc_1$ Complex): Receives electrons from coenzyme Q ( $ubiquinone$ ) and transfers them to Cytochrome c. Pumps $4$ protons ( $H^+$ ).
Complex IV (Cytochrome Oxidase): Transfers electrons from Cytochrome c to oxygen ( $O_2$ ). Pumps $2$ protons ( $H^+$ ).
ATP Synthase: Uses the positive charge build-up (proton gradient) to couple a phosphate group to $ADP$ to produce $ATP$ .

ATP Yield Variation and Shuttle Systems

Complete oxidation of glucose yields roughly $30$ to $32\,ATP$ . The variation depends on how $NADH$ produced in the cytosol (glycolysis) enters the mitochondria.
Malate-Aspartate Shuttle System:
- Found in the heart, liver, and kidneys.
- More efficient; transfers electrons to mitochondrial $NADH$ .
- Yields a total of $32\,ATP$ per glucose.
Glycerol-3-Phosphate Shuttle System:
- Found in the brain and skeletal muscles.
- Electrons are transferred to $FADH_2$ , resulting in energy loss.
- Yields a total of $30\,ATP$ per glucose.

Differentiating Phosphorylation Methods

Substrate-Level Phosphorylation:
- Mechanism: Direct transfer of a phosphate group from a high-energy substrate (e.g., fructose-1,6-bisphosphate) to $ADP$ .
- Location: Cytosol (Glycolysis) and mitochondrial matrix (TCA).
- Oxygen: Not required.
- Yield: Total of $4\,ATP$ per glucose ( $2$ from glycolysis, $2$ from TCA cycle as $GTP$ ).
Oxidative Phosphorylation:
- Mechanism: Indirect production of $ATP$ via a proton gradient generated by the electron transport from $NADH$ and $FADH_2$ to oxygen.
- Location: Inner mitochondrial membrane.
- Oxygen: Essential as the final electron acceptor.
- Yield: $26$ to $28\,ATP$ per glucose.

The Fates of Glucose in the Cell

Storage: Converted into polymeric forms like glycogen (animals) or starch and sucrose (plants).
Oxidation via Glycolysis: Burned for immediate energy ( $ATP$ ).
Synthesis of Structural Polymers: Used for the extracellular matrix and cell wall polysaccharides.
Pentose Phosphate Pathway (PPP): Used to produce NADPH and pentose sugars for biosynthesis.

Pentose Phosphate Pathway (PPP)

Also known as the Phosphogluconate Pathway or Hexose Monophosphate Pathway.
Occurs in the cytosol and does not require oxygen.
Highly active in the liver, adipose tissue, Red Blood Cells ( $RBCs$ ), adrenal cortex, and lactating mammary glands.
Two Independent Phases:

1. Oxidative Phase (Irreversible)

Purpose: Produces $NADPH$ and pentose phosphates.
Process: Glucose-6-phosphate ( $6$ carbons) is converted to Ribulose-5-phosphate ( $5$ carbons).
Key Reaction: $1\,CO_2$ is released and $2\,NADH$ are produced.
Rate-Limiting Enzyme: Glucose-6-phosphate Dehydrogenase (G6PD).
Regulation: High levels of $NADP^+$ activate the pathway; high levels of $NADPH$ inhibit it (negative feedback).

2. Non-Oxidative Phase (Reversible)

Purpose: Sugar rearrangement to link back to glycolysis.
Process: Interconverts $-3$ , $-4$ , $-5$ , $-6$ , and $-7$ carbon sugars.
Key Reactions:
- Transketolase: Transfers $2$ -carbon units. Requires thiamine pyrophosphate (Vitamin $B_1$ ) as a cofactor.
- Transaldolase: Transfers $3$ -carbon units.
Outcomes: Produces Ribose-5-phosphate (for nucleotides) or recycles intermediates back into Fructose-6-phosphate and Glyceraldehyde-3-phosphate for glycolysis.

Functional Roles of PPP Products

NADPH (Nicotinamide Adenine Dinucleotide Phosphate):
- Biosynthesis: Acts as an electron donor for fatty acid, cholesterol, and steroid hormone synthesis.
- Antioxidant Defense: Maintains reduced glutathione to protect cells (especially $RBCs$ ) from oxidative damage.
- Detoxification: Involved in Cytochrome $P450$ reactions and immune functions (phagocytosis/respiratory burst).
Pentose Sugars (Ribose-5-phosphate):
- Essential building blocks for nucleotides used in DNA and RNA synthesis.
- Synthesis of coenzymes ( $ATP$ , $NADH$ , $FADH_2$ , and Coenzyme A).

Clinical Relevance

G6PD Deficiency: Leads to hemolytic anemia because the cell cannot produce enough $NADPH$ to maintain reduced glutathione, leaving $RBCs$ vulnerable to oxidative stress.
Thiamine Deficiency: Interferes with Transketolase activity, which can disrupt the non-oxidative phase of the PPP and general glucose metabolism.

Questions & Discussion

Question: Why is the final ATP yield in cellular respiration 30 or 32?
Answer: It depends on the shuttle system used to transport cytosolic $NADH$ into the mitochondria. The malate-aspartate shuttle (heart, liver, kidneys) yields $32\,ATP$ , while the glycerol-3-phosphate shuttle (brain, skeletal muscle) yields $30\,ATP$ .
Question: What is the difference between oxidative and substrate-level phosphorylation?
Answer: Substrate-level is a direct phosphate transfer from a substrate to $ADP$ in the cytosol or matrix without oxygen. Oxidative is an indirect process using the proton gradient and oxygen in the inner mitochondrial membrane.
Question: Is the conversion of Ribulose-5-phosphate to Ribose-5-phosphate oxidative or non-oxidative?
Answer: It is a non-oxidative rearrangement. The oxidative phase ends at the production of Ribulose-5-phosphate, after which sugar interconversion (rearrangement) occurs in the non-oxidative phase.