4.4 Gluconeogenesis & Pentose Phosphate Pathway

Overview & Physiological Purpose of Gluconeogenesis

Gluconeogenesis is the anabolic pathway that synthesizes new glucose from non-carbohydrate precursors. Taking place primarily in the liver ($\approx 90%$) and to a lesser extent in the cortex of the kidneys ($\approx 10%$), gluconeogenesis becomes vital during extended fasting ($>12\text{ hours}$), starvation, intense exercise, or low-carbohydrate diets to maintain blood glucose levels within normal limits ($\approx 70-100\text{ mg/dL}$).

The brain, red blood cells, renal medulla, and testes rely heavily on glucose as an essential metabolic fuel. Because hepatic glycogen stores are depleted after 18–24 hours of fasting, gluconeogenesis provides the sole endogenous mechanism for glucose supply.

Non-Carbohydrate Precursors

1. Lactate: Released by red blood cells and anaerobic muscle; converted to pyruvate via LDH in the liver (Cori Cycle).
2. Glycerol: Released from adipose tissue triacylglycerol lipolysis; converted to DHAP via glycerol kinase and glycerol-3-phosphate dehydrogenase.
3. Glucogenic Amino Acids: Derived from muscle protein breakdown; Alanine is transaminated to pyruvate in the liver (Glucose-Alanine Cycle). $\alpha$-Ketoglutarate and oxaloacetate are derived from glutamate and aspartate.

AAMC MCAT Trap: Acetyl-CoA CANNOT be net converted into glucose in humans because the PDH complex reaction is irreversible, and two carbons enter the TCA cycle as acetyl-CoA while two carbons are lost as $\text{CO}_2$.

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Overcoming Irreversible Glycolytic Steps: The Four Bypass Enzymes

Gluconeogenesis is not simply the exact reverse of glycolysis. Glycolysis contains three thermodynamically irreversible steps (Steps 1, 3, and 10) with large negative $\Delta G^{\circ\prime}$ values. Gluconeogenesis utilizes four unique bypass enzymes to circumvent these barriers.

 Glycolytic Irreversible Step 10:  PEP ───────────────► Pyruvate  (Pyruvate Kinase)
 Gluconeogenic Bypass 1:           Pyruvate ──► OAA ──► PEP       (Pyruvate Carboxylase + PEPCK)
 
 Glycolytic Irreversible Step 3:   F6P ────────────────► F1,6BP    (PFK-1)
 Gluconeogenic Bypass 2:           F1,6BP ─────────────► F6P       (F1,6-Bisphosphatase)
 
 Glycolytic Irreversible Step 1:   Glucose ────────────► G6P       (Hexokinase)
 Gluconeogenic Bypass 3:           G6P ────────────────► Glucose   (Glucose-6-Phosphatase)

Bypass 1: Conversion of Pyruvate to Phosphoenolpyruvate (PEP)

Circumvents Pyruvate Kinase (Step 10) using two enzymes and two high-energy phosphate bonds per pyruvate:

1. Pyruvate Carboxylase (Mitochondrial Matrix): $\text{Pyruvate} + \text{HCO}_3^- + \text{ATP} \xrightarrow{\text{Biotin, Acetyl-CoA}} \text{Oxaloacetate} + \text{ADP} + \text{P}_i$

   • Requires Biotin (Vitamin $\text{B}_7$) as a covalent prosthetic group carrying $\text{CO}_2$.
   • Possesses an absolute requirement for Acetyl-CoA as an obligatory allosteric activator. Elevated acetyl-CoA (from fatty acid oxidation) signals that energy is available to build glucose.
   • Mitochondrial Shuttle: Oxaloacetate cannot cross the inner mitochondrial membrane directly; it is reduced to malate by mitochondrial MDH, exported to the cytosol, and reoxidized back to oxaloacetate by cytosolic MDH.

2. Phosphoenolpyruvate Carboxykinase (PEPCK, Cytosol): $\text{Oxaloacetate} + \text{GTP} \xrightarrow{\text{PEPCK}} \text{Phosphoenolpyruvate (PEP)} + \text{CO}_2 + \text{GDP}$

   • Decarboxylation of oxaloacetate drives the phosphorylation reaction using GTP.

Bypass 2: Conversion of Fructose-1,6-Bisphosphate to Fructose-6-Phosphate

Circumvents PFK-1 (Step 3):

$\text{F1,6BP} + \text{H}_2\text{O} \xrightarrow{\text{F16BPase}} \text{Fructose-6-Phosphate} + \text{P}_i \quad (\Delta G^{\circ\prime} = -16.7\text{ kJ/mol})$

Fructose-1,6-Bisphosphatase (F16BPase) is the primary rate-limiting enzyme of gluconeogenesis.

• Allosteric Activators: ATP and Citrate.
• Allosteric Inhibitors: AMP and Fructose-2,6-bisphosphate (F2,6BP).

Bypass 3: Conversion of Glucose-6-Phosphate to Free Glucose

Circumvents Hexokinase/Glucokinase (Step 1):

$\text{Glucose-6-Phosphate} + \text{H}_2\text{O} \xrightarrow{\text{G6Pase}} \text{Glucose} + \text{P}_i$

Glucose-6-Phosphatase (G6Pase) is embedded in the lumenal membrane of the Endoplasmic Reticulum (ER) of hepatocytes and renal cortex cells.

• G6P is translocated into the ER lumen, hydrolyzed by G6Pase, and free glucose is transported back into the cytosol before being exported into the bloodstream via GLUT2 transporters.
• Skeletal Muscle Lacks G6Pase: Muscle cells cannot release free glucose into the blood; they retain G6P for internal glycogen synthesis or glycolysis.

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Summary Table: Glycolysis vs. Gluconeogenesis Bypasses & Regulation

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Reciprocal Regulation & Hormonal Control via Fructose-2,6-Bisphosphate

To prevent a wasteful futile cycle (where ATP is hydrolyzed without net work), glycolysis and gluconeogenesis are reciprocally regulated:

 FED STATE (Insulin):       Insulin ──► Dephosphorylates PFK-2/FBPase-2
                            ──► PFK-2 Active ──► [F2,6BP] Rises
                            ──► Stimulates PFK-1 (Glycolysis ON)
                            ──► Inhibits F16BPase (Gluconeogenesis OFF)

 FASTED STATE (Glucagon):  Glucagon ──► cAMP ──► PKA ──► Phosphorylates PFK-2/FBPase-2
                            ──► FBPase-2 Active ──► [F2,6BP] Drops
                            ──► Removes inhibition on F16BPase (Gluconeogenesis ON)
                            ──► Inactivates PFK-1 (Glycolysis OFF)

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The Pentose Phosphate Pathway (PPP / HMP Shunt)

The Pentose Phosphate Pathway (also known as the Hexose Monophosphate Shunt) takes place entirely in the cytosol. Operating parallel to glycolysis, it oxidizes Glucose-6-Phosphate without generating ATP.

Primary Functions of the PPP:

1. Generation of NADPH for reductive biosynthesis and ROS defense.
2. Synthesis of Ribose-5-Phosphate for nucleotide and nucleic acid (RNA/DNA) synthesis.
3. Interconversion of dietary 5-carbon sugars with 3-carbon and 6-carbon glycolytic intermediates.

Phase 1: Oxidative Phase (Irreversible NADPH Generation)

1. Glucose-6-Phosphate Dehydrogenase (G6PDH) [Rate-Limiting Step]: $\text{G6P} + \text{NADP}^+ \xrightarrow{\text{G6PDH}} \text{6-Phosphoglucono-}\delta\text{-lactone} + \text{NADPH} + \text{H}^+$ G6PDH is strongly inhibited by high $[\text{NADPH}]/[\text{NADP}^+]$ ratios.
2. 6-Phosphogluconolactonase hydrolyzes the lactone to 6-Phosphogluconate.
3. 6-Phosphogluconate Dehydrogenase: $\text{6-Phosphogluconate} + \text{NADP}^+ \rightarrow \text{Ribulose-5-Phosphate} + \text{NADPH} + \text{CO}_2$

$\text{Net Oxidative Yield per G6P:} \quad 2\text{ NADPH} + 1\text{ CO}_2 + 1\text{ Ribulose-5-Phosphate}$

Phase 2: Non-Oxidative Phase (Reversible Sugar Interconversions)

Ribulose-5-Phosphate is isomerized to Ribose-5-Phosphate (for nucleotide synthesis) or epimerized to Xylulose-5-Phosphate.

Under conditions where nucleotide demand is low but NADPH demand is high, the non-oxidative enzymes Transketolase (requires TPP / Vitamin $\text{B}_1$) and Transaldolase convert 3 molecules of pentose phosphate (15 carbons) into 2 molecules of Fructose-6-Phosphate and 1 molecule of Glyceraldehyde-3-Phosphate, recycling carbons back into glycolysis or gluconeogenesis.

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Distinct Physiological Functions of NADPH vs. NADH

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Glutathione Reduction, Oxidative Stress & G6PD Deficiency

Red blood cells encounter continuous oxidative stress from hemoglobin autoxidation, forming Reactive Oxygen Species (ROS) such as hydrogen peroxide ($\text{H}_2\text{O}_2$).

• Glutathione Peroxidase neutralizes $\text{H}_2\text{O}_2$ by oxidizing reduced monomeric glutathione (GSH) to glutathione disulfide (GSSG): $2\text{ GSH} + \text{H}_2\text{O}_2 \xrightarrow{\text{Glutathione Peroxidase}} \text{GSSG} + 2\text{ H}_2\text{O}$
• Glutathione Reductase uses NADPH generated by G6PDH to reduce GSSG back to active GSH: $\text{GSSG} + \text{NADPH} + \text{H}^+ \xrightarrow{\text{Glutathione Reductase}} 2\text{ GSH} + \text{NADP}^+$

                 G6PDH (PPP): NADP+ ───────────► NADPH
                                                   │
                                                   ▼ Glutathione Reductase
 ROS / H2O2 ──► 2 H2O  <======== 2 GSH ◄──────── GSSG
               (Glutathione Peroxidase)

G6PD Deficiency Pathophysiology

• Inheritance: X-linked recessive disorder; most common enzymatic deficiency worldwide.
• Mechanism: Impaired G6PDH activity lowers cytosolic NADPH production in erythrocytes. Under oxidative triggers (fava beans, sulfa drugs, antimalarials like primaquine, or infections), GSH cannot be regenerated.
• Pathology: ROS accumulate, crosslinking sulfhydryl groups in hemoglobin, forming insoluble intracellular precipitates called Heinz bodies. As red blood cells pass through the spleen, splenic macrophages pluck out these inclusions, producing characteristic Bite cells and triggering acute intravascular hemolytic anemia.