Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway: Exhaustive Study Guide

Major Pathways of Glucose Utilization

Glucose occupies a central position in the metabolism of plants, animals, and many microorganisms. It is a versatile precursor capable of being used in several major pathways:

• Storage: When glucose is abundant, it is stored as polysaccharides (glycogen in animals and microorganisms, starch in plants) or sucrose.
• Oxidation via Glycolysis: Glucose is degraded into the three-carbon compound pyruvate to yield energy in the form of $ATP$ and $NADH$.
• Oxidation via Pentose Phosphate Pathway: This pathway produces ribose 5-phosphate for nucleic acid synthesis and $NADPH$ for reductive biosynthetic processes.
• Synthesis of Structural Polymers: Glucose is used to create extracellular matrix and cell wall polysaccharides.

Glycolysis: The Universal Central Pathway

Glycolysis is a nearly universal 10-step metabolic process in which one molecule of glucose is degraded into two molecules of pyruvate. This process occurs in two distinct phases: the Preparatory Phase and the Payoff Phase.

The Preparatory Phase (Investment Phase)

In this phase, energy is invested to activate glucose and prepare the carbon chain for cleavage.

1. ATP Investment: Two molecules of $ATP$ are consumed to phosphorylate the hexose intermediates.
2. Raising Free Energy: The phosphorylation increases the free energy ($\Delta G$) of the intermediates.
3. Cleavage: The hexose carbon chain is converted into two molecules of the triose phosphate, glyceraldehyde 3-phosphate.

The Payoff Phase

Energy is extracted from the triose phosphates through oxidation and phosphorylation.

1. Energy Yield: This phase produces four molecules of $ATP$ and two molecules of $NADH$. Since two $ATP$ were invested in the first phase, the net yield is $2 \, ATP$ and $2 \, NADH$ per glucose.
2. Product: Two molecules of pyruvate are the final carbon products.

Chemical Transformations of Glycolysis

Glycolysis involves three noteworthy chemical changes:

1. Carbon Skeleton Degradation: The conversion of the six-carbon glucose to two three-carbon pyruvates.
2. Phosphorylation of ADP to ATP: Compounds with high phosphoryl group transfer potential, generated during the pathway, transfer phosphate groups to $ADP$ (substrate-level phosphorylation).
3. Hydride Ion Transfer: A hydride ion ($:H^-$) is transferred to $NAD^+$ to form $NADH$.

The 10 Steps of Glycolysis

Step 1: Phosphorylation of Glucose

• Enzyme: Hexokinase.
• Reaction: Glucose + $ATP \rightarrow$ Glucose 6-phosphate + $ADP$.
• Logic: Phosphorylation traps glucose inside the cell because the plasma membrane lacks transporters for phosphorylated sugars. It also activates the glucose for further reactions.
• Details: Hexokinase requires $Mg^{2+}$ for activity. In humans, there are four isozymes (Hexokinase I-IV).

Step 2: Conversion of Glucose 6-Phosphate to Fructose 6-Phosphate

• Enzyme: Phosphohexose isomerase (also known as phosphoglucose isomerase).
• Reaction: Glucose 6-phosphate $\rightleftharpoons$ Fructose 6-phosphate.
• Logic: Moving the carbonyl from $C-1$ to $C-2$ is a prerequisite for the subsequent phosphorylation and the C-C bond cleavage in Step 4.
• Mechanism: Involves a cis-enediol intermediate.

Step 3: Phosphorylation of Fructose 6-Phosphate

• Enzyme: Phosphofructokinase-1 (PFK-1).
• Reaction: Fructose 6-phosphate + $ATP \rightarrow$ Fructose 1,6-bisphosphate + $ADP$.
• Logic: This is the first "committed" step of glycolysis. $PFK-1$ is a major regulatory enzyme, activated by $ADP/AMP$ and inhibited by $ATP$ and citrate.

Step 4: Cleavage of Fructose 1,6-Bisphosphate

• Enzyme: Aldolase (Fructose 1,6-bisphosphate aldolase).
• Reaction: Fructose 1,6-bisphosphate $\rightleftharpoons$ Dihydroxyacetone phosphate (DHAP) + Glyceraldehyde 3-phosphate (G3P).
• Class I Aldolases: Found in animals and plants; they form a Schiff base intermediate.
• Class II Aldolases: Found in fungi and bacteria; they use a metal ion (often $Zn^{2+}$) instead of a Schiff base.

Step 5: Interconversion of Triose Phosphates

• Enzyme: Triose phosphate isomerase.
• Reaction: DHAP $\rightleftharpoons$ G3P.
• Logic: This ensures that both products of the aldolase reaction can proceed through the same payoff pathway.

Step 6: Oxidation of Glyceraldehyde 3-Phosphate

• Enzyme: Glyceraldehyde 3-phosphate dehydrogenase (G3PDH).
• Reaction: G3P + $P_i$ + $NAD^+ \rightleftharpoons$ 1,3-Bisphosphoglycerate + $NADH$ + $H^+$.
• Logic: This is an energy-conserving reaction where the oxidation of the aldehyde group is coupled to the formation of an acyl phosphate, which has a very high standard free energy of hydrolysis ($\Delta G'^{\circ} = -49.3 \, kJ/mol$).
• Mechanism: Involves a covalent thiohemiacetal and then a thioester linkage with a Cys residue in the active site.

Step 7: Phosphoryl Transfer to ADP

• Enzyme: Phosphoglycerate kinase.
• Reaction: 1,3-Bisphosphoglycerate + $ADP \rightleftharpoons$ 3-Phosphoglycerate + $ATP$.
• Mechanism: Substrate-level phosphorylation. Steps 6 and 7 combined have a cumulative $\Delta G'^{\circ} = -12.2 \, kJ/mol$.

Step 8: Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate

• Enzyme: Phosphoglycerate mutase.
• Reaction: 3-Phosphoglycerate $\rightleftharpoons$ 2-Phosphoglycerate.
• Logic: The phosphoryl group is moved from $C-3$ to $C-2$ to set up for Step 9.
• Mechanism: Involves a 2,3-bisphosphoglycerate ($2,3-BPG$) intermediate and phosphorylated Histidine residues.

Step 9: Dehydration of 2-Phosphoglycerate

• Enzyme: Enolase.
• Reaction: 2-Phosphoglycerate $\rightleftharpoons$ Phosphoenolpyruvate (PEP) + $H_2O$.
• Logic: Dehydration creates a compound (PEP) with a very high phosphoryl group transfer potential.

Step 10: Transfer of Phosphoryl Group to ADP

• Enzyme: Pyruvate kinase.
• Reaction: PEP + $ADP \rightarrow$ Pyruvate + $ATP$.
• Logic: Final substrate-level phosphorylation. Pyruvate is initially formed in its enol form and then spontaneously tautomerizes to the more stable keto form.
• Requirements: Needs $K^+$ and either $Mg^{2+}$ or $Mn^{2+}$.

Overall Balance and Energetics of Glycolysis

The net equation for glycolysis is: $\text{glucose} + 2NAD^+ + 2ADP + 2P_i \rightarrow 2 \text{ pyruvate} + 2NADH + 2H^+ + 2ATP + 2H_2O$

Free-Energy Changes

• The conversion of glucose to pyruvate is exergonic: $\Delta G'^{\circ}_1 = -146 \, kJ/mol$.
• The formation of $ATP$ from $ADP$ and $P_i$ is endergonic: $\Delta G'^{\circ}_2 = 61.0 \, kJ/mol$.
• The sum of these processes results in $\Delta G'^{\circ}_{Sum} = -85 \, kJ/mol$, making glycolysis essentially irreversible under cellular conditions.

Feeder Pathways for Glycolysis

Various carbohydrates can enter the glycolytic pathway at different points:

• Endogenous Glycogen and Starch: These are mobilized by glycogen/starch phosphorylase, which uses inorganic phosphate ($P_i$) to produce glucose 1-phosphate (phosphorolysis). This is then converted to glucose 6-phosphate by phosphoglucomutase.
• Dietary Polysaccharides: $\alpha$-amylase in the mouth and small intestine hydrolyzes $(\alpha 1 \rightarrow 4)$ glycosidic linkages to produce maltose and maltotriotriose.
• Disaccharides: Hydrolyzed by membrane-bound enzymes in the intestine:     * Lactose: Hydrolyzed by lactase to glucose and galactose.     * Sucrose: Hydrolyzed by sucrase to glucose and fructose.     * Trehalose: Hydrolyzed by trehalase to glucose.
• Lactose Persistence and Intolerance: Lactase is usually absent in adults (standard phenotype), leading to lactose intolerance. Persistence of the enzyme into adulthood is the "lactase persistence" phenotype.
• Galactose Metabolism: Galactose is phosphorylated by galactokinase and then converted to glucose 1-phosphate via $UDP$-glucose and $UDP$-galactose intermediates. Defects in this pathway cause galactosemia.
• Fructose and Mannose:     * In the small intestine, hexokinase phosphorylates fructose to fructose 6-phosphate.     * In the liver, fructokinase phosphorylates fructose to fructose 1-phosphate, which is then cleaved by fructose 1-phosphate aldolase into DHAP and glyceraldehyde.     * Mannose is phosphorylated by hexokinase to mannose 6-phosphate and then isomerized to fructose 6-phosphate.

Fates of Pyruvate

Pyruvate represents a metabolic junction with three primary fates:

1. Aerobic Conditions: Pyruvate is oxidized to acetyl-CoA, which enters the Citric Acid Cycle for complete oxidation to $CO_2$. This yields significantly more $ATP$ ($30$ or $32$ per glucose).
2. Lactic Acid Fermentation (Anaerobic): Under low oxygen (hypoxia) or in organisms lacking mitochondria (like erythrocytes), pyruvate is reduced to lactate by lactate dehydrogenase. This reaction oxidizes $NADH$ back to $NAD^+$, allowing glycolysis to continue.
3. Ethanol Fermentation (Anaerobic): In yeast and certain microorganisms, pyruvate is converted to ethanol and $CO_2$ in two steps:     * Pyruvate decarboxylase: Converts pyruvate to acetaldehyde. Requires $Mg^{2+}$ and thiamine pyrophosphate (TPP).     * Alcohol dehydrogenase: Reduces acetaldehyde to ethanol, regenerating $NAD^+$.

Distinctive Metabolic Effects

• Pasteur Effect: The observation that glucose consumption is much faster under anaerobic conditions because the $ATP$ yield is lower ($2 \, ATP$ vs. $30/32 \, ATP$).
• Warburg Effect: Tumor cells exhibit high rates of glycolysis and lactate fermentation even in the presence of oxygen. This is the basis for PET scans using glucose analogs.

Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors like pyruvate, lactate, and certain amino acids. It occurs primarily in the liver in mammals.

The Three Bypass Reactions

Gluconeogenesis uses seven of the ten glycolytic enzymes; however, it must bypass the three highly exergonic, irreversible steps of glycolysis:

1. First Bypass (Pyruvate to PEP):     * Pyruvate enters mitochondria and is converted to oxaloacetate by pyruvate carboxylase (requires biotin and $ATP$).     * Oxaloacetate is reduced to malate by mitochondrial malate dehydrogenase, transported to the cytosol, and reoxidized to oxaloacetate.     * PEP carboxykinase converts oxaloacetate to PEP using $GTP$.     * Alternative: If lactate is the precursor, PEP carboxykinase acts within the mitochondria.
2. Second Bypass (Fructose 1,6-bisphosphate to Fructose 6-phosphate):     * Catalyzed by fructose 1,6-bisphosphatase (FBPase-1). This is a hydrolysis reaction requiring $Mg^{2+}$.
3. Third Bypass (Glucose 6-phosphate to Glucose):     * Catalyzed by glucose 6-phosphatase, found only in the lumen of the endoplasmic reticulum (ER) of hepatocytes, renal cells, and small intestine epithelial cells.

Summary Net Equation:

$2 \text{ pyruvate} + 4ATP + 2GTP + 2NADH + 2H^+ + 4H_2O \rightarrow \text{glucose} + 4ADP + 2GDP + 6P_i + 2NAD^+$

Precursors

• Glucogenic Amino Acids: Alanine, arginine, asparagine, etc., can be converted to glucose.
• Note on Fatty Acids: Mammals cannot convert acetyl-CoA from fatty acids into glucose; plants and microorganisms use the glyoxylate pathway for this.
• Glyceroneogenesis: Adipocytes convert pyruvate to DHAP and then to glycerol 3-phosphate.

Coordinated Regulation of Glycolysis and Gluconeogenesis

The two pathways are reciprocally regulated to prevent "futile cycles" (simultaneous operation that wastes energy).

Regulation of Hexokinase

• Hexokinase I, II, III: Inhibited by their product, glucose 6-phosphate.
• Hexokinase IV (Glucokinase): Found in the liver, it has a high $K_m$ for glucose ($\approx 10 \, mM$) and is not inhibited by glucose 6-phosphate. It is regulated by sequestration in the nucleus by a regulatory protein, triggered by fructose 6-phosphate and reversed by glucose.

Reciprocal Regulation by PFK-1 and FBPase-1

• PFK-1 (Glycolysis): Activated by $ADP$, $AMP$, and fructose 2,6-bisphosphate ($F2,6BP$). Inhibited by $ATP$ and citrate.
• FBPase-1 (Gluconeogenesis): Inhibited by $AMP$ and fructose 2,6-bisphosphate.

Fructose 2,6-Bisphosphate ($F2,6BP$)

This is the most potent allosteric regulator. Its levels are controlled by a bifunctional enzyme with two activities:

• Phosphofructokinase-2 (PFK-2): Synthesizes $F2,6BP$ from Fructose 6-phosphate.
• Fructose 2,6-bisphosphatase (FBPase-2): Breaks down $F2,6BP$.
• Hormonal Control:     * Glucagon: Simulates cAMP-dependent protein kinase ($PKA$), which phosphorylates the bifunctional enzyme, activating its phosphatase activity ($FBPase-2$). This lowers $F2,6BP$, inhibiting glycolysis and stimulating gluconeogenesis.     * Insulin: Activates a phosphoprotein phosphatase that dephosphorylates the enzyme, activating its kinase activity ($PFK-2$). This raises $F2,6BP$, stimulating glycolysis and inhibiting gluconeogenesis.

Other Regulatory Points

• Pyruvate Kinase: Allosterically inhibited by $ATP$, acetyl-CoA, and long-chain fatty acids. In the liver only, it is also inhibited by phosphorylation via $PKA$ (stimulated by glucagon).
• Transcriptional Regulation: Insulin and glucagon regulate the expression of genes like PEP carboxykinase and hexokinase. ChREBP (Carbohydrate Response Element Binding Protein) is a transcription factor that coordinates glucose and fat metabolism.

The Pentose Phosphate Pathway (PPP)

This pathway, also called the phosphogluconate pathway, oxidizes glucose 6-phosphate to produce pentoses and $NADPH$.

Oxidative Phase

1. Dehydrogenation: Glucose 6-phosphate is oxidized to 6-phosphoglucono-$\delta$-lactone by glucose 6-phosphate dehydrogenase (G6PD), yielding $NADPH$.
2. Hydrolysis: Lactonase converts the lactone to 6-phosphogluconate.
3. Oxidative Decarboxylation: 6-phosphogluconate dehydrogenase forms ribulose 5-phosphate and a second $NADPH$.
4. Isomerization: Phosphopentose isomerase converts ribulose 5-phosphate to ribose 5-phosphate.

Nonoxidative Phase (Recycling)

When cells need $NADPH$ but not pentoses, the pentose phosphates are recycled back to glucose 6-phosphate via several enzymes:

• Ribulose 5-phosphate epimerase: Converts ribulose 5-phosphate to xylulose 5-phosphate.
• Transketolase: Transfers a two-carbon fragment. Requires TPP.
• Transaldolase: Transfers a three-carbon fragment.

Tissue Distribution

• Biosynthetic Tissues: Tissues synthesizing fatty acids, cholesterol, or steroid hormones (liver, adipose, adrenal glands) use the PPP for $NADPH$.
• Dividing Cells: Use ribose 5-phosphate for DNA and RNA synthesis.

Thiamine Deficiency

Thiamine is the precursor for TPP, a vital cofactor for transketolase and pyruvate decarboxylase. Deficiency results in:

• Beriberi: Severe pain, swelling, and paralysis.
• Wernicke-Korsakoff Syndrome: Common in heavy drinkers; involves motor coordination issues.