Comprehensive Biochemistry Notes: HMP Pathway and the Cori Cycle

Overview of the Hexose Monophosphate (HMP) Pathway

• Definition: The Hexose Monophosphate Pathway (HMP), also known as the Pentose Phosphate Pathway (PPP), is a glucose metabolic pathway where five-carbon sugars (pentoses) and $NADPH$ are synthesized in the cytosol of the cell.

• Alternative Names:

  • Phosphogluconic Acid Pathway (PAP).

  • Warburg-Dickens Pathway.

  • 6-phosphogluconate Pathway.

• Pathway Characteristics:

  • It is a pathway of hexose oxidation where $\text{glucose-6-phosphate}$ is metabolized.

  • It serves as an alternative route for the oxidation of glucose compared to glycolysis.

  • Oxygen Requirements: The pathway does not use molecular oxygen ($O_2$).

  • Oxidizing Agent: It requires $NADP^+$ as the oxidizing agent.

  • Energy Production: The pathway does not require or generate $ATP$.

  • Location: Occurs within the cytosol of the cell.

Biological Distribution and Tissue Specificity

• Active Tissues: The HMP shunt is highly active in tissues that require $NADPH$ for the synthesis of fatty acids and steroids. These lipid-synthesizing tissues include:

  • Adrenal cortex.

  • Mammary glands (specifically during lactation).

  • Adipose tissues.

  • Testis.

  • Liver.

• Inactive Tissues: The pathway is rarely detected in skeletal muscles because the synthesis of lipids rarely occurs in this tissue.

• Role in Red Blood Cells (RBCs):

  • The HMP is active in human red blood cells.

  • The generated $NADPH$ protects unsaturated fatty acids in the cell membrane from oxidation.

  • It maintains the iron atoms of haemoglobin in their normal reduced ferrous state ($Fe^{2+}$).

Primary Roles and Functions of HMP

1. Generation of $NADPH$: Nicotinamide adenine dinucleotide phosphate ($NADPH$) is produced for use in metabolic reactions such as fatty acid synthesis.

2. Synthesis of Ribose 5-phosphate: This pentose is an essential precursor for nucleotide and nucleic acid synthesis.

3. Production of Erythrose 4-phosphate: This sugar intermediate is required for the synthesis of aromatic amino acids.

4. Interconversion of Sugars: The pathway yields metabolic intermediates that can be channeled into glycolysis and gluconeogenesis pathways.

Sequence of Reactions: The Oxidative Phase

• The oxidative phase involves two successive oxidations by $NADP^+$ and a final oxidative decarboxylation to form a pentose phosphate.

• Step 1: Oxidation of Glucose-6-phosphate:

  • Reaction: Oxidation of $\text{glucose-6-phosphate}$ at carbon one to yield $\text{6-phosphogluconolactone}$.

  • Enzyme: $\text{Glucose-6-phosphate dehydrogenase (G6PD)}$.

  • Mechanism: Involves the removal of 2 hydrogen atoms.

  • Cofactor: First molecule of $NADPH$ is produced.

  • Regulation: This is the major control point and the rate-limiting step of the oxidative phase. $G6PD$ is strongly inhibited by its product, $NADPH$.

• Step 2: Hydrolysis of 6-phosphogluconolactone:

  • Reaction: The unstable lactone is hydrolyzed to form $\text{6-phosphogluconic acid}$.

  • Enzyme: $\text{Gluconolactone hydrolase}$.

  • Note: This reaction is reversible and may also occur spontaneously without the enzyme.

• Step 3: Oxidative Decarboxylation of 6-phosphogluconic acid:

  • Reaction: $\text{6-phosphogluconate}$ is decarboxylated to form $\text{ribulose-5-phosphate}$.

  • Enzyme: $\text{6-phosphogluconate dehydrogenase}$.

  • Cofactor: The second molecule of $NADPH$ is generated here.

Sequence of Reactions: The Non-Oxidative Phase

• Interconversion of Pentose Sugars:

  • $\text{Ribulose-5-phosphate}$ (a ketopentose) is isomerized to $\text{ribose-5-phosphate}$ (an aldopentose) by the enzyme $\text{phosphopentose isomerase}$ (or $\text{phosphor-riboseisomerase}$). This product is used for nucleotide synthesis.

  • $\text{Ribulose-5-phosphate}$ is epimerized on carbon 3 to $\text{xylulose-5-phosphate}$ by the enzyme $\text{phosphopentose epimerase}$ (or $\text{phosphoketopentose epimerase}$).

  • These reactions are freely reversible; thus, $\text{ribulose-5-phosphate}$, $\text{ribose-5-phosphate}$, and $\text{xylulose-5-phosphate}$ exist in equilibrium.

• Interconversion of Other Sugars:

  • First Reaction: The enzyme $\text{transketolase}$ transfers a glycoaldehyde group from the keto sugar $\text{xylulose-5-phosphate}$ to the aldo sugar $\text{ribose-5-phosphate}$. This forms $\text{sedoheptulose-7-phosphate}$ (7 carbons) and $\text{glyceraldehyde-3-phosphate}$ (3 carbons).

  • Second Reaction: The enzyme $\text{transaldolase}$ transfers a three-carbon dihydroxyacetone phosphate group from $\text{sedoheptulose-7-phosphate}$ to $\text{glyceraldehyde-3-phosphate}$. This forms $\text{fructose-6-phosphate}$ and $\text{erythrose-4-phosphate}$.

  • These trioses ($\text{glyceraldehyde-3-phosphate}$) can be channeled into the glycolytic pathway for $ATP$ generation via the TCA cycle.

Gestational Diabetes and Insulin Management

• Gestational Diabetes: This condition occurs in pregnant women and usually disappears after the birth of the baby. Women with a history of gestational diabetes are at an increased risk of developing Type 2 diabetes later in life.

• Management:

  • Consultation with a dietitian for healthy eating strategies.

  • Management of blood glucose levels.

  • Regular exercise.

• Types of Insulin:

  • Rapid-acting insulin.

  • Short-acting insulin.

  • Intermediate-acting insulin.

  • Mixed insulin.

  • Long-acting insulin.

The Cori Cycle (Lactic Acid Cycle)

• Definition: Named after Carl Ferdinand Cori and Gerty Cori, this cycle describes the metabolic pathway where lactate produced by anaerobic glycolysis in muscles is transported to the liver, converted back to glucose, and returned to the muscles.

• Mechanism in Muscle:

  • Muscular activity requires $ATP$ provided by the breakdown of glycogen (glycogenolysis).

  • $\text{Glycogen}$ releases $\text{glucose 1-phosphate (G1P)}$, which is converted to $\text{glucose 6-phosphate (G6P)}$ by $\text{phosphoglucomutase}$.

  • $G6P$ enters glycolysis to provide $ATP$ for muscular contraction.

  • When oxygen supply is insufficient (intense activity), pyruvate is converted to lactate by $\text{lactate dehydrogenase}$. This step regenerates $NAD^+$ to allow glycolysis to continue.

• Mechanism in Liver:

  • Lactate is released from muscles, enters the bloodstream, and is taken up by the liver.

  • Gluconeogenesis: The liver reverses the process by converting lactate back to pyruvate and then back to glucose.

• Metabolic Fate of Liver Glucose:

  • Supplied to muscles via the bloodstream for further glycolysis.

  • If muscle activity has ceased, used to replenish glycogen stores via glycogenesis.

• Energy Balance:

  • Glycolysis produces $2$ molecules of $ATP$.

  • Gluconeogenesis consumes $6$ molecules of $ATP$.

  • Net consumption: $4$ molecules of $ATP$ per iteration.

  • The cycle cannot be sustained indefinitely and shifts the metabolic burden from the muscles to the liver.