Carbohydrate Metabolism — Study Notes

1. Glycolysis (Embden-Meyerhof-Parnas Pathway)

Glycolysis is a universal, cytosolic pathway that breaks down one molecule of glucose ($6\text{C}$) into two molecules of pyruvate ($3\text{C}$), generating net $2\text{ ATP}$ and $2\text{ NADH}$. It occurs in both aerobic and anaerobic conditions.

graph TD
    A[Glucose] -->|Hexokinase / ATP| B[Glucose-6-Phosphate]
    B --> C[Fructose-6-Phosphate]
    C -->|PFK-1 / ATP| D[Fructose-1,6-Bisphosphate]
    D --> E[DHAP + GAP]
    E --> F[2 x GAP]
    F -->|GAPDH / 2 NADH| G[2 x 1,3-BPG]
    G -->|Phosphoglycerate Kinase / 2 ATP| H[2 x 3-PG]
    H --> I[2 x 2-PG]
    I --> J[2 x PEP]
    J -->|Pyruvate Kinase / 2 ATP| K[2 x Pyruvate]

The Ten Reactions

Glycolysis is divided into two phases: - Preparatory Phase (energy investment, uses $2\text{ ATP}$): 1. Phosphorylation: Glucose $\rightarrow$ Glucose-6-phosphate (G6P) by Hexokinase (or glucokinase in liver). Irreversible committed step. 2. Isomerization: G6P $\rightarrow$ Fructose-6-phosphate (F6P) by Phosphoglucose isomerase. 3. Phosphorylation: F6P $\rightarrow$ Fructose-1,6-bisphosphate (F1,6BP) by Phosphofructokinase-1 (PFK-1). Key rate-limiting regulatory step. Uses second ATP. 4. Cleavage: F1,6BP is split into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (GAP) by Aldolase. 5. Isomerization: DHAP $\rightarrow$ GAP by Triose phosphate isomerase.

• Payoff Phase (energy generating, produces $4\text{ ATP}$ and $2\text{ NADH}$):
• Oxidation: GAP $\rightarrow$ 1,3-Bisphosphoglycerate (1,3-BPG) by GAP Dehydrogenase. Generates $2\text{ NADH}$.
• Substrate-level Phosphorylation: 1,3-BPG + ADP $\rightarrow$ 3-Phosphoglycerate + ATP by Phosphoglycerate kinase.
• Shift: 3-Phosphoglycerate $\rightarrow$ 2-Phosphoglycerate by Phosphoglycerate mutase.
• Dehydration: 2-Phosphoglycerate $\rightarrow$ Phosphoenolpyruvate (PEP) by Enolase.
• Substrate-level Phosphorylation: PEP + ADP $\rightarrow$ Pyruvate + ATP by Pyruvate Kinase. Irreversible step.

Net Yield: $\text{Glucose} + 2\text{ NAD}^+ + 2\text{ ADP} + 2\text{ P}_i \rightarrow 2\text{ Pyruvate} + 2\text{ NADH} + 2\text{ H}^+ + 2\text{ ATP} + 2\text{ H}_2\text{O}$

2. The Citric Acid Cycle (Krebs / TCA Cycle)

Under aerobic conditions, pyruvate enters the mitochondria. First, it is oxidatively decarboxylated to Acetyl-CoA by the Pyruvate Dehydrogenase (PDH) Complex, releasing $CO_2$ and generating $NADH$.

The TCA cycle occurs in the mitochondrial matrix. It oxidizes Acetyl-CoA ($2\text{C}$) to $2\text{ CO}_2$, conserving energy as $NADH$, $FADH_2$, and $GTP$.

The Eight Steps

1. Condensation: Acetyl-CoA ($2\text{C}$) + Oxaloacetate ($4\text{C}$) $\rightarrow$ Citrate ($6\text{C}$) by Citrate Synthase.
2. Isomerization: Citrate $\rightarrow$ Isocitrate by Aconitase.
3. Oxidative Decarboxylation: Isocitrate $\rightarrow$ $\alpha$-Ketoglutarate by Isocitrate Dehydrogenase. Rate-limiting step. Releases $CO_2$, generates $NADH$.
4. Oxidative Decarboxylation: $\alpha$-Ketoglutarate $\rightarrow$ Succinyl-CoA by $\alpha$-Ketoglutarate Dehydrogenase Complex. Releases $CO_2$, generates $NADH$.
5. Substrate-level Phosphorylation: Succinyl-CoA $\rightarrow$ Succinate by Succinyl-CoA synthetase. Generates $GTP$ (which converts to $ATP$).
6. Dehydrogenation: Succinate $\rightarrow$ Fumarate by Succinate Dehydrogenase (located on inner mitochondrial membrane; acts as Complex II of ETC). Generates $FADH_2$.
7. Hydration: Fumarate $\rightarrow$ Malate by Fumarase.
8. Dehydrogenation: Malate $\rightarrow$ Oxaloacetate by Malate Dehydrogenase. Generates $NADH$.

Yield per Acetyl-CoA: $3\text{ NADH} + 1\text{ FADH}_2 + 1\text{ GTP} + 2\text{ CO}_2$. (Double this for one glucose molecule).

3. Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (lactate, glycerol, glucogenic amino acids, propionate). It occurs primarily in the liver (and some in the kidneys) during fasting to maintain blood glucose levels.

It is NOT a simple reversal of glycolysis. It bypasses the three irreversible glycolytic steps using four specialized enzymes:

1. Bypass of Pyruvate Kinase: Pyruvate is carboxylated to Oxaloacetate by Pyruvate Carboxylase (mitochondrial, requires biotin and ATP). Oxaloacetate is then converted to PEP by PEP Carboxykinase (PEPCK) (cytosolic, requires GTP).
2. Bypass of PFK-1: Fructose-1,6-bisphosphate is converted to Fructose-6-phosphate by Fructose-1,6-bisphosphatase-1 (FBPase-1).
3. Bypass of Hexokinase: Glucose-6-phosphate is hydrolyzed to free Glucose by Glucose-6-phosphatase (located in the endoplasmic reticulum membrane).

Energetic Cost: Synthesizing 1 glucose from 2 pyruvales requires $4\text{ ATP}$, $2\text{ GTP}$, and $2\text{ NADH}$.

4. Glycogen Metabolism

Glycogen is a highly branched polymer of glucose stored in liver and muscle.

• Glycogenesis (Synthesis): Glucose $\rightarrow$ G6P $\rightarrow$ G1P. G1P reacts with UTP to form active UDP-Glucose. Glycogen Synthase transfers glucose from UDP-glucose to form $\alpha(1\rightarrow4)$ linkages. Key rate-limiting enzyme. Branching Enzyme transfers segments of 7 residues to create $\alpha(1\rightarrow6)$ branch points.

• Glycogenolysis (Breakdown): Glycogen Phosphorylase uses inorganic phosphate to cleave $\alpha(1\rightarrow4)$ bonds, releasing Glucose-1-phosphate. Key rate-limiting enzyme. Debranching Enzyme transfers remaining glucose chains at a branch and hydrolyzes the final $\alpha(1\rightarrow6)$ linkage, releasing free glucose. G1P is converted to G6P by phosphoglucomutase. In the liver, G6P is converted to glucose by Glucose-6-phosphatase to raise blood sugar. Muscle lacks this enzyme; G6P enters glycolysis directly for energy.

5. Pentose Phosphate Pathway (PPP)

Also known as the Hexose Monophosphate (HMP) Shunt, the PPP takes place in the cytosol. It does not generate ATP.

Functions

• Production of NADPH: Used as reducing equivalent for anabolic pathways (fatty acid, cholesterol synthesis) and keeping glutathione reduced to prevent oxidative damage.
• Production of Ribose-5-phosphate: Used for nucleotide and nucleic acid synthesis.

Phases

1. Oxidative Phase (Irreversible): G6P is oxidized by Glucose-6-Phosphate Dehydrogenase (G6PDH) (rate-limiting step) and subsequent enzymes, generating $2\text{ NADPH}$ and $CO_2$, yielding Ribulose-5-phosphate.
2. Non-Oxidative Phase (Reversible): Ribulose-5-phosphate is converted into Ribose-5-phosphate or recycled into glycolytic intermediates (Fructose-6-phosphate and Glyceraldehyde-3-phosphate) via transketolase and transaldolase.

6. Regulation of Carbohydrate Metabolism

Metabolism is regulated coordinately by allosteric effectors and hormones (insulin, glucagon, epinephrine).

Hormonal Regulation

• Insulin (high blood glucose): Promotes glucose uptake, glycolysis, glycogenesis, and lipogenesis. Dephosphorylates and activates glycogen synthase and PFK-1 (via fructose-2,6-bisphosphate).
• Glucagon (low blood glucose, liver only) / Epinephrine (stress, muscle & liver): Activates adenylate cyclase to raise cAMP, activating PKA. PKA phosphorylates and inactivates Glycogen Synthase, while activating Glycogen Phosphorylase (promoting glycogenolysis) and FBPase-2 (inhibiting glycolysis, promoting gluconeogenesis).

Allosteric Control of PFK-1 vs FBPase-1

PFK-1 is activated by $AMP$ and Fructose-2,6-bisphosphate (F2,6BP) (the most potent regulator), and inhibited by $ATP$ and $Citrate$. FBPase-1 is inhibited by $AMP$ and $F2,6BP$. This prevents futile cycling.