13.2 Gluconeogenesis, Glycogen, Lipogenesis, Ketogenesis & Hormonal Control of Fuel Metabolism

Why this area matters

The TOS carbohydrate area includes "ketogenesis, lipidogenesis and disorders" and "diseases of hormonal metabolism". These mechanisms explain fasting hypoglycemia in infants, diabetic ketoacidosis, the fatty liver of insulin resistance and the glycogen storage diseases. They also connect to Medicine (diabetes), Pediatrics (inborn errors) and Pharmacology (insulin and oral agents).

Gluconeogenesis: The Four Irreversible Thermodynamic Bypasses

Gluconeogenesis maintains blood glucose during prolonged fasting ($>12\text{ hours}$), primarily in hepatocytes and to a lesser extent renal cortex tubular cells. Gluconeogenesis utilizes lactate, glycerol, and glucogenic amino acids (chiefly alanine). Gluconeogenesis circumvents the three irreversible glycolytic steps using four dedicated enzymes:

1. Pyruvate Carboxylase (Mitochondria): Converts pyruvate to oxaloacetate ($OAA$). Consumes $1\text{ ATP}$. Requires Biotin (Vitamin B7) and $CO_2$. Obligatory allosteric activator: Acetyl-CoA (signals abundant lipid breakdown).
2. Phosphoenolpyruvate Carboxykinase (PEPCK, Cytosol): Converts $OAA$ to phosphoenolpyruvate ($PEP$). Consumes $1\text{ GTP}$ and releases $CO_2$. Induced by glucagon and cortisol.
3. Fructose-1,6-Bisphosphatase-1 (Cytosol): Hydrolyzes F-1,6-BP to Fructose-6-phosphate. Rate-limiting enzyme of gluconeogenesis. Allosterically inhibited by AMP and F-2,6-BP; stimulated by citrate.
4. Glucose-6-Phosphatase (Endoplasmic Reticulum): Dephosphorylates G6P to free glucose in hepatocytes and enterocytes. Skeletal muscle lacks this enzyme, which prevents muscle glycogen from releasing free glucose into systemic circulation.

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Glycogen Metabolism and Glycogen Storage Diseases (GSDs)

Glycogen is a branched polymer of glucose featuring $\alpha(1\to 4)$ glycosidic linkages with $\alpha(1\to 6)$ branches every 8 to 12 residues. Glycogen synthesis is catalyzed by Glycogen Synthase (activated by insulin via dephosphorylation), whereas breakdown is catalyzed by Glycogen Phosphorylase (activated by glucagon, epinephrine, and AMP via phosphorylation).

Clinical Profiles of High-Yield Glycogen Storage Diseases

The Cori cycle and the glucose-alanine cycle

• Cori cycle: red cells and exercising muscle produce lactate. The liver converts it back to glucose by gluconeogenesis, at a net cost of 4 ATP per glucose.
• Glucose-alanine cycle: muscle transfers amino nitrogen to pyruvate to make alanine. The liver uses the carbon for glucose and the nitrogen for urea.

Ethanol raises the hepatic NADH-to-NAD+ ratio, which pushes pyruvate to lactate and oxaloacetate to malate. This blocks gluconeogenesis, so alcohol can cause fasting hypoglycemia and lactic acidosis in malnourished drinkers.

Fatty Acid Biosynthesis: The Citrate Shuttle and Acetyl-CoA Carboxylase

Fatty acid synthesis occurs predominantly in the cytoplasm of hepatocytes, lactating mammary glands, and adipose tissue during energy surplus. It converts excess carbohydrate-derived acetyl-CoA into saturated fatty acids, primarily palmitate ($16\text{C}$).

The Citrate Shuttle

Acetyl-CoA generated inside the mitochondrial matrix cannot cross the impermeable inner mitochondrial membrane. To exit into the cytosol, acetyl-CoA condenses with oxaloacetate to form citrate, catalyzed by mitochondrial citrate synthase. When intramitochondrial energy charge is high (ATP and NADH elevated), isocitrate dehydrogenase is inhibited, causing citrate to accumulate and exit via the tricarboxylate carrier into the cytosol (Mnemonic: "SYnthesis needs CITrate").

In the cytoplasm, ATP-Citrate Lyase cleaves citrate back into acetyl-CoA and oxaloacetate, consuming $1\text{ ATP}$:

$\text{Citrate} + \text{ATP} + \text{CoA-SH} + \text{H}_2\text{O} \xrightarrow{\text{ATP-Citrate Lyase}} \text{Acetyl-CoA} + \text{Oxaloacetate} + \text{ADP} + P_i$

Cytosolic oxaloacetate is reduced to malate by malate dehydrogenase. Malate is then converted to pyruvate by Malic Enzyme, generating $1\text{ NADPH}$ per cycle. Pyruvate re-enters the mitochondrion.

Acetyl-CoA Carboxylase (ACC): The Rate-Limiting Control Point

Cytosolic acetyl-CoA is carboxylated to malonyl-CoA ($3\text{C}$), catalyzed by Acetyl-CoA Carboxylase (ACC). This is the committed, rate-limiting reaction of lipogenesis:

$\text{Acetyl-CoA} + \text{HCO}_3^- + \text{ATP} \xrightarrow{\text{ACC}} \text{Malonyl-CoA} + \text{ADP} + P_i$

• Cofactor: Requires Biotin (Vitamin B7), ATP, and $CO_2$ (bicarbonate).
• Allosteric Activation: Citrate stimulates polymer formation of ACC subunits.
• Allosteric Inhibition: Long-chain fatty acyl-CoA (e.g., palmitoyl-CoA) feeds back to depolymerize and inhibit ACC.
• Hormonal Regulation: Insulin promotes protein phosphatase-2A activity, which dephosphorylates and activates ACC. Glucagon and epinephrine stimulate AMP-activated protein kinase (AMPK), which phosphorylates and inactivates ACC.

Fatty Acid Synthase (FAS) Multi-Enzyme Complex

The homodimeric Fatty Acid Synthase (FAS) complex contains an Acyl Carrier Protein (ACP) arm carrying a 4'-phosphopantetheine prosthetic group (derived from Vitamin B5). Sequential additions of two-carbon units from malonyl-CoA, driven by decarboxylation and consuming $2\text{ NADPH}$ per elongation cycle, elongate the chain until the $16\text{C}$ saturated fatty acid palmitate is liberated by palmitoyl thioesterase. The required NADPH is supplied by the pentose phosphate pathway (G6PD) and cytosolic malic enzyme.

Hepatic Ketogenesis and Diabetic Ketoacidosis (DKA) Biochemistry

When $\beta$-oxidation generates immense quantities of acetyl-CoA during prolonged fasting or untreated Type 1 Diabetes Mellitus, oxaloacetate is consumed by gluconeogenesis. Excess acetyl-CoA is redirected into ketogenesis, occurring exclusively in hepatic mitochondria.

Enzymatic Cascade of Ketogenesis

1. Two acetyl-CoA molecules condense to acetoacetyl-CoA via mitochondrial thiolase.
2. Acetoacetyl-CoA condenses with a third acetyl-CoA to form $\beta$-hydroxy-$\beta$-methylglutaryl-CoA (HMG-CoA), catalyzed by Mitochondrial HMG-CoA Synthase (the rate-limiting enzyme of ketogenesis).
3. HMG-CoA lyase cleaves HMG-CoA to yield Acetoacetate and acetyl-CoA.
4. Acetoacetate is either reduced to $\beta$-Hydroxybutyrate by $\beta$-hydroxybutyrate dehydrogenase (consuming NADH) or spontaneously decarboxylates to volatile Acetone (causing sweet, fruity breath).

In severe ketoacidosis, a high mitochondrial $\text{NADH}/\text{NAD}^+$ ratio shifts the equilibrium heavily toward $\beta$-hydroxybutyrate. Standard nitroprusside urine test strips detect only acetoacetate, often underestimating the true severity of acidosis.

Peripheral Utilization and Hepatic Non-Utilization

Ketone bodies diffuse into the bloodstream to fuel extrahepatic tissues (cardiac muscle, skeletal muscle, renal cortex, and the brain during prolonged starvation). Peripheral tissues utilize $\beta$-Ketoacyl-CoA Transferase (Thiophorase) to transfer CoA from succinyl-CoA to acetoacetate, generating acetoacetyl-CoA. Because hepatocytes lack thiophorase, the liver cannot utilize the ketone bodies it synthesizes, ensuring complete export to peripheral tissues.

Hormonal control: fed, fasting and starvation

Insulin actions:

• stimulates glycogen synthase, PFK-2, acetyl-CoA carboxylase, lipoprotein lipase and protein synthesis;
• inhibits glycogen phosphorylase, gluconeogenic enzymes, hormone-sensitive lipase and ketogenesis;
• drives potassium into cells, which is why insulin treats hyperkalemia.

Counter-regulatory hormones (glucagon, epinephrine, cortisol, growth hormone) do the opposite. Glucagon acts mainly on the liver through cAMP and protein kinase A.

Disorders of hormonal fuel metabolism

• Type 1 diabetes and diabetic ketoacidosis: absolute insulin deficiency plus high glucagon causes uncontrolled lipolysis, hepatic ketogenesis and gluconeogenesis.
  • Hyperglycemia leads to osmotic diuresis and dehydration.
  • Ketoacids cause a high anion gap acidosis with Kussmaul breathing.
  • Total-body potassium is depleted even when serum K+ looks normal or high, so potassium must be replaced as insulin is given.
• Type 2 diabetes: insulin resistance with relative deficiency. Enough insulin remains to suppress ketogenesis, so the typical crisis is the hyperosmolar hyperglycemic state.
• Insulinoma: fasting hypoglycemia with high insulin and high C-peptide. Exogenous (factitious) insulin gives high insulin with low C-peptide. Sulfonylurea misuse gives high C-peptide and a positive drug screen.
• Cushing syndrome, acromegaly and pheochromocytoma cause secondary hyperglycemia through their counter-regulatory hormones.
• Hypoglycemia in infants:
  • von Gierke disease: with lactic acidosis and hepatomegaly;
  • MCAD deficiency: hypoketotic hypoglycemia;
  • hyperinsulinism: no ketones, needs a high glucose infusion;
  • hormone deficiency.

Biotin deficiency (from eating many raw egg whites, which contain avidin) impairs the carboxylases: pyruvate carboxylase, acetyl-CoA carboxylase and propionyl-CoA carboxylase.