---

Integrated Metabolism: Carbohydrates, Proteins, and Lipids

THE CORE CONCEPT

• Pyruvate - crossroads of carbohydrate, amino acid, and lactate metabolism
• Acetyl-CoA - final common pathway entry into the TCA cycle; also precursor for ketone bodies and fatty acid synthesis
• Oxaloacetate (OAA) - TCA cycle intermediate; also a gluconeogenic substrate
• Alpha-ketoglutarate (alpha-KG) - links amino acid catabolism to TCA cycle
• Glucose-6-phosphate - branch point for glycolysis, glycogen synthesis, and pentose phosphate pathway

---

SECTION 1: THE TCA (KREBS/CITRIC ACID) CYCLE - THE CENTRAL HUB

Entry points into TCA cycle

Exam tip (NEET PG/INICET/USMLE): Leucine and Lysine are the ONLY purely ketogenic amino acids (no net glucose production). All others are either purely glucogenic or mixed.

---

SECTION 2: THE MAJOR INTERCONNECTING CYCLES

2A. THE CORI CYCLE (Lactic Acid Cycle)

1. Muscle and RBCs metabolize glucose via glycolysis -> lactate (anaerobic; RBCs have no mitochondria)
2. Lactate is exported into blood
3. Liver takes up lactate -> converts to pyruvate (lactate dehydrogenase)
4. Pyruvate enters gluconeogenesis -> glucose
5. Glucose re-enters blood for use by muscle/RBCs

• Muscle generates 2 ATP from glucose -> lactate
• Liver uses 6 ATP to convert lactate -> glucose (gluconeogenesis)
• Net: energy transfer from liver to muscle (liver subsidizes anaerobic muscle work)
• Neither cycle yields NEW carbon skeletons - carbon is recycled

---

2B. THE GLUCOSE-ALANINE CYCLE (Cahill Cycle)

1. Muscle proteins are catabolized during fasting -> amino acids
2. Amino groups transferred to pyruvate (from glycolysis) via alanine aminotransferase (ALT/SGPT) -> alanine + alpha-ketoglutarate
3. Alanine exported from muscle into blood
4. Liver takes up alanine -> transamination back to pyruvate + glutamate
5. Pyruvate -> gluconeogenesis -> glucose (released back to blood)
6. Glutamate -> urea cycle -> urea excreted by kidney

• Provides gluconeogenic substrate to liver (carbon)
• Transfers nitrogen from muscle to liver for urea synthesis (nitrogen detoxification)
• Uses alanine as a non-toxic nitrogen carrier (unlike free ammonia)

• Cori cycle: energy transfer only
• Glucose-alanine cycle: energy transfer + nitrogen transfer

Exam memory hook: ALT (alanine aminotransferase) is elevated in liver disease - this is the enzyme that converts alanine to pyruvate (and vice versa) in the glucose-alanine cycle. ALT is a liver-specific marker because the liver is where this transamination is most active.

---

2C. GLUCONEOGENESIS - THE MASTER INTEGRATION POINT

USMLE/NEET PG key: Von Gierke disease = glucose-6-phosphatase deficiency -> cannot release free glucose from liver -> severe fasting hypoglycemia + hepatomegaly + lactic acidosis.

---

SECTION 3: LIPID-CARBOHYDRATE INTEGRATION

Fatty Acid Synthesis from Carbohydrates

1. Glucose -> Glycolysis -> Pyruvate
2. Pyruvate -> Acetyl-CoA (pyruvate dehydrogenase, mitochondrial)
3. Acetyl-CoA condenses with OAA -> Citrate
4. Citrate shuttle: Citrate exits mitochondria -> cytoplasm
5. ATP-citrate lyase cleaves citrate -> Acetyl-CoA (cytoplasm) + OAA
6. Cytoplasmic Acetyl-CoA is used for fatty acid synthesis (acetyl-CoA carboxylase -> malonyl-CoA -> palmitate)

• Acetyl-CoA (from beta-oxidation of even-chain fatty acids) CANNOT be converted to pyruvate or OAA NET
• Acetyl-CoA enters TCA but the 2 carbons added are lost as 2 CO2 per turn
• Therefore: fat cannot contribute net carbons to gluconeogenesis (except glycerol and odd-chain fatty acids)
• Glyoxylate cycle allows this in plants/bacteria/yeast (isocitrate lyase + malate synthase) but HUMANS LACK THIS CYCLE

High-yield exam point: This is why prolonged starvation/DKA leads to muscle protein catabolism - fat alone cannot maintain glucose levels for the brain, so protein must be broken down.

Ketone Body Formation (Ketogenesis)

1. Lipolysis in adipocytes -> FFAs released into blood
2. Liver beta-oxidation: FFAs -> Acetyl-CoA
3. With low carbohydrate intake: OAA is depleted (pulled into gluconeogenesis)
4. Acetyl-CoA cannot enter TCA (no OAA partner) -> accumulates
5. 2 Acetyl-CoA -> Acetoacetyl-CoA -> HMG-CoA (HMG-CoA synthase) -> Acetoacetate + beta-hydroxybutyrate + Acetone
6. Ketone bodies exported to brain, heart, muscle as alternative fuel

• Glucagon activates PEPCK -> OAA -> PEP (for gluconeogenesis)
• OAA is depleted from TCA, leaving acetyl-CoA "stranded"
• This is the biochemical basis of ketosis

---

SECTION 4: PROTEIN-LIPID INTEGRATION

Lipogenic amino acids

• Glucogenic amino acids -> glucose -> can be stored as glycogen or converted to fat via Acetyl-CoA
• Ketogenic amino acids -> Acetyl-CoA -> directly enter FA synthesis or ketogenesis

Cholesterol synthesis

• Acetyl-CoA (from all three macronutrients) -> HMG-CoA (cytoplasmic) -> Mevalonate -> Cholesterol
• Rate-limiting enzyme: HMG-CoA reductase (target of statins)
• HMG-CoA has two fates:
  • Cytoplasmic HMG-CoA -> cholesterol (via HMG-CoA reductase)
  • Mitochondrial HMG-CoA -> ketone bodies (via HMG-CoA lyase)

Amino acids as lipid precursors

• Serine -> Phospholipids (phosphatidylserine)
• Glycine + Succinyl-CoA -> Heme synthesis
• Methionine -> SAM -> methylation of lipids (phosphatidylcholine)

---

SECTION 5: THE UREA CYCLE AND ITS INTEGRATION

1. Aspartate (from OAA + glutamate via transamination) donates a nitrogen to argininosuccinate in the urea cycle
2. Fumarate is released from argininosuccinate -> enters TCA cycle directly
3. TCA: Fumarate -> Malate -> OAA
4. OAA + glutamate -> Aspartate + alpha-KG (regenerating aspartate for next urea cycle turn)

---

SECTION 6: METABOLIC STATE-BASED INTEGRATION

Fed State (Post-prandial, Insulin dominant)

• Key enzyme activated: Acetyl-CoA carboxylase (FA synthesis), glycogen synthase, pyruvate kinase
• Key enzyme inhibited: Hormone-sensitive lipase, gluconeogenesis enzymes

Fasting State (Post-absorptive, Glucagon dominant)

Prolonged Starvation

1. 0-4 hr: Glycogenolysis
2. 4-16 hr: Gluconeogenesis from lactate, alanine, glycerol
3. 16 hr - 2 days: Gluconeogenesis from muscle protein (major); fat oxidation + ketogenesis intensifies
4. 3-7 days onwards: Brain adapts to ketones -> spares muscle protein (reduces gluconeogenesis demand)

---

SECTION 7: HORMONAL INTEGRATION

---

SECTION 8: CLINICAL CORRELATIONS (High-yield for NEET PG/USMLE/INICET)

1. Diabetic Ketoacidosis (DKA)

• Pathophysiology: Absolute insulin deficiency (Type 1 DM) -> glucagon unopposed
  • Lipolysis maximal -> FFAs flood liver
  • Gluconeogenesis maximal -> OAA depleted into PEP
  • Acetyl-CoA accumulates (no OAA for TCA) -> ketone body formation
  • Result: hyperglycemia + ketonemia + metabolic acidosis (HAGMA)
• Signs: Kussmaul respirations (compensatory respiratory alkalosis to blow off CO2), acetone breath, osmotic diuresis, dehydration
• Lab: Urine nitroprusside test detects acetoacetate (NOT beta-hydroxybutyrate - the predominant ketone in blood); low bicarbonate; elevated anion gap
• Tx: Insulin + IV fluids + potassium correction

2. Starvation Ketosis vs. DKA

3. Von Gierke Disease (Type Ia Glycogen Storage Disease)

• Defect: Glucose-6-phosphatase (liver, kidney, intestine)
• Cannot release free glucose from G6P -> cannot complete gluconeogenesis or glycogenolysis
• Features: Severe fasting hypoglycemia, hepatomegaly, lactic acidosis (lactate cannot be cleared), hyperuricemia (Cori cycle backs up -> increased lactate -> competes with urate for renal excretion), hyperlipidemia

4. McArdle Disease (Type V GSD)

• Defect: Muscle phosphorylase
• Cannot use muscle glycogen -> exercise intolerance, myoglobinuria
• Lactate does NOT rise with exercise (no lactate from muscle) -> used in forearm ischemic exercise test

5. Hyperammonemia / Urea Cycle Defects

• Ornithine transcarbamylase (OTC) deficiency: most common urea cycle defect (X-linked)
• Accumulation of ammonia -> neurological damage (ammonia inhibits alpha-KG -> blocks TCA -> CNS energy failure)
• Treatment: low-protein diet, nitrogen scavengers (sodium benzoate binds glycine; sodium phenylacetate binds glutamine)

6. Phenylketonuria (PKU)

• Defect: Phenylalanine hydroxylase (converts Phe -> Tyr)
• Phe accumulates -> converted to phenylpyruvate, phenylacetate, phenyllactate
• Blocks aromatic amino acid metabolism; competes with large neutral amino acids for brain transport
• Mental retardation, mousy/musty odor, hypopigmentation (reduced tyrosine -> less melanin)
• Treatment: phenylalanine-restricted diet; BH4 supplementation for mild forms

7. Maple Syrup Urine Disease (MSUD)

• Defect: Branched-chain alpha-keto acid dehydrogenase
• Accumulation of leucine, isoleucine, valine (and their keto acids)
• Maple syrup odor in urine, CNS toxicity, neonatal encephalopathy
• Leucine is most toxic (ketogenic -> excess ketones)

8. Methylmalonic Acidemia

• Defect: Methylmalonyl-CoA mutase (requires adenosyl-B12/cobalamin as cofactor)
• Propionyl-CoA -> methylmalonyl-CoA cannot be converted to succinyl-CoA -> cannot enter TCA
• Features: metabolic acidosis, hyperammonemia, ketosis, hypoglycemia
• Odd-chain fatty acids, isoleucine, valine, methionine, threonine are all affected (all feed into propionyl-CoA)

9. Alcoholic Ketoacidosis

• Ethanol metabolism produces excess NADH (alcohol dehydrogenase + aldehyde dehydrogenase)
• High NADH ratio: shifts OAA -> malate, pyruvate -> lactate
• OAA depleted -> TCA slows -> Acetyl-CoA accumulates -> ketogenesis
• Glucose does NOT rise (unlike DKA) because gluconeogenesis is also impaired (pyruvate/OAA depleted)
• Features: ketosis + lactic acidosis + hypoglycemia (no hyperglycemia)

10. Statin-Induced Myopathy

• HMG-CoA reductase inhibited -> blocks cholesterol synthesis
• Also depletes CoQ10 (ubiquinone, part of mevalonate pathway) -> mitochondrial dysfunction in muscle -> myopathy/rhabdomyolysis

11. Obesity and Metabolic Syndrome

• Chronic excess caloric intake: excess Acetyl-CoA -> fatty acid synthesis -> triglyceride storage
• Insulin resistance: hyperinsulinemia, hyperglycemia, dyslipidemia (high TG, low HDL, high LDL)
• Ectopic fat deposition in liver -> NAFLD/NASH

---

SECTION 9: PENTOSE PHOSPHATE PATHWAY INTEGRATION

• G6P can go to pentose phosphate pathway (PPP) instead of glycolysis
• PPP generates:
  • NADPH: for FA synthesis, glutathione reductase (RBC oxidative defense), cytochrome P450
  • Ribose-5-phosphate: for nucleotide synthesis
• G6PD deficiency: most common enzyme deficiency worldwide (X-linked); RBCs cannot regenerate NADPH -> oxidative hemolysis triggered by drugs (primaquine, dapsone), infections, fava beans
• Clinical: Heinz bodies (denatured Hb) + bite cells on smear; normal between episodes

---

SECTION 10: SUMMARY TABLE - KEY REGULATORY ENZYMES

---

QUICK REVISION - EXAM MEMORY AIDS

• Even-chain fatty acids (Acetyl-CoA has no net contribution)
• Leucine, Lysine (purely ketogenic)
• Ethanol carbon directly

• ALL amino acids EXCEPT Leu and Lys
• Lactate, Pyruvate, Alanine
• Glycerol (from fat hydrolysis)
• Propionate/Odd-chain fatty acids (via succinyl-CoA)

1. Glycogenolysis (0-6 hrs)
2. Gluconeogenesis from lactate/alanine (6-24 hrs)
3. Gluconeogenesis from muscle protein + Ketogenesis (1-3 days)
4. Ketone adaptation by brain (3+ days) -> protein sparing

• Fat CANNOT make net glucose (even-chain FA) - no glyoxylate cycle in humans
• RBCs CANNOT do beta-oxidation (no mitochondria)
• Muscle CANNOT release free glucose (no glucose-6-phosphatase)
• Brain CANNOT use fatty acids directly (FFA don't cross BBB well; uses glucose and after adaptation, ketones)
• Kidney CAN do gluconeogenesis (has glucose-6-phosphatase) - particularly during acidosis (uses glutamine)

---