22.5 Tissue-Specific Biochemistry & Metabolic Integration

Tissue-Specific Biochemistry & Metabolic Integration

Metabolism is not uniform across the body. Each tissue expresses a characteristic enzyme suite that fits its function, and endocrine signals coordinate fuel storage, release, and utilization across fed, fasted, and exercise states. The PA-CAT Bulletin of Information, rev. 20240815 lists biochemistry of tissues as a Biochemistry topic; this section integrates pathways from earlier chapters (glycolysis, TCA, β-oxidation, urea cycle, ketogenesis) into a whole-body view.

Liver — The Metabolic Hub

The liver is the central exchange for carbohydrate, lipid, and nitrogen metabolism. In glycogen metabolism, it stores glycogen (~100 g) and releases glucose via glycogen phosphorylase and debranching enzyme. Crucially, hepatic glucose-6-phosphatase — absent in muscle — frees glucose into blood, so the liver is the main source of blood glucose during short fasting. The liver performs gluconeogenesis from lactate (Cori cycle), glycerol, alanine (glucose-alanine cycle), and glucogenic amino acids; the key enzymes are PEPCK, fructose-1,6-bisphosphatase, and glucose-6-phosphatase, induced by glucagon and cortisol and suppressed by insulin.

In lipid metabolism, the liver synthesizes fatty acids from excess carbohydrate (acetyl-CoA carboxylase, fatty acid synthase), esterifies them into triglycerides, and packages VLDL. It performs ketogenesis from fatty-acid-derived acetyl-CoA when carbohydrate is scarce: HMG-CoA synthase (mitochondrial) and HMG-CoA lyase yield acetoacetate and β-hydroxybutyrate, fuel for brain, heart, and muscle during fasting. (Note: HMG-CoA reductase in the cytosol governs cholesterol synthesis; HMG-CoA synthase in mitochondria governs ketogenesis — same intermediate, different fates.)

The liver runs the urea cycle (carbamoyl phosphate synthetase I, ornithine transcarbamoylase, argininosuccinate synthetase, argininosuccinate lyase, arginase), disposing of ammonia from amino acid catabolism; defects cause hyperammonemia. It synthesizes most plasma proteins (albumin, clotting factors — vitamin K dependent), and carries out detoxification via cytochrome P450 oxidation and phase II conjugation (glucuronidation, sulfation, glutathione, acetylation).

Skeletal and Cardiac Muscle

Skeletal muscle stores glycogen (~400 g) but lacks glucose-6-phosphatase, so its glycogen is for local use — phosphorylated glucose enters glycolysis within the myocyte. At rest, muscle uses fatty acids; during sprint, it relies on phosphocreatine and anaerobic glycolysis, producing lactate that the liver reconverts to glucose (Cori cycle). During prolonged exercise, fatty acid β-oxidation dominates. Muscle takes up glucose via GLUT4 in an insulin-dependent manner.

Cardiac muscle is oxidative and continuously active, preferring fatty acids (60–90% of energy), with ketones and lactate as additional substrates. It has abundant mitochondria and little glycogen reserve. Because it cannot tolerate oxygen debt, ischemia rapidly shifts it to anaerobic glycolysis with lactate accumulation — a biochemical signature of myocardial infarction. The heart expresses GLUT1 and GLUT4, with GLUT4 translocation driven by both insulin and contraction (via AMPK).

Adipose Tissue

White adipose tissue is the long-term fuel depot. In the fed state, insulin induces GLUT4 translocation (glucose uptake), drives lipogenesis, and upregulates lipoprotein lipase on the capillary endothelium, which hydrolyzes chylomicron and VLDL triglycerides so free fatty acids enter the adipocyte for esterification. Insulin also inhibits hormone-sensitive lipase (via dephosphorylation).

In fasting, glucagon has little effect on human adipocytes; the dominant lipolytic signals are catecholamines (β-adrenergic, Gs-cAMP-PKA) and growth hormone and cortisol. PKA phosphorylates hormone-sensitive lipase and perilipin, freeing fatty acids and glycerol into blood. Glycerol cannot be reused by adipocytes (no glycerol kinase), so it travels to liver for gluconeogenesis — a key link between fat and carbohydrate metabolism. Brown adipose tissue expresses uncoupling protein 1 (UCP1), which dissipates the proton gradient as heat — non-shivering thermogenesis, important in neonates and hibernators.

Brain and Erythrocytes

The brain consumes ~120 g glucose per day at rest and ~20% of total body oxygen. It cannot use fatty acids (they do not cross the blood-brain barrier well) and depends on glucose, with GLUT1 at the blood-brain barrier and GLUT3 on neurons. After ~3–5 days of fasting, the brain shifts partly to ketone bodies (acetoacetate, β-hydroxybutyrate), which cross via monocarboxylate transporters; after prolonged fasting, ketones supply up to two-thirds of brain energy, sparing glucose and reducing muscle protein catabolism.

Mature erythrocytes lack mitochondria and so cannot perform oxidative phosphorylation, β-oxidation, or the TCA cycle. They rely entirely on anaerobic glycolysis (2 ATP per glucose) with lactate as the end product, and on the pentose phosphate pathway to generate NADPH, which maintains glutathione in the reduced state and protects the red cell from oxidative damage. Glucose-6-phosphate dehydrogenase deficiency is the most common enzymopathy and causes hemolytic anemia on oxidative stress (fava beans, sulfa drugs, infections) because NADPH cannot regenerate reduced glutathione.

Fed, Fasted, and Exercise States

In the fed (absorptive) state, elevated insulin drives glycogen synthesis (liver and muscle), glycolysis, fatty acid and triglyceride synthesis (liver and adipose), and protein synthesis; GLUT4 translocation increases muscle/adipose glucose uptake. Insulin activates phosphofructokinase-2 (raises F-2,6-BP, stimulating glycolysis and inhibiting gluconeogenesis) and acetyl-CoA carboxylase (malonyl-CoA inhibits carnitine palmitoyltransferase-1, shutting off β-oxidation when energy is plentiful).

In the fasted state, glucagon (and low insulin) activates hepatic glycogen phosphorylase (via cAMP-PKA), induces PEPCK and fructose-1,6-bisphosphatase for gluconeogenesis, inhibits acetyl-CoA carboxylase (lower malonyl-CoA releases CPT-1, enabling β-oxidation and ketogenesis), and drives hormone-sensitive lipase in adipose via catecholamines. Cortisol supports gluconeogenesis by inducing muscle protein catabolism (alanine export) and adipose lipolysis.

During exercise, AMPK (activated by rising AMP/ATP) stimulates glucose uptake (GLUT4 translocation independent of insulin), fatty acid oxidation (inhibits ACC, lowering malonyl-CoA), and mitochondrial biogenesis; epinephrine drives glycogenolysis and lipolysis. Creatine kinase buffers ATP in muscle by transferring phosphate from phosphocreatine to ADP.

Metabolic Regulation and Sensing

Key control points: PFK-1 (rate-limiting for glycolysis; activated by F-2,6-BP and AMP; inhibited by ATP, citrate), fructose-1,6-bisphosphatase (rate-limiting for gluconeogenesis; activated by citrate; inhibited by F-2,6-BP and AMP), isocitrate dehydrogenase (rate-limiting TCA; activated by ADP, Ca2+; inhibited by ATP, NADH), acetyl-CoA carboxylase (rate-limiting fatty acid synthesis; activated by insulin/citrate; inhibited by glucagon/AMPK/palmitoyl-CoA), and carnitine palmitoyltransferase-1 (rate-limiting β-oxidation; inhibited by malonyl-CoA). mTOR senses amino acids and insulin to drive anabolism; AMPK senses energy stress to drive catabolism; SIRT1 senses NAD+/NADH to adapt to fasting. These sensors integrate signals across tissues and are increasingly tested at the biochemical level on the PA-CAT.