18.4 Blood Chemistry: Heme Metabolism, Hemoglobin & Plasma Proteins

18.4 Blood Chemistry: Heme Metabolism, Hemoglobin & Plasma Proteins

The biochemistry outline lists blood chemistry: heme metabolism, hemostasis and blood coagulation, plasma composition, and molecular aspects of erythrocytes and hemoglobin. Oxygen-hemoglobin dissociation is covered in 16.1, red cell physiology and the coagulation cascade in 16.5, and hemoglobinopathies as diseases in 9.1. This section covers the molecular basis.

Heme Synthesis

Heme (protoporphyrin IX plus Fe2+) is made mostly in the bone marrow (for hemoglobin) and the liver (for cytochromes, including the CYP450 enzymes).

Porphyrias and Lead Poisoning

• Acute intermittent porphyria: autosomal dominant with variable penetrance. Attacks cause severe abdominal pain, a predominantly motor peripheral neuropathy, psychiatric symptoms, tachycardia and dark or red urine with high urinary porphobilinogen. Triggers include CYP-inducing drugs (barbiturates, some anticonvulsants, sulfonamides), alcohol, fasting and progesterone. Treatment is IV hemin and glucose, which suppress ALA synthase.
• Porphyria cutanea tarda: the most common porphyria. It causes photosensitive blistering and fragility on the backs of the hands, hypertrichosis and scarring, and is associated with hepatitis C, alcohol, estrogens and hemochromatosis. It is treated with phlebotomy or low-dose hydroxychloroquine.
• Lead poisoning: inhibits ALA dehydratase and ferrochelatase, causing microcytic anemia with basophilic stippling and ringed sideroblasts, abdominal colic, wrist or foot drop (motor neuropathy) and encephalopathy in children. Chelation uses succimer, edetate calcium disodium or dimercaprol.

Heme Degradation and Bilirubin

1. Macrophages (mainly in the spleen) open the heme ring with heme oxygenase, releasing Fe2+ (recycled), CO and biliverdin (green).
2. Biliverdin reductase forms unconjugated bilirubin (yellow), which is insoluble and carried to the liver on albumin.
3. Hepatocytes conjugate it with glucuronic acid using UGT1A1, forming water-soluble conjugated bilirubin that is secreted into bile.
4. Gut bacteria convert it to urobilinogen. Some is reabsorbed and excreted in urine as urobilin, and the rest becomes stercobilin, which colors stool.

A bruise changes color as hemoglobin breaks down: red-blue or purple at first, then green (biliverdin) and yellow-brown (bilirubin, hemosiderin) over several days. This sequence is seen around ankle sprains and after foot surgery.

Hemoglobin: Structure and Molecular Aspects

• Cooperativity: hemoglobin moves between a low-affinity taut (T) state and a high-affinity relaxed (R) state, producing the sigmoid dissociation curve.
• Bohr effect: H+ and CO2 stabilize the T state and release oxygen in active tissues.
• 2,3-BPG, made in red cells by the Rapoport-Luebering shunt off glycolysis, binds the central cavity of deoxyhemoglobin and shifts the curve right. It rises with chronic anemia and high altitude.
• CO2 transport: about 70% as bicarbonate (carbonic anhydrase in red cells with the chloride shift), about 20–25% as carbaminohemoglobin, and about 5–10% dissolved. The Haldane effect describes how deoxygenated hemoglobin carries more CO2.
• Methemoglobin (Fe3+) cannot carry oxygen. It is reduced by NADH-cytochrome b5 reductase, and in emergencies by methylene blue through NADPH (11.4).

Molecular hemoglobinopathies:

• HbS: beta-6 glutamate to valine. The hydrophobic valine allows deoxygenated HbS to polymerize.
• HbC: beta-6 glutamate to lysine. It causes milder disease with target cells.
• Alpha-thalassemia usually results from gene deletions (four alpha genes): one deletion is a silent carrier, two cause trait, three cause HbH disease and four cause hydrops fetalis with Hb Barts.
• Beta-thalassemia results from point mutations, often affecting splicing or promoters (beta-zero or beta-plus alleles).

Glycated hemoglobin (HbA1c) forms by nonenzymatic attachment of glucose to the N-terminal valine of the beta chain. It reflects average glucose over the red cell lifespan, roughly the prior 3 months. Conditions that shorten red cell survival (hemolysis, blood loss, transfusion) or alter hemoglobin can make HbA1c misleading.

Erythrocyte Metabolism

Red cells lack mitochondria, so they rely on anaerobic glycolysis (net 2 ATP per glucose). Pyruvate kinase deficiency starves the Na+/K+-ATPase and causes hemolysis. The hexose monophosphate shunt supplies NADPH to keep glutathione reduced; G6PD deficiency therefore causes oxidant hemolysis with Heinz bodies (17.3).

Plasma Composition

Plasma is blood without cells; serum is plasma without clotting factors (fibrinogen is consumed in the clot).

Acute-phase reactants are driven by IL-6:

• Positive (rise): CRP, fibrinogen, ferritin, hepcidin, serum amyloid A, haptoglobin
• Negative (fall): albumin, transferrin, transthyretin (prealbumin)

Low albumin in an infected or inflamed patient therefore reflects inflammation as well as nutrition (18.5).

Plasma osmolality can be estimated from routine chemistry (sodium in mEq/L; glucose and BUN in mg/dL):

$$\text{Osm}_{\text{calc}} = 2[\text{Na}^+] + \frac{\text{glucose}}{18} + \frac{\text{BUN}}{2.8}$$

With Na+ 140, glucose 90 and BUN 14: 280 + 5 + 5 = 290 mOsm/kg. A measured value well above the calculated value (an osmolal gap) suggests unmeasured osmoles such as ethanol, methanol or ethylene glycol.

Biochemistry of Hemostasis

• Vitamin K-dependent gamma-carboxylation: vitamin K hydroquinone serves as cofactor for gamma-glutamyl carboxylase, adding a second carboxyl group to glutamate residues on factors II, VII, IX and X and proteins C and S. The resulting Gla residues bind Ca2+, anchoring the factors to phospholipid membranes. Vitamin K epoxide reductase (VKORC1) regenerates the cofactor and is the target of warfarin.
• Factor XIIIa is a transglutaminase that forms covalent links between glutamine and lysine residues of adjacent fibrin monomers, stabilizing the clot.
• von Willebrand factor is a large multimer that bridges platelets to collagen and carries factor VIII in plasma, protecting it from degradation.
• Antithrombin is a serine protease inhibitor (serpin) activated by heparin's pentasaccharide sequence.