Understand hemoglobin structure, oxygen dissociation curve, and hemoglobin metabolism for ASCP BOC hematology.

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Chapter 4 โ€” HEMOGLOBIN METABOLISM

ASCP Board Exam ยท Lab-Focused Study Guide
Structure ยท Heme & Globin Synthesis ยท Ontogeny ยท Oโ‚‚ Dissociation Curve ยท COโ‚‚ Transport ยท Dyshemoglobins ยท Hemoglobin Measurement ยท Practice Questions

1 ยท Priority Table & Quick Foundation

Topic Priority Why It’s Tested
Hemoglobin structure (2 pairs of unlike chains, heme = protoporphyrin IX + Feยฒโบ) HIGH Opening item of almost every Hb question set
Normal adult and newborn Hb fractions (A >95%, Aโ‚‚ <3.5%, F 1โ€“2%; newborn F 60โ€“90%) HIGH Interpreting HPLC/electrophoresis; thalassemia screening
Embryonic/fetal/adult hemoglobins and their chains HIGH Gower-1, Gower-2, Portland, F, A, Aโ‚‚ recall
Oxygen dissociation curve: sigmoid shape, Pโ‚…โ‚€, left vs right shifts, Bohr effect, 2,3-BPG, Hb F HIGH The most-tested physiology concept in this chapter
Dyshemoglobins: methemoglobin, sulfhemoglobin, carboxyhemoglobin HIGH Cyanosis/CO poisoning cases; methylene blue; toxic levels
Hemoglobin measurement: cyanmethemoglobin reference method, SLS method HIGH Method principle, wavelength, reagents, sulfhemoglobin exception
Heme synthesis: ALA synthase (rate-limiting), mitochondria vs cytoplasm, ferrochelatase HIGH Links to porphyrias, lead poisoning, sideroblastic anemia
Globin genes: chromosome 16 (ฮฑ, ฮถ) vs 11 (ฮต, ฮณ, ฮด, ฮฒ); gene copy numbers MEDIUM Foundation for thalassemia genetics
COโ‚‚ transport: carbonic anhydrase, bicarbonate, chloride shift, carbamino MEDIUM Physiology recall
T (tense) vs R (relaxed) forms; 2,3-BPG binding between ฮฒ chains MEDIUM Mechanism behind the curve shifts
Regulation: heme feedback, ฮณโ†’ฮฒ switching (KLF1, BCL11A, LCR) MEDIUM Occasional recall; basis of Hb F induction therapy
Glycated Hb (Hb Aโ‚c) MEDIUM Crossover with chemistry
Myoglobin vs hemoglobin curve LOW Hyperbolic vs sigmoid; urine dipstick
Helix nomenclature (Aโ€“H), amino acid counts, nitric oxide transport LOW Rarely tested beyond E7/F8 histidines
๐ŸŽฏ KEY POINT โ€” Where to spend your time

Focus: normal Hb fractions by age, the oxygen dissociation curve and everything that shifts it, the three dyshemoglobins (and which can be reversed), the cyanmethemoglobin method, and ALA synthase/ferrochelatase in heme synthesis. Skim: helix labels, amino acid numbers, transcription-factor names beyond KLF1/BCL11A, and nitric oxide transport.

Quick Foundation โ€” from zero

  • Hemoglobin fills ~95% of the RBC’s cytoplasm (~34 g/dL inside the cell). Its job: carry Oโ‚‚ from lungs to tissues and help carry COโ‚‚ back.
  • Each Hb molecule = 4 globin chains (two pairs of different chains) + 4 heme groups. Each heme is a ring (protoporphyrin IX) with one ferrous iron (Feยฒโบ) that holds one Oโ‚‚.
  • If the iron is oxidized to ferric (Feยณโบ), the molecule becomes methemoglobin and can’t carry Oโ‚‚.
  • Heme is built partly in mitochondria and partly in the cytoplasm; globin is made on ribosomes. Both happen only in precursors and reticulocytes โ€” mature RBCs can’t make Hb.
  • Which globin chains are made changes with age: embryo โ†’ fetus (Hb F) โ†’ adult (Hb A).
  • Hb grabs Oโ‚‚ where Oโ‚‚ is plentiful (lungs) and lets it go where it is scarce (tissues). The S-shaped dissociation curve describes this; acid, heat, COโ‚‚ and 2,3-BPG push it right (easier release).
  • The lab measures Hb by converting all forms to a single stable colored product and reading its absorbance.
๐Ÿ’ก Analogy

Hemoglobin is a team of four porters holding hands. When one picks up a parcel (Oโ‚‚), the others find it easier to pick theirs up (cooperativity) โ€” so in the lungs they load fully and fast. In the tissues, acid, heat and 2,3-BPG are like a boss yelling “drop it!” โ€” the team lets go more readily (right shift). CO is a parcel glued to the hand โ€” never released, and it makes the others grip their parcels tighter too.

๐Ÿฅ Why this matters clinically

The same Hb value can mean very different oxygen delivery. A patient with CO poisoning or methemoglobinemia can have a normal Hb and normal PaOโ‚‚ but be severely hypoxic โ€” only the right test (co-oximetry) shows it.

Concept Plain-English Explanation
Heme Protoporphyrin IX ring + central ferrous iron (Feยฒโบ); one per globin chain
Globin Protein chain (ฮฑ 141 aa; ฮฒ, ฮณ, ฮด, ฮต 146 aa) that holds the heme in a pocket
Tetramer Complete Hb: 2 ฮฑ + 2 non-ฮฑ chains (e.g., ฮฑโ‚‚ฮฒโ‚‚ = Hb A)
Pโ‚…โ‚€ POโ‚‚ at which Hb is 50% saturated (normal โ‰ˆ 27 mm Hg). Lower Pโ‚…โ‚€ = higher affinity
Left shift Hb holds Oโ‚‚ tighter (high affinity) โ†’ less Oโ‚‚ released to tissues
Right shift Hb releases Oโ‚‚ more easily (low affinity) โ†’ more Oโ‚‚ to tissues
Dyshemoglobin Hb that cannot carry Oโ‚‚: methemoglobin, sulfhemoglobin, carboxyhemoglobin
Ontogeny The age-dependent sequence of hemoglobins from embryo to adult

2 ยท Core Content Tables

2.1 Hemoglobin Structure

Feature Details
Definition Globular tetramer of two pairs of unlike globin chains, each carrying one heme; MW โ‰ˆ 64,000 Da; can carry up to 4 Oโ‚‚
Heme Protoporphyrin IX + central Feยฒโบ. Sits in a hydrophobic cleft between the E and F helices. Iron is linked to the proximal histidine (F8); Oโ‚‚ binds on the other side near the distal histidine (E7)
Globin chains ฮฑ and ฮถ = 141 amino acids; ฮฒ, ฮณ, ฮด, ฮต = 146. ฮณ exists as Gฮณ (glycine at 136) and Aฮณ (alanine at 136)
Secondary/tertiary 8 helices (Aโ€“H) joined by 7 nonhelical segments; folded ‘pretzel’ shape. Hydrophobic interior keeps iron ferrous; hydrophilic exterior makes Hb water-soluble
Quaternary Two ฮฑฮฒ dimers. ฮฑโ‚ฮฒโ‚ bonds are strong (stable dimers); ฮฑโ‚ฮฒโ‚‚ contacts shift during oxygenation/deoxygenation
Oโ‚‚ capacity ~1.34 mL Oโ‚‚ per gram of Hb
Glycation Nonenzymatic sugar attachment. Hb Aโ‚c = glucose on N-terminal valine of ฮฒ chain; normally ~4โ€“6%; reflects mean glucose over the previous 2โ€“3 months
Other functions Buffers Hโบ (acid-base), carries COโ‚‚, binds/transports nitric oxide (S-nitrosohemoglobin at ฮฒ-chain cysteine)

Why hemoglobin is kept inside RBCs: free Hb is quickly denatured and lost through the kidney. Once released by hemolysis it is salvaged by haptoglobin/hemopexin (Chapter 2).

๐Ÿงช PRE-ANALYTICAL โ€” Hb Aโ‚c*
  • EDTA whole blood; stable for days refrigerated
  • Falsely low when RBC life span is shortened (hemolysis, blood loss, recent transfusion) โ€” fewer days of glucose exposure
  • Hb variants (S, C, E) and high Hb F can interfere depending on method โ€” HPLC chromatograms may flag them

2.2 Heme Biosynthesis

Feature Details
Where / when Erythroid precursors from the pronormoblast through the reticulocyte (polychromatic erythrocyte); stops when mitochondria are lost
Step 1 (mitochondria) Glycine + succinyl CoA โ†’ ALA by ALA synthase โ€” the rate-limiting step
Cytoplasm steps ALA โ†’ porphobilinogen (PBG) by ALA dehydratase (PBG synthase) โ†’ hydroxymethylbilane (PBG deaminase) โ†’ uroporphyrinogen III (synthase) โ†’ coproporphyrinogen III (decarboxylase)
Back to mitochondria Coproporphyrinogen oxidase โ†’ protoporphyrinogen IX โ†’ protoporphyrinogen oxidase โ†’ protoporphyrin IX โ†’ ferrochelatase (heme synthase) inserts Feยฒโบ โ†’ heme
Iron delivery Plasma transferrin carries Feยณโบ โ†’ binds transferrin receptors โ†’ endosome โ†’ acidification releases iron โ†’ reduced to Feยฒโบ in mitochondria
Regulation (negative feedback) Heme represses ALA synthase transcription and inhibits ALA dehydratase and PBG deaminase; heme/protoporphyrin IX may inhibit ferrochelatase. โ†‘ heme demand โ†’ โ†‘ ALA synthase
Clinical significance* Enzyme defects โ†’ porphyrias; lead inhibits ALA dehydratase and ferrochelatase (โ†‘ zinc protoporphyrin, basophilic stippling); ALA synthase defects โ†’ sideroblastic anemia

Heme leaves the mitochondrion and joins globin chains in the cytoplasm. Heme also promotes globin translation โ€” so heme and globin production stay balanced.

2.3 Globin Biosynthesis & Assembly

Feature Details
Genes Chromosome 16: ฮถ and ฮฑ (ฮฑ-cluster). Chromosome 11: ฮต, ฮณ, ฮด, ฮฒ (ฮฒ-cluster)
Copy number Two ฮฑ and two ฮณ genes per chromosome โ†’ 4 ฮฑ and 4 ฮณ genes per diploid cell; one ฮฒ and one ฮด per chromosome โ†’ 2 ฮฒ and 2 ฮด per diploid cell
Where / when Transcription in nucleus, translation on cytoplasmic ribosomes; pronormoblast through reticulocyte, not mature RBC
Balance ฮฑ genes make more mRNA but it is translated less efficiently โ†’ ฮฑ and ฮฒ chains produced in ~equal amounts
Assembly Each chain binds heme โ†’ ฮฑ + non-ฮฑ heterodimer โ†’ two dimers โ†’ tetramer. ฮฑ (positive charge) prefers ฮฒ (most negative) > ฮณ > ฮด
Why Hb Aโ‚‚ is low ฮด-globin gene has a promoter mutation โ†’ ฮด chains made poorly (<3.5% Aโ‚‚)
Regulation Locus control region (LCR), >20 kb upstream, loops to the active gene’s promoter with transcription factors (KLF1, GATA1, etc.). KLF1 drives ฮฒ transcription and ฮณโ†’ฮฒ switching; represses ฮณ via BCL11A
Clinical significance Imbalanced chain production โ†’ thalassemias; structurally abnormal chains โ†’ hemoglobinopathies (Hb S, C, E, Hb M)
๐Ÿงช PRE-ANALYTICAL โ€” Hb fractionation (HPLC / electrophoresis)*
  • EDTA whole blood; ask about transfusion in the last ~3 months โ€” donor Hb A masks the patient’s pattern
  • Interpret against age: newborns normally have 60โ€“90% Hb F; adult values apply after ~2 years
  • Iron deficiency can lower Hb Aโ‚‚ and mask ฮฒ-thalassemia trait
  • Definitive variant identification requires DNA testing

2.4 Hemoglobin Ontogeny โ€” Normal Hemoglobins

Stage Hemoglobin Chains Amount / Timing
Early embryo (yolk sac erythroblasts) Gower-1 ฮถโ‚‚ฮตโ‚‚ First ~3 months only
Early embryo Gower-2 ฮฑโ‚‚ฮตโ‚‚ First ~3 months only
Early embryo Portland ฮถโ‚‚ฮณโ‚‚ First ~3 months only
Fetus (2ndโ€“3rd trimester) Hb F ฮฑโ‚‚ฮณโ‚‚ Begins in early embryo, peaks 3rd trimester, declines just before birth
Birth Hb F / Hb A ฮฑโ‚‚ฮณโ‚‚ / ฮฑโ‚‚ฮฒโ‚‚ F 60โ€“90%, A 10โ€“40%
~6 months Hb A predominant ฮฑโ‚‚ฮฒโ‚‚ ฮณโ†’ฮฒ switch largely complete
2 years โ†’ adult Hb A ฮฑโ‚‚ฮฒโ‚‚ >95%
2 years โ†’ adult Hb Aโ‚‚ ฮฑโ‚‚ฮดโ‚‚ <3.5%
2 years โ†’ adult Hb F ฮฑโ‚‚ฮณโ‚‚ 1โ€“2%, in a few ‘F cells’ (A/F cells)

ฮถ and ฮต appear only in the first 3 months; ฮฑ replaces ฮถ on chromosome 16, and ฮต โ†’ ฮณ โ†’ ฮฒ/ฮด on chromosome 11. Chapter 1 connected these embryonic Hbs to the yolk sac phase.

2.5 Hemoglobin Reference Intervals

Feature Details
Men 14โ€“18 g/dL (140โ€“180 g/L)
Women 12โ€“15 g/dL (120โ€“150 g/L)
Newborns 16.5โ€“21.5 g/dL (165โ€“215 g/L)
Why newborns are higher Hb F’s high Oโ‚‚ affinity delivers Oโ‚‚ less readily โ†’ mild tissue hypoxia โ†’ โ†‘ EPO โ†’ more RBCs; values fall to adult-like levels by ~6 months as Hb A replaces Hb F
Altitude Slightly higher Hb as compensation for thin air
Note Each lab establishes its own intervals by method and population

2.6 Oxygen Transport & the Dissociation Curve

Feature Details
Curve shape Sigmoid โ€” low affinity at low POโ‚‚, high affinity at high POโ‚‚, because of cooperativity: each Oโ‚‚ bound changes tetramer conformation and raises the affinity of the remaining hemes
Pโ‚…โ‚€ Normal โ‰ˆ 27 mm Hg. Left shift โ†’ Pโ‚…โ‚€ < 27; right shift โ†’ Pโ‚…โ‚€ > 27
Normal values Arterial SaOโ‚‚ 96โ€“100%; arterial POโ‚‚ 80โ€“100 mm Hg; venous POโ‚‚ 30โ€“50 mm Hg
T vs R Tense (T) = deoxy form, stabilized by one 2,3-BPG between the ฮฒ chains with salt bridges โ†’ low affinity. Relaxed (R) = oxy form; binding Oโ‚‚ breaks salt bridges, releases 2,3-BPG, and rotates ฮฑโ‚ฮฒโ‚ vs ฮฑโ‚‚ฮฒโ‚‚ dimers by 15ยฐ at the ฮฑโ‚ฮฒโ‚‚ contact
Bohr effect โ†“ pH (โ†‘ Hโบ) and โ†‘ PCOโ‚‚ in tissues โ†’ right shift โ†’ Oโ‚‚ released
Right shift causes โ†‘ temperature ยท acidosis ยท โ†‘ PCOโ‚‚ ยท โ†‘ 2,3-BPG (altitude, pulmonary insufficiency, CHF, severe anemia) ยท low-affinity Hb variants
Left shift causes โ†“ temperature ยท alkalosis ยท โ†“ PCOโ‚‚ ยท โ†“ 2,3-BPG (multiple transfusions of stored blood) ยท high-affinity Hb variants ยท Hb F ยท CO binding
Hb F Pโ‚…โ‚€ 19โ€“21 mm Hg (left shift) because ฮณ chains bind 2,3-BPG weakly (one amino acid difference at the binding site) โ†’ good Oโ‚‚ uptake from mother, poorer release to fetal tissues
Myoglobin Muscle monomer (~17,000 Da); hyperbolic curve, very high affinity โ€” releases Oโ‚‚ only at very low POโ‚‚. Released in MI, trauma, rhabdomyolysis; positive urine dipstick for ‘blood’
๐Ÿงช PRE-ANALYTICAL โ€” Blood gases & oxygen saturation*
  • Arterial blood in heparinized syringe, no air bubbles (air raises POโ‚‚), capped, analyzed promptly (cells keep consuming Oโ‚‚)
  • Calculated SaOโ‚‚ from POโ‚‚ assumes a normal curve โ€” it is falsely normal with CO or MetHb. Use co-oximetry for measured fractions
  • Pulse oximetry misreads COHb as Oโ‚‚Hb and trends toward ~85% with high MetHb

2.7 Carbon Dioxide & Nitric Oxide Transport

Feature Details
In tissues COโ‚‚ enters RBC + Hโ‚‚O โ†’ Hโ‚‚COโ‚ƒ (carbonic anhydrase) โ†’ Hโบ + HCOโ‚ƒโป. Hโบ binds HbOโ‚‚ โ†’ Oโ‚‚ released (Bohr). HCOโ‚ƒโป leaves the cell; Clโป enters to keep electroneutrality = chloride shift
In lungs Oโ‚‚ binds deoxyHb โ†’ Hโบ released โ†’ combines with HCOโ‚ƒโป โ†’ Hโ‚‚COโ‚ƒ โ†’ COโ‚‚ + Hโ‚‚O โ†’ COโ‚‚ exhaled; HCOโ‚ƒโป moves in, Clโป moves out
Proportions ~85% of tissue COโ‚‚ carried via Hb buffering of Hโบ (as bicarbonate); a small amount dissolved in cytoplasm; the rest as carbamino groups on globin
Band 3 link HCOโ‚ƒโป/Clโป exchange uses the band 3 anion exchanger (Chapter 3)
Nitric oxide Endothelial NO relaxes vascular smooth muscle; some binds ฮฒ-chain cysteine (S-nitrosohemoglobin) โ€” Hb may release NO in hypoxic areas (proposed; not universally accepted)

2.8 Dyshemoglobins

Feature Methemoglobin (MetHb) Sulfhemoglobin (SulfHb) Carboxyhemoglobin (COHb)
Change Heme iron oxidized to Feยณโบ (reversible) Sulfur atom added to the pyrrole ring (irreversible) CO bound to heme iron; affinity 240ร— Oโ‚‚
Normal level ~1% (b5 reductase keeps it low) Not normally present <2% (endogenous); smokers up to ~15%
Causes Acquired (toxic): nitrites, primaquine, dapsone, benzocaine. Hereditary: cytochrome b5 reductase 3 (CYB5R3) deficiency (autosomal recessive) or Hb M (globin mutation, autosomal dominant, 30โ€“50% MetHb) Sulfonamides, phenacetin, nitrites, phenylhydrazine; industrial sulfur chemicals Car exhaust, tobacco smoke, industrial pollution, coal/gas/charcoal burning, house fires
Effect on curve Can’t carry Oโ‚‚ Can’t carry Oโ‚‚ Left shift โ€” remaining hemes hold Oโ‚‚ tighter
Symptoms / levels <25% usually asymptomatic ยท >30% cyanosis + hypoxia ยท >50% coma/death Cyanosis; usually mild 20โ€“30% headache, dizziness, confusion ยท >40% coma, seizures, arrhythmias, death
Blood color Chocolate brown โ€” doesn’t turn red with Oโ‚‚ Greenish pigment Cherry red (skin may be cherry red)
Spectral peak 630 nm; disappears when cyanide added Similar to MetHb, but does not shift with cyanide Detected by spectral absorption (co-oximetry)
Treatment Remove oxidant; IV methylene blue if โ‰ฅ30% (works via NADPH โ€” G6PD needed); exchange transfusion if life-threatening No treatment; persists for life of the RBC โ€” avoid the agent Remove from source; 100% Oโ‚‚; hyperbaric Oโ‚‚ considered if COHb >25% (controversial)
Diagnosis Co-oximetry; Hb M by electrophoresis/HPLC/DNA; b5R3 by enzyme assay and DNA Co-oximetry with cyanide step COHb >3% in nonsmokers or >10% in smokers

Heterozygotes for CYB5R3 mutations have normal MetHb at rest but develop methemoglobinemia with oxidant drugs. Smokers may have higher Hct (compensatory polycythemia) from chronic COHb.

๐Ÿงช PRE-ANALYTICAL โ€” Co-oximetry (MetHb, COHb, SulfHb)
  • Heparinized arterial or venous whole blood; tightly capped, analyzed promptly โ€” MetHb changes on standing*
  • COHb can be measured on venous blood; draw as soon as possible โ€” COHb falls once the patient is on 100% Oโ‚‚*
  • SulfHb is distinguished from MetHb because its peak does not disappear after adding cyanide
  • Chocolate-brown blood that stays brown when shaken in air suggests MetHb

2.9 Hemoglobin Measurement โ€” Cyanmethemoglobin (Reference) Method

Feature Details
Status Reference method (CLSI H15) for Hb concentration
Principle Lysing agent frees Hb โ†’ potassium ferricyanide oxidizes Feยฒโบ โ†’ Feยณโบ (MetHb) โ†’ potassium cyanide converts MetHb โ†’ stable cyanmethemoglobin โ†’ absorbance at 540 nm, compared with a standard
What it measures All forms except sulfhemoglobin (not converted)*
Automation Adapted to automated analyzers
Alternative Sodium lauryl sulfate (SLS) method โ†’ SLS-methemoglobin; no cyanide, no toxic waste; widely used on analyzers
Separation methods Electrophoresis and HPLC separate Hb A, Aโ‚‚, F and variants; DNA testing confirms
๐Ÿงช PRE-ANALYTICAL โ€” Hemoglobin concentration*
  • EDTA whole blood, well mixed
  • Falsely โ†‘ (turbidity): lipemia, very high WBC (>20 ร— 10โน/L), high platelet count, cryoglobulins/paraproteins, unlysed RBCs (Hb S, Hb C) โ€” correct lipemia with a plasma blank
  • COHb converts slowly in some reagents โ€” allow full reaction time
  • Cyanide reagent is toxic โ€” follow disposal rules

*From general laboratory practice, not stated in Rodak Ch. 10.

3 ยท Visual Memory Hooks

๐Ÿง  Heme building blocks
โ€œGlycine Sucks Coffee in the Mitochondriaโ€ โ†’ Glycine + Succinyl CoA โ†’ ALA by ALA synthase (rate-limiting, mitochondria).
๐Ÿง  Heme location path
โ€œMito โ†’ Cyto โ†’ Mitoโ€ โ†’ starts and ends in mitochondria; middle steps in cytoplasm. Ferrochelatase = last step, adds Feยฒโบ.
๐Ÿง  Globin chromosomes
โ€œAlpha is Sweet 16; Beta plays for Team 11โ€ โ†’ ฮฑ, ฮถ on chr 16 ยท ฮต, ฮณ, ฮด, ฮฒ on chr 11.
๐Ÿง  Gene copies
โ€œAlpha and Gamma come in 4s; Beta and Delta in 2sโ€ (per diploid cell).
๐Ÿง  Embryonic Hbs
โ€œGower-1 is ZEta-Epsilon; Gower-2 is Alpha-Epsilon; Portland is Zeta-Gammaโ€ โ†’ ฮถโ‚‚ฮตโ‚‚, ฮฑโ‚‚ฮตโ‚‚, ฮถโ‚‚ฮณโ‚‚.
๐Ÿง  Adult fractions
โ€œ95 โ€“ 3.5 โ€“ 2โ€ โ†’ A >95%, Aโ‚‚ <3.5%, F 1โ€“2%.
๐Ÿง  Right shift
โ€œCADET, face RIGHT!โ€ โ†’ โ†‘ COโ‚‚, Acid, 2,3-DPG/BPG, Exercise, Temperature โ†’ right shift, Oโ‚‚ released.
๐Ÿง  Left shift
โ€œLeft = Loves oxygen (holds on)โ€ โ†’ alkalosis, cold, โ†“ 2,3-BPG (stored blood), Hb F, CO, high-affinity variants.
๐Ÿง  T vs R
โ€œTense Tissues, Relaxed Respirationโ€ โ†’ T = deoxy (with 2,3-BPG) in tissues; R = oxy in lungs.
๐Ÿง  Chloride shift
โ€œBicarb Bails out, Chloride Comes inโ€ (in tissues).
๐Ÿง  Dyshemoglobin colors
โ€œChocolate Met, Cherry CO, Green Sulfโ€.
๐Ÿง  Reversibility
โ€œSulf is Stuckโ€ โ†’ sulfhemoglobin is irreversible and ignores cyanide; MetHb reverses with methylene blue.
๐Ÿง  CO numbers
โ€œ240 ร— stronger; >3 non-smoker, >10 smokerโ€.
๐Ÿง  Reference method
โ€œFerri-Cyanide at 540โ€ โ†’ ferricyanide oxidizes, cyanide stabilizes, read at 540 nm.

Pattern Triggers

If you seeโ€ฆ Thinkโ€ฆ Becauseโ€ฆ
Newborn with 88% Hb F and Hb 20 g/dL Normal newborn ฮณโ†’ฮฒ switch not complete; Hb F’s left shift drives more RBC production
Adult with Hb F 1%, Aโ‚‚ 2%, A 97% Normal adult pattern Within >95 / <3.5 / 1โ€“2%
Cyanosis, normal PaOโ‚‚, brown blood that stays brown in air Methemoglobinemia Feยณโบ can’t bind Oโ‚‚
Cyanosis starting after dapsone/benzocaine/nitrites Toxic (acquired) methemoglobinemia Oxidant overwhelms b5 reductase
Lifelong cyanosis, autosomal dominant, MetHb 30โ€“50% Hb M Globin mutation stabilizes ferric iron
MetHb not falling after methylene blue in a G6PD-deficient patient No NADPH for methylene blue pathway Methylene blue needs G6PD
Cyanosis, abnormal peak that doesn’t shift with cyanide Sulfhemoglobin Irreversible; not converted
Headache, confusion in a family in winter; cherry-red skin; normal PaOโ‚‚ Carbon monoxide poisoning CO displaces Oโ‚‚ and left-shifts the curve
Heavy smoker with high Hct Chronic COHb โ†’ compensatory polycythemia Tissue hypoxia โ†’ โ†‘ EPO
Hypoxic patient with severe anemia or at altitude, curve right-shifted โ†‘ 2,3-BPG Adaptation to unload more Oโ‚‚
Patient after massive transfusion of old blood, curve left-shifted โ†“ 2,3-BPG in stored RBCs Stored cells lose 2,3-BPG
Positive urine blood dipstick, no RBCs, muscle injury Myoglobinuria (rhabdomyolysis) Myoglobin reacts like Hb on the dipstick

Commonly Confused Pairs

Feature A B
Hb A vs Hb F ฮฑโ‚‚ฮฒโ‚‚; Pโ‚…โ‚€ ~27; binds 2,3-BPG well ฮฑโ‚‚ฮณโ‚‚; Pโ‚…โ‚€ 19โ€“21; binds 2,3-BPG weakly (left shift)
Hb Aโ‚‚ vs Hb F (adult) ฮฑโ‚‚ฮดโ‚‚, <3.5% ฮฑโ‚‚ฮณโ‚‚, 1โ€“2% in F cells
Gower-1 vs Gower-2 ฮถโ‚‚ฮตโ‚‚ (no ฮฑ) ฮฑโ‚‚ฮตโ‚‚
Tense vs relaxed Deoxy, low affinity, 2,3-BPG bound Oxy, high affinity, 2,3-BPG released
Hemoglobin vs myoglobin Tetramer, sigmoid, cooperative Monomer, hyperbolic, very high affinity
MetHb vs SulfHb Reversible; peak shifts with cyanide; brown Irreversible; peak does not shift; green
Hereditary MetHb: b5R3 deficiency vs Hb M Autosomal recessive, enzyme defect Autosomal dominant, globin mutation; no effective therapy
ALA synthase vs ferrochelatase First, rate-limiting step (mitochondria) Last step, inserts Feยฒโบ (mitochondria)
Cyanmethemoglobin vs SLS method Reference; cyanide; 540 nm Non-toxic; SLS-MetHb; automated
Bohr effect vs chloride shift pH/COโ‚‚ effect on Oโ‚‚ affinity HCOโ‚ƒโป/Clโป exchange across membrane

Timeline Table โ€” Hemoglobin Across Life

Marker Onset Peak Duration / End Best Exam Use
ฮถ and ฮต chains (Gower-1, Gower-2, Portland) Early embryo First weeks Gone after ~3 months’ gestation Embryonic Hb recall
ฮณ chains (Hb F) Early embryo 3rd trimester Declines before birth; 1โ€“2% adult Newborn pattern
ฮฒ chains (Hb A) Low in fetal life Rises after birth Predominant by 6 months; >95% by 2 years ฮณโ†’ฮฒ switch
ฮด chains (Hb Aโ‚‚) Around birth Adult level <3.5% lifelong ฮฒ-thal screening
Hb F at birth โ€” 60โ€“90% Falls over first months Age-appropriate interpretation
Hb Aโ‚c Reflects glucose โ€” Previous 2โ€“3 months Diabetes monitoring
Sulfhemoglobin After exposure โ€” Life of the RBC Why it can’t be treated
COHb after removal from CO โ€” โ€” Falls faster on 100% Oโ‚‚ Draw early

4 ยท Diagnostic Flowcharts

Flowchart 1 โ€” Cyanosis or hypoxia with a normal PaOโ‚‚

Symptoms of hypoxia, PaOโ‚‚ normal, pulse oximetry discordant โ†’ Order co-oximetry
โ†‘ COHb (>3% nonsmoker / >10% smoker) โ†’ CO poisoning โ†’ 100% Oโ‚‚
โ†‘ MetHb, peak at 630 nm disappears with cyanide โ†’ Methemoglobinemia โ†’ drug history, family history
Abnormal peak that persists after cyanide โ†’ Sulfhemoglobinemia โ†’ stop offending drug
All fractions normal โ†’ Look for cardiac/pulmonary causes
โš ๏ธ Common mistake: trusting a calculated SaOโ‚‚ from the blood gas. It assumes all Hb is normal and misses CO, MetHb and SulfHb.

Flowchart 2 โ€” Methemoglobinemia: acquired or hereditary?

โ†‘ MetHb confirmed โ†’ Recent oxidant exposure? (nitrites, dapsone, primaquine, benzocaine)
Yes โ†’ Toxic methemoglobinemia โ†’ remove agent; methylene blue if โ‰ฅ30%
No; lifelong; siblings affected; responds to reducing agents โ†’ CYB5R3 deficiency (autosomal recessive) โ†’ enzyme assay/DNA
No; parent affected (dominant); MetHb 30โ€“50%; unresponsive โ†’ Hb M โ†’ electrophoresis/HPLC/DNA
Methylene blue fails โ†’ Check G6PD status (no NADPH) or suspect Hb M/SulfHb
โš ๏ธ Common mistake: giving methylene blue without considering G6PD โ€” it depends on the NADPH (HMP) pathway.

Flowchart 3 โ€” Interpreting Hb fractions by age (timing)

Hb F 60โ€“90%, A 10โ€“40% โ†’ Normal newborn
Hb F still high at 6โ€“12 months โ†’ Switch not complete โ€” recheck with age; consider hemoglobinopathy if persistent
Age โ‰ฅ2 years: A >95%, Aโ‚‚ <3.5%, F 1โ€“2% โ†’ Normal adult pattern
Adult Aโ‚‚ >3.5% โ†’ Suspect ฮฒ-thalassemia trait* (Chapter on thalassemia)
Recent transfusion โ†’ Repeat after ~3 months*
โš ๏ธ Common mistake: applying adult reference intervals to a newborn’s HPLC result.

Flowchart 4 โ€” Method selection for hemoglobin testing

Need Hb concentration โ†’ Cyanmethemoglobin (reference) or SLS method on analyzer
Need to separate Hb A, Aโ‚‚, F and variants โ†’ HPLC or electrophoresis
Need to identify a specific variant (e.g., Hb M) โ†’ DNA/molecular testing
Need MetHb, COHb, SulfHb fractions โ†’ Co-oximetry (spectral absorption)
Need long-term glucose control โ†’ Hb Aโ‚c
โš ๏ธ Common mistake: using the cyanmethemoglobin result to detect dyshemoglobins โ€” it converts MetHb and COHb into the same product, so they are counted as ‘hemoglobin’.

Flowchart 5 โ€” Why is the oxygen dissociation curve shifted?

Pโ‚…โ‚€ > 27 mm Hg (right shift) โ†’ Check: acidosis, fever, โ†‘ PCOโ‚‚, โ†‘ 2,3-BPG (anemia, altitude, CHF), low-affinity variant
Pโ‚…โ‚€ < 27 mm Hg (left shift) โ†’ Check: alkalosis, hypothermia, stored-blood transfusion, CO, Hb F, high-affinity variant
Left shift + unexplained erythrocytosis + normal EPO-producing kidney โ†’ High-affinity Hb variant โ†’ hypoxia โ†’ โ†‘ EPO*
Right shift in chronic anemia โ†’ Compensatory โ†‘ 2,3-BPG โ€” adaptive
โš ๏ธ Common mistake: memorizing ‘right = better’. Right shift helps tissue delivery but reduces loading in the lungs if extreme.

5 ยท 25 High-Yield Facts

# Fact
1 Hemoglobin makes up about 95% of the RBC’s cytoplasmic protein, at about 34 g/dL inside the cell.
2 Each hemoglobin molecule has two pairs of unlike globin chains and four heme groups.
3 Heme consists of protoporphyrin IX with a central ferrous (Feยฒโบ) iron.
4 Ferric (Feยณโบ) iron cannot bind oxygen; this form is methemoglobin.
5 Heme sits between the E and F helices, linked to the proximal histidine F8.
6 Each gram of hemoglobin binds about 1.34 mL of oxygen.
7 ALA synthase, joining glycine and succinyl CoA in the mitochondria, is the rate-limiting step of heme synthesis.
8 Ferrochelatase inserts ferrous iron into protoporphyrin IX to form heme.
9 Heme inhibits ALA synthase transcription as negative feedback.
10 ฮฑ- and ฮถ-globin genes are on chromosome 16; ฮต, ฮณ, ฮด and ฮฒ genes are on chromosome 11.
11 There are four ฮฑ and four ฮณ genes but two ฮฒ and two ฮด genes per diploid cell.
12 Hemoglobin synthesis continues through the reticulocyte stage but not in the mature RBC.
13 Gower-1 is ฮถโ‚‚ฮตโ‚‚, Gower-2 is ฮฑโ‚‚ฮตโ‚‚, and Portland is ฮถโ‚‚ฮณโ‚‚.
14 Hb F (ฮฑโ‚‚ฮณโ‚‚) is 60โ€“90% of hemoglobin at birth.
15 Normal adults have more than 95% Hb A, less than 3.5% Hb Aโ‚‚, and 1โ€“2% Hb F.
16 Adult hemoglobin reference intervals are 14โ€“18 g/dL for men and 12โ€“15 g/dL for women; newborns 16.5โ€“21.5 g/dL.
17 The oxygen dissociation curve is sigmoid because of cooperativity between subunits.
18 The normal Pโ‚…โ‚€ is about 27 mm Hg.
19 Acidosis, fever, increased PCOโ‚‚ and increased 2,3-BPG shift the curve to the right.
20 2,3-BPG binds between the ฮฒ chains of deoxyhemoglobin, stabilizing the tense form.
21 Hb F has a Pโ‚…โ‚€ of 19โ€“21 mm Hg because it binds 2,3-BPG weakly.
22 In tissues, bicarbonate leaves the RBC in exchange for chloride (the chloride shift).
23 Carbon monoxide binds hemoglobin 240 times more strongly than oxygen and shifts the curve to the left.
24 Sulfhemoglobin is irreversible and its absorbance peak does not shift when cyanide is added.
25 The cyanmethemoglobin method, read at 540 nm, is the reference method for hemoglobin concentration.
โญ EXAM TIP โ€” The single most testable pattern

Curve shifts: right = โ†‘ Hโบ, โ†‘ COโ‚‚, โ†‘ temperature, โ†‘ 2,3-BPG โ†’ easier Oโ‚‚ release. Left = alkalosis, cold, โ†“ 2,3-BPG, Hb F, CO โ†’ Oโ‚‚ held tighter. Pair it with normal fractions (A >95%, Aโ‚‚ <3.5%, F 1โ€“2%; newborn F 60โ€“90%).

Quick Interpretation Table

Lab Finding Clinical Interpretation
Newborn: 88% Hb F, 12% Hb A; Hb 20 g/dL Normal newborn
Adult: 97% A, 2% Aโ‚‚, 1% F Normal adult
MetHb 35%, cyanosis, brown blood Symptomatic methemoglobinemia โ€” consider methylene blue
COHb 18% in a nonsmoker with headache CO poisoning
COHb 8% in a heavy smoker, no symptoms Consistent with smoking (below 10% smoker threshold)
Pโ‚…โ‚€ 20 mm Hg in an adult Left shift โ€” high-affinity state or variant
Hb Aโ‚c 9% Poor glucose control over prior 2โ€“3 months

๐ŸŽด Flashcards โ€” Active Recall Practice

How it works: tap the card to flip it. Then mark it โœ“ Knew It or โœ— Review Again (buttons unlock after flipping). Use ๐Ÿ”€ Shuffle to randomize and ๐Ÿ Finish anytime โ€” your summary appears at the end, and you can drill only the cards you missed.

๐Ÿ“ Practice Quiz โ€” Test Your Knowledge

How it works: tap an answer for each case. You’ll get instant feedback with the full explanation and the examiner’s trap. Your final score appears once all 10 are answered.

๐Ÿชค Trap Questions โ€” Beat the Examiner

Harder on purpose. Each question targets a real misconception. After answering you’ll see the correct reasoning, the trap explained, why the examiner wrote it, and the wrong answer most students choose.

8 ยท Rapid Revision Sheet

๐Ÿ“– READ THIS 2 HOURS BEFORE YOUR EXAM

Structure Numbers

  • 4 globin + 4 heme ยท MW ~64,000
  • Heme = protoporphyrin IX + Feยฒโบ
  • ฮฑ 141 aa ยท ฮฒ/ฮณ/ฮด/ฮต 146
  • 1.34 mL Oโ‚‚/g Hb
  • Heme between E and F; proximal His F8

Heme Synthesis

  • Glycine + succinyl CoA โ†’ ALA (ALA synthase, rate-limiting, mito)
  • ALA dehydratase โ†’ PBG (cyto)
  • Ferrochelatase + Feยฒโบ โ†’ heme (mito)
  • Heme represses ALA synthase
  • Transferrin brings Feยณโบ

Globin Genes

  • Chr 16: ฮฑ, ฮถ ยท Chr 11: ฮต, ฮณ, ฮด, ฮฒ
  • 4 ฮฑ + 4 ฮณ ยท 2 ฮฒ + 2 ฮด per cell
  • ฮฑ binds ฮฒ > ฮณ > ฮด
  • ฮด promoter mutation โ†’ low Aโ‚‚
  • KLF1 โ†’ ฮณโ†’ฮฒ switch; BCL11A represses ฮณ

Normal Hbs

  • Gower-1 ฮถโ‚‚ฮตโ‚‚ ยท Gower-2 ฮฑโ‚‚ฮตโ‚‚ ยท Portland ฮถโ‚‚ฮณโ‚‚
  • F ฮฑโ‚‚ฮณโ‚‚ ยท A ฮฑโ‚‚ฮฒโ‚‚ ยท Aโ‚‚ ฮฑโ‚‚ฮดโ‚‚
  • Birth: F 60โ€“90%, A 10โ€“40%
  • Adult: A >95%, Aโ‚‚ <3.5%, F 1โ€“2%
  • A predominant by 6 months

Hb Reference

  • Men 14โ€“18 g/dL
  • Women 12โ€“15 g/dL
  • Newborn 16.5โ€“21.5 g/dL
  • Hb Aโ‚c 4โ€“6% ยท 2โ€“3 months

Oโ‚‚ Curve

  • Sigmoid ยท Pโ‚…โ‚€ 27 mm Hg
  • Hb F Pโ‚…โ‚€ 19โ€“21
  • SaOโ‚‚ 96โ€“100% ยท PaOโ‚‚ 80โ€“100 ยท PvOโ‚‚ 30โ€“50
  • T = deoxy + 2,3-BPG ยท R = oxy
  • Myoglobin = hyperbolic

Shifts (Opposites)

  • RIGHT: โ†‘ Hโบ, โ†‘ COโ‚‚, โ†‘ temp, โ†‘ 2,3-BPG
  • LEFT: alkalosis, cold, โ†“ 2,3-BPG, Hb F, CO
  • Stored blood โ†’ left
  • Altitude/anemia/CHF โ†’ right

Dyshemoglobins

  • MetHb Feยณโบ ยท 630 nm ยท brown ยท >30% cyanosis ยท methylene blue
  • SulfHb irreversible ยท green ยท no cyanide shift
  • COHb 240ร— ยท cherry red ยท left shift
  • COHb dx: >3% nonsmoker, >10% smoker
  • CYB5R3 AR ยท Hb M AD

COโ‚‚ Transport

  • Carbonic anhydrase: COโ‚‚ + Hโ‚‚O โ†’ Hโ‚‚COโ‚ƒ
  • ~85% via Hโบ/HCOโ‚ƒโป
  • Chloride shift (band 3)
  • Carbamino on globin

Measurement

  • Cyanmethemoglobin = reference ยท 540 nm
  • Ferricyanide oxidizes, cyanide stabilizes
  • SLS = non-toxic alternative
  • HPLC/electrophoresis separate fractions
  • Turbidity (WBC, lipemia) โ†’ false โ†‘ Hb

9 ยท Final Memory Map

  • ๐Ÿฉธ HEMOGLOBIN METABOLISM
    • Structure
      • Tetramer: 2 pairs of unlike chains + 4 heme
      • Heme = protoporphyrin IX + Feยฒโบ (Eโ€“F pocket, His F8)
      • ฮฑโ‚ฮฒโ‚ strong ยท ฮฑโ‚ฮฒโ‚‚ moves on oxygenation ยท Hb Aโ‚c
    • Heme Synthesis
      • ALA synthase (rate-limiting, mitochondria)
      • Cytoplasmic middle steps โ†’ back to mitochondria
      • Ferrochelatase inserts Feยฒโบ ยท heme feedback
    • Globin Synthesis
      • Chr 16 ฮฑ/ฮถ ยท Chr 11 ฮต/ฮณ/ฮด/ฮฒ
      • 4 ฮฑ, 4 ฮณ, 2 ฮฒ, 2 ฮด genes
      • LCR, KLF1, BCL11A ยท heterodimer โ†’ tetramer
    • Ontogeny
      • Embryonic: Gower-1, Gower-2, Portland
      • Fetal/newborn: Hb F 60โ€“90%
      • Adult: A >95%, Aโ‚‚ <3.5%, F 1โ€“2%
    • Oxygen Transport
      • Sigmoid curve, cooperativity, Pโ‚…โ‚€ 27
      • Right: acid, COโ‚‚, heat, 2,3-BPG ยท Left: alkali, cold, Hb F, CO
      • T vs R ยท Hb F Pโ‚…โ‚€ 19โ€“21 ยท myoglobin hyperbolic
    • COโ‚‚ & NO
      • Carbonic anhydrase โ†’ HCOโ‚ƒโป ยท chloride shift
      • 85% via Hb buffering ยท carbamino
      • S-nitrosohemoglobin (ฮฒ cysteine)
    • Dyshemoglobins
      • MetHb: Feยณโบ, 630 nm, methylene blue, CYB5R3 (AR), Hb M (AD)
      • SulfHb: irreversible, no cyanide shift
      • COHb: 240ร—, left shift, 100% Oโ‚‚
    • Measurement
      • Cyanmethemoglobin 540 nm (reference)
      • SLS method
      • HPLC/electrophoresis/DNA ยท co-oximetry



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