Master RBC membrane proteins, lipid bilayer structure, and their clinical significance for ASCP hematology questions.

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Chapter 3 โ€” RBC METABOLISM & MEMBRANE STRUCTURE

ASCP Board Exam ยท Lab-Focused Study Guide
Glycolysis & Shunts ยท G6PD ยท Methemoglobin ยท 2,3-BPG ยท Lipids ยท Transmembrane & Skeletal Proteins ยท Osmotic Balance ยท Practice Questions

1 ยท Priority Table & Quick Foundation

Topic Priority Why It’s Tested
The four metabolic pathways โ€” product and purpose of each HIGH ‘Which pathway generates ATP / NADPH / 2,3-BPG / keeps iron ferrous?’ is a guaranteed item
HMP shunt, G6PD, glutathione, Heinz bodies HIGH G6PD deficiency is the most common inherited RBC enzyme defect worldwide
Methemoglobin reductase (cytochrome b5 reductase) & cyanosis HIGH Cyanosis case questions; NADH vs NADPH confusion
Rapoport-Luebering shunt & 2,3-BPG (Oโ‚‚ release, ATP cost) HIGH Links to the oxygen dissociation curve and altitude/anemia adaptation
Vertical vs horizontal membrane linkages โ†’ hereditary spherocytosis vs elliptocytosis HIGH Mechanism questions for the two commonest membrane disorders
Transmembrane (integral) vs skeletal (peripheral) proteins HIGH ‘Which is an integral protein?’ โ€” glycophorin A vs ankyrin/spectrin/actin
Osmotic balance: cation pumps, Naโบ/Kโบ gradients, colloid osmotic hemolysis HIGH Explains why ATP failure โ†’ spherocytes โ†’ hemolysis
GPI anchor, CD55/CD59, PIGA โ†’ PNH HIGH Flow cytometry diagnosis of PNH is a classic board topic
Lipid asymmetry, flippases, PS exposure; cholesterol exchange โ†’ target cells/acanthocytes MEDIUM Senescence signal and acquired shape changes
Deformability determinants (surface area, MCHC/viscosity, elasticity) MEDIUM Why high MCHC or spheres shorten survival
Blood-group-bearing proteins (GPA MN, GPB Ss, GPC Gerbich, Rh/RhAG, Kell, Duffy, Kidd) MEDIUM Crossover with blood bank; RhAG โ†’ Rh-null
Glycolysis enzyme sequence (hexokinase, PFK, PK, etc.) MEDIUM PK deficiency; ATP-consuming vs ATP-generating steps
SDS-PAGE band numbering, molecular weights, copy numbers LOW Recall only that lower band number = higher molecular weight
Individual phospholipid chemistry, spectrin repeat counts LOW Not tested beyond outer vs inner leaflet
๐ŸŽฏ KEY POINT โ€” Where to spend your time

Focus: one product + one purpose + one disease for each pathway; vertical (โ†’ spherocytes) vs horizontal (โ†’ elliptocytes); integral vs skeletal proteins; the Naโบ/Kโบ/Caยฒโบ pump story; GPI-anchored CD55/CD59 and PNH. Skim: molecular weights, copy numbers, spectrin repeats, and every glycolytic intermediate past the key enzymes.

Quick Foundation โ€” from zero

  • A mature RBC has no nucleus, no mitochondria, no ribosomes (lost 24โ€“48 h after release). It can’t make proteins or burn fuel with oxygen.
  • Its only energy source is anaerobic glycolysis of plasma glucose, which enters through Glut-1 without spending energy (no insulin needed).
  • Carrying Oโ‚‚ and COโ‚‚ is passive. What needs energy is protecting the cell: pumping ions, keeping hemoglobin iron ferrous, fighting oxidants, and keeping the membrane flexible.
  • Three side roads branch off glycolysis: the HMP shunt (antioxidant NADPH), the methemoglobin reductase pathway (keeps iron Feยฒโบ), and the Rapoport-Luebering shunt (makes 2,3-BPG to unload Oโ‚‚).
  • The membrane is a lipid bilayer (barrier) stitched onto a protein net underneath (skeleton). Transmembrane proteins pin the bilayer to the skeleton โ€” the vertical links. Spectrin-to-spectrin links make the net โ€” the horizontal links.
  • Lose vertical links โ†’ the bilayer flakes off โ†’ spherocytes. Weaken horizontal links โ†’ the net can’t spring back โ†’ elliptocytes.
  • When ATP runs out, pumps fail, Naโบ and water rush in, the cell rounds up and bursts (colloid osmotic hemolysis).
๐Ÿ’ก Analogy

The RBC is a tent. The fabric is the lipid bilayer; the poles and ropes underneath are spectrin and actin (horizontal frame); the stakes that nail the fabric to the frame are band 3โ€“ankyrin and glycophorin Cโ€“4.1 (vertical). Pull out the stakes and pieces of fabric blow away โ€” the tent shrinks into a ball (spherocyte). Snap the ropes and the frame sags into an oval (elliptocyte). The generator (glycolysis) runs the pumps that keep the tent from flooding.

๐Ÿฅ Why this matters clinically

Most inherited hemolytic anemias are either an enzyme defect (G6PD, PK โ€” often normal shape, ‘nonspherocytic’) or a membrane defect (HS, HE). Knowing which pathway or linkage failed tells you which test to order.

Concept Plain-English Explanation
Anaerobic glycolysis (EMP) Glucose โ†’ lactate without oxygen; the RBC’s only ATP source
NADPH vs NADH NADPH (from HMP/G6PD) fights peroxide via glutathione; NADH (from glycolysis) reduces methemoglobin
Methemoglobin Hemoglobin with iron in the ferric (Feยณโบ) state โ€” can’t carry Oโ‚‚
2,3-BPG Small molecule that makes Hb let go of Oโ‚‚ in the tissues
Integral (transmembrane) protein Crosses the bilayer (band 3, glycophorins); carries antigens and transports
Peripheral (skeletal) protein Lies under the bilayer only (spectrin, ankyrin, actin, 4.1)
Semipermeable membrane Lets water and some small anions through; blocks cations and large molecules
GPI anchor Lipid ‘hook’ holding ~30 proteins (incl. CD55, CD59) on the outer surface

2 ยท Core Content Tables

2.1 Energy Needs of the RBC

Feature Details
Processes that REQUIRE energy Cation gradient maintenance ยท phospholipid asymmetry ยท skeletal protein deformability ยท keeping Hb iron ferrous ยท protecting proteins from oxidative denaturation ยท starting glycolysis ยท glutathione synthesis ยท nucleotide salvage
Process that does NOT require energy Oxygen (and COโ‚‚) transport โ€” passive binding and release by hemoglobin
Glucose entry Glut-1 transmembrane protein; no energy; independent of insulin
Why no reserve No glycogen, no mitochondria โ†’ depends minute-to-minute on plasma glucose
Clinical significance As enzymes decay, ATP falls โ†’ cell becomes senescent and is removed (~120 days). Inherited glycolytic enzyme defects โ†’ hereditary nonspherocytic hemolytic anemia

2.2 Embden-Meyerhof Pathway (Anaerobic Glycolysis)

Feature Details
Share of glucose ~90% (McKenzie: 90โ€“95%)
ATP balance Consumes 2 ATP (hexokinase, phosphofructokinase) ยท generates 4 ATP (phosphoglycerate kinase ร—2, pyruvate kinase ร—2) ยท net +2 ATP per glucose
Phase 1 Glucose โ†’ G6P (hexokinase) โ†’ F6P (G6P isomerase) โ†’ F1,6-BP (PFK) โ†’ G3P + DHAP (aldolase). Hexokinase and PFK steps limit the rate
Phase 2 G3P โ†’ 1,3-BPG (G3PD, generates NADH) โ†’ 3-PG + 2 ATP (phosphoglycerate kinase)
Phase 3 3-PG โ†’ 2-PG (mutase) โ†’ PEP (enolase) โ†’ pyruvate + 2 ATP (pyruvate kinase). PK is activated allosterically by F1,6-BP
End step Pyruvate โ†’ lactate (lactate dehydrogenase) regenerates NADโบ so glycolysis can continue; lactate/pyruvate leave the cell
Clinical significance PK deficiency = most common glycolytic enzyme defect โ†’ โ†“ ATP โ†’ rigid, dehydrated cells โ†’ chronic hemolysis (often โ†‘ 2,3-BPG upstream, so anemia is tolerated relatively well)*

Glycolysis is the RBC’s generator. Everything else in this chapter either branches off it (shunts) or runs on its ATP (pumps, flippases, skeleton).

๐Ÿงช PRE-ANALYTICAL โ€” Pyruvate kinase assay
  • EDTA or heparin whole blood; test before transfusion or wait ~3 months after (donor cells mask deficiency)*
  • Remove leukocytes and platelets (filtration) โ€” WBCs have high PK activity and can falsely normalize the result*
  • High reticulocyte counts raise enzyme activity (young cells) โ€” interpret against the retic count*

2.3 Hexose Monophosphate Shunt (Pentose Phosphate / Aerobic Glycolysis)

Feature Details
Share of glucose 5โ€“10% of G6P at rest; can rise up to ~30-fold during oxidative challenge
Key enzymes G6PD (G6P โ†’ 6-phosphogluconate, NADPโบ โ†’ NADPH) โ†’ 6-phosphogluconate dehydrogenase (โ†’ ribulose-5-P + COโ‚‚ + NADPH). Pentoses re-enter glycolysis
Mechanism of protection NADPH โ†’ glutathione reductase converts GSSG โ†’ GSH โ†’ glutathione peroxidase reduces Hโ‚‚Oโ‚‚ โ†’ Hโ‚‚O. Protects heme iron, globin โ€“SH groups, enzymes, and membrane lipids/thiols
Unique role G6PD is the only source of NADPH in the RBC
When it fails Peroxide oxidizes Hb โ†’ denatured Hb precipitates as Heinz bodies on the inner membrane โ†’ splenic macrophages remove them (‘bite cells’) or the whole cell โ†’ hemolysis. Membrane โ€“SH oxidation โ†’ leaky membranes
Triggers in G6PD deficiency Oxidant drugs (primaquine, sulfonamides, dapsone), infection, fava beans
Clinical significance G6PD deficiency = most common inherited RBC enzyme deficiency worldwide; X-linked* โ†’ mostly males; episodic hemolysis
๐Ÿงช PRE-ANALYTICAL โ€” G6PD testing
  • Fluorescent spot test (screen): NADPH fluoresces under UV โ€” deficient blood fails to fluoresce; quantitative spectrophotometric assay confirms
  • EDTA/heparin/ACD whole blood; avoid testing during or right after a hemolytic episode (old deficient cells destroyed; retics have more G6PD โ†’ false normal) and after transfusion โ€” repeat in ~2โ€“3 months*
  • Heinz bodies need a supravital stain (crystal violet, brilliant cresyl blue) โ€” invisible on Wright stain
  • Heterozygous females can be missed by screening tests because of mixed normal/deficient populations*

2.4 Methemoglobin Reductase Pathway

Feature Details
Why needed Heme iron is constantly oxidized by Oโ‚‚ and peroxide from Feยฒโบ to Feยณโบ (methemoglobin), which cannot bind Oโ‚‚. The HMP prevents oxidation but can’t efficiently reverse it
Enzyme Methemoglobin reductase = cytochrome b5 reductase (NADH diaphorase)
Mechanism Uses NADH generated when G3P โ†’ 1,3-BPG (G3PD step) โ†’ electron carrier cytochrome b5 โ†’ returns Feยณโบ to Feยฒโบ. Provides >65% of MetHb-reducing capacity. NADPH can reduce MetHb only slowly
Normal load ~2% of Hb oxidized daily; without the pathway MetHb would accumulate to 20โ€“40% (McKenzie)
Clinical significance Hereditary cytochrome b5 reductase deficiency (autosomal recessive โ€” e.g., affected brothers) or oxidant drugs/chemicals (nitrites, dapsone, local anesthetics*) โ†’ methemoglobinemia โ†’ cyanosis despite normal PaOโ‚‚, chocolate-brown blood
Treatment clue Ascorbic acid (vitamin C), a reducing agent, reduces MetHb non-enzymatically โ€” it turned Deeny’s ‘blue men’ pink. Methylene blue* works via NADPH (so it fails in G6PD deficiency)
๐Ÿงช PRE-ANALYTICAL โ€” Methemoglobin
  • Co-oximetry (multi-wavelength spectrophotometry) on arterial or venous blood โ€” pulse oximetry is unreliable*
  • Analyze promptly โ€” MetHb levels change on standing*
  • Sulfhemoglobin also causes cyanosis but is not reversed by reducing agents*

2.5 Rapoport-Luebering Pathway & 2,3-BPG

Feature Details
Reaction 1,3-BPG โ†’ 2,3-BPG (bisphosphoglycerate mutase) โ†’ 3-PG (bisphosphoglycerate phosphatase), bypassing phosphoglycerate kinase
Function 2,3-BPG (2,3-DPG) competes for the Oโ‚‚-binding site of Hb โ†’ โ†“ Hbโ€“Oโ‚‚ affinity โ†’ โ†‘ Oโ‚‚ delivery to tissues (right shift)
ATP cost Skips the PGK step โ†’ the cell loses the 2 ATP that step would make; net ATP from that glucose can drop toward zero โ†’ ATP deficit
Regulation Acidic pH and low 3-PG/2-PG inhibit the mutase; acidic pH and low ATP activate the phosphatase โ€” both push 1,3-BPG back toward ATP production
Increased 2,3-BPG High altitude, anemia, hypoxia (adaptive โ€” more Oโ‚‚ unloaded)
Clinical significance Deficient pathway โ†’ โ†‘ Oโ‚‚ affinity โ†’ tissue hypoxia โ†’ compensatory erythrocytosis. Stored blood loses 2,3-BPG*

2.6 Four Pathways at a Glance

Pathway % Glucose Key Enzyme Product Protects / Provides Defect โ†’ Disease
Embden-Meyerhof ~90% PK, PFK, hexokinase ATP (net 2) Energy for pumps, shape, flippases PK deficiency โ†’ hemolytic anemia
Hexose monophosphate 5โ€“10% G6PD NADPH โ†’ GSH Oxidant protection of Hb and membrane G6PD deficiency โ†’ Heinz body hemolysis
Methemoglobin reductase Branch of EMP Cytochrome b5 reductase Uses NADH Iron kept Feยฒโบ Methemoglobinemia โ†’ cyanosis
Rapoport-Luebering Branch of EMP BPG mutase 2,3-BPG Oโ‚‚ release to tissues โ†‘ Oโ‚‚ affinity โ†’ hypoxia/erythrocytosis

2.7 RBC Deformability

Feature Details
Numbers Biconcave; volume ~90 fL; surface area ~140 ยตmยฒ = ~40% more surface than a 90 fL sphere; stretches up to 2.5ร— resting diameter; passes ~2 ยตm splenic pores (McKenzie: ~3 ยตm) and capillaries
Membrane properties ~100ร— more elastic than a comparable latex membrane, yet laterally stronger than steel
Determinants โ‘  Geometry (excess surface-to-volume) โ‘ก Cytoplasmic viscosity (MCHC 32โ€“36%) โ‘ข Membrane elasticity (skeleton + lipids)
Mechanism of loss Aging RBCs lose membrane surface but keep Hb โ†’ MCHC rises โ†’ viscosity rises. MCHC >36% (spherocytes) โ†’ can’t pass splenic pores โ†’ removed by macrophages
Clinical significance Spherocytes, sickle cells, and dehydrated cells have short survival because they can’t deform

2.8 Membrane Lipids

Feature Details
Overall composition ~52% protein, ~40% lipid, ~8% carbohydrate
Bilayer Phospholipids with hydrophilic heads facing plasma and cytoplasm and hydrophobic tails hidden inside โ†’ impermeable barrier; reseals rapidly when torn
Cholesterol โ‰ˆ 1 molecule per phospholipid, equally in both leaflets; confers tensile strength. โ†‘ cholesterol โ†’ stronger but less elastic
Cholesterol exchange Continuous exchange with plasma lipoproteins (RBC can’t synthesize lipid); affected by plasma bile salts and LCAT. Abnormal cholesterol content/distribution โ†’ acanthocytes and target cells (liver disease)
Phospholipid asymmetry Outer: PC, SM ยท Inner: PS, PE (+ PI). Maintained by energy-dependent flippases, floppases, scramblases
PS exposure PS (the only negatively charged phospholipid) flips to the outer layer in senescent RBCs, sickle cell anemia, thalassemia โ†’ macrophage PS receptors โ†’ destruction
Glycolipids ~5% of outer leaflet; carbohydrate chains form the negatively charged glycocalyx (prevents adhesion, protects from microbes); carry ABH and Lewis antigens
Leaflet imbalance (McKenzie) Outer leaflet expansion โ†’ echinocytes ยท inner leaflet expansion โ†’ stomatocytes

2.9 Transmembrane (Integral) Proteins

Protein Function Blood Group / Note
Band 3 (anion exchanger 1) Clโป/HCOโ‚ƒโป exchange; most abundant transmembrane protein (~27% of protein, ~1.2 million copies); anchors ankyrin complex Carries most ABH determinants; Diego
Glut-1 (band 4.5) Glucose transport (no energy) Supports ABH antigens
Glycophorin A (PAS-1) ~85% of glycophorins; sialic acid โ†’ negative charge MN
Glycophorin B (PAS-4) ~10% of glycophorins Ss (with GPA = MNSs system)
Glycophorin C (PAS-2) ~4%; links to protein 4.1 complex Gerbich
Rh proteins Multipass (12ร—) lipoproteins D and CcEe antigens
RhAG Required for Rh antigen expression; probable COโ‚‚ gas transporter Loss โ†’ Rh-null with morphologic abnormalities (stomatocytes)*
Kell Znยฒโบ-binding endopeptidase Kell antigens (XK protein nearby)
Duffy G-proteinโ€“coupled (chemokine) receptor Duffy antigens
Kidd Urea transporter Kidd antigens
Aquaporin 1 Water channel Colton*
Pumps & cotransporters Naโบ/Kโบ-ATPase, Caยฒโบ-ATPase, Kโบ-Clโป, Naโบ-Kโบ-2Clโป etc. Osmotic balance
ICAM-4, CD47 Adhesion; CD47 ‘don’t eat me’/signal receptor LW (ICAM-4)

Transmembrane proteins cluster into two complexes named by their skeletal anchor: the ankyrin complex (band 3, protein 4.2, RhAG, Rh, GPA, CD47, LW) and the 4.1 complex (band 3, Rh, Duffy, Kell/XK, GPC, Glut-1 + dematin, adducin, actin, tropomyosin, tropomodulin). These are the vertical connections.

2.10 GPI Anchor & PNH

Feature Details
Structure Phosphatidylinositol (PI) in the outer leaflet + glycan core โ†’ glycosylphosphatidylinositol (GPI) anchor
Proteins held >30, including CD55 (DAF, decay-accelerating factor) and CD59 (MIRL, membrane inhibitor of reactive lysis) โ€” both protect against complement
Gene PIGA encodes a glycosyltransferase needed early in GPI synthesis
Mechanism of disease Acquired PIGA mutation in a stem cell clone โ†’ no GPI anchor โ†’ CD55/CD59 absent โ†’ complement-mediated intravascular hemolysis = paroxysmal nocturnal hemoglobinuria (PNH)
Lab diagnosis* Flow cytometry: loss of CD55/CD59 on RBCs and FLAER/GPI-linked markers on granulocytes/monocytes (replaced the older sucrose hemolysis and Ham acid tests)
๐Ÿงช PRE-ANALYTICAL โ€” PNH flow cytometry*
  • EDTA peripheral blood; process within the lab’s validated window (typically โ‰ค48 h)
  • Granulocytes/monocytes are preferred for clone size: RBC clones are underestimated after hemolysis or transfusion (the GPI-deficient cells are lysed or diluted)
  • Report RBC type II (partial) and type III (complete deficiency) populations separately

2.11 Cytoskeletal (Peripheral) Proteins

Protein SDS-PAGE Band Function
ฮฑ-spectrin 1 Antiparallel heterodimer with ฮฒ-spectrin; primary skeletal protein (~16% of protein)
ฮฒ-spectrin 2 Binds ankyrin and actin/4.1 (~14%)
Ankyrin 2.1 Anchors spectrin to band 3 and protein 4.2 (ankyrin complex)
Adducin 2.9 Caps actin filaments; links to band 3
Protein 4.1 4.1 Stabilizes spectrinโ€“actin junction; anchors 4.1 complex (GPC)
Protein 4.2 4.2 Strengthens ankyrinโ€“band 3 binding
Dematin 4.9 Actin bundling; links to Glut-1
Actin 5 Short filaments of 14โ€“16 monomers; binds ฮฒ-spectrin
Tropomodulin 5 Caps actin filaments
G3PD 6 Glycolytic enzyme bound to membrane
Tropomyosin 7 Regulates actin filament length

Spectrin heterodimers (ฮฑ + ฮฒ, antiparallel) join head-to-head into tetramers, and tetramer tails meet at actinโ€“4.1 junctional complexes, forming a hexagonal lattice just under the bilayer โ€” the horizontal (lateral) support. During deformation spectrin dimer bonds open and close and its helical repeats unfold and refold, then spring back.

Linkage Proteins Involved Defect โ†’ Disease Mechanism
Vertical (bilayer โ†” skeleton) Band 3, ankyrin, protein 4.2, ฮฑ/ฮฒ-spectrin (spectrinโ€“ankyrinโ€“band 3) Hereditary spherocytosis (mostly autosomal dominant) Too few anchors โ†’ lipid bilayer lost as small blebs/vesicles, volume kept โ†’ โ†“ surface-to-volume โ†’ spherocyte
Horizontal (within skeleton) Spectrin dimerโ€“dimer bonds, spectrinโ€“ankyrinโ€“4.1 junction Hereditary elliptocytosis (ovalocytosis; autosomal dominant) Membrane fails to rebound after deformation โ†’ cells progressively elongate; mild to severe hemolysis
๐Ÿงช PRE-ANALYTICAL โ€” Membrane defect testing*
  • Osmotic fragility: fresh heparinized blood; spherocytes lyse in less-hypotonic saline (โ†‘ fragility); incubated (24 h, 37 ยฐC) test increases sensitivity; results falsely normal after transfusion or with many retics
  • EMA (eosin-5โ€ฒ-maleimide) binding by flow cytometry: โ†“ fluorescence in HS (EMA binds band 3); EDTA blood, stable several days refrigerated
  • SDS-PAGE of membrane proteins (reference labs): Coomassie blue stains proteins, PAS stains glycophorins; lower band number = higher molecular weight

2.12 Osmotic Balance & Permeability

Feature Details
Permeable to Water (via aquaporin 1) and anions HCOโ‚ƒโป and Clโป (via band 3)
Impermeable to Naโบ, Kโบ, Caยฒโบ and large molecules (proteins) โ€” the membrane is semipermeable
Gradients Intracellular : extracellular Naโบ 1 : 12 and Kโบ 25 : 1 (McKenzie: RBC Naโบ 5.4โ€“7 vs plasma 135โ€“145; RBC Kโบ 98โ€“106 vs plasma 3.6โ€“5.0 mmol/L). Intracellular Caยฒโบ only 5โ€“10 ยตmol/L
Pumps ATP-dependent Naโบ-ATPase / Kโบ-ATPase (Naโบ/Kโบ pump: 3 Naโบ out, 2 Kโบ in) ยท Caยฒโบ-ATPase (controlled by calmodulin; needs Mgยฒโบ) ยท pumps use ~15% of RBC ATP
Mechanism of failure ATP loss or pump damage โ†’ Caยฒโบ and Naโบ influx โ†’ water follows โ†’ cell swells โ†’ spheroid โ†’ ruptures = colloid osmotic hemolysis
Calcium โ†‘ Caยฒโบ โ†’ cross-links skeletal proteins (rigid cells, e.g., irreversibly sickled cells) and opens the Gรกrdos Kโบ channel โ†’ Kโบ and water loss โ†’ dehydration (McKenzie)
Clinical example Sickle cell disease: crystallized HbS deforms the membrane โ†’ โ†‘ Naโบ, Kโบ, especially Caยฒโบ permeability โ†’ hemolysis
๐Ÿงช PRE-ANALYTICAL โ€” Potassium & hemolysis
  • Because RBC Kโบ is ~25ร— plasma Kโบ, in-vitro hemolysis falsely raises serum potassium โ€” check the hemolysis index before reporting
  • Delayed separation or refrigerated whole blood lets Kโบ leak out of cells (pumps stop without glucose/warmth)*

*From general hematology/laboratory practice, not stated in the Rodak Ch. 9 or McKenzie source pages.

3 ยท Visual Memory Hooks

๐Ÿง  The four pathways
โ€œEvery Hungry Man Runsโ€ โ†’ EMP = Energy (ATP) ยท HMP = Hero against oxidants (NADPH) ยท MetHb reductase = Maintains Feยฒโบ (NADH) ยท R-L = Releases Oโ‚‚ (2,3-BPG).
๐Ÿง  NADPH vs NADH
โ€œP for Protection, H for Hemeโ€ โ†’ NADPH from HMP Protects via glutathione; NADH from glycolysis fixes Heme iron via MetHb reductase.
๐Ÿง  Glycolysis ATP
โ€œSpend 2, Earn 4, Keep 2โ€ โ†’ hexokinase + PFK spend; PGK + PK earn; net +2.
๐Ÿง  G6PD triggers
โ€œFava, Fever, and Pharmacyโ€ โ†’ fava beans, infection, oxidant drugs (primaquine, sulfa, dapsone).
๐Ÿง  2,3-BPG
โ€œBPG = Bye-bye Precious Gasโ€ โ†’ more 2,3-BPG = Hb lets Oโ‚‚ go. Costs 2 ATP.
๐Ÿง  Methemoglobin
โ€œFerric is Frozenโ€ โ†’ Feยณโบ can’t carry Oโ‚‚ โ†’ blue patient, brown blood; Vitamin C thaws it.
๐Ÿง  Energy-free process
โ€œOxygen rides freeโ€ โ†’ Oโ‚‚/COโ‚‚ transport is the only listed RBC process that needs no energy.
๐Ÿง  Phospholipid leaflets
โ€œPC and SM Sit outside; PS and PE Party insideโ€ โ†’ PS outside = ‘eat me’.
๐Ÿง  Glycophorins
โ€œA-MN, B-Ss, C-Gerbich (Aโ†’M, Bโ†’S, Cโ†’G)โ€.
๐Ÿง  Vertical vs horizontal
โ€œVertical Vesicles โ†’ Spheres; Horizontal Holds shape โ†’ Ellipsesโ€ โ†’ vertical defect = HS; horizontal defect = HE.
๐Ÿง  Ankyrin complex
โ€œBand 3 Asks Ankyrin (with 4.2) to Hug Spectrinโ€ โ†’ the major vertical anchor.
๐Ÿง  Band numbering
โ€œBig proteins finish firstโ€ โ†’ lowest band number = heaviest protein (spectrin = bands 1, 2).
๐Ÿง  Ion gradients
โ€œSodium Stays Out (1:12), Potassium Parties In (25:1)โ€ โ†’ pump: 3 Naโบ out, 2 Kโบ in.
๐Ÿง  PNH
โ€œPIGA Pulls the anchor โ†’ 55 and 59 go missingโ€ โ†’ no GPI โ†’ no CD55/CD59 โ†’ complement lysis.

Pattern Triggers

If you seeโ€ฆ Thinkโ€ฆ Becauseโ€ฆ
Episodic hemolysis after primaquine/sulfa/fava; Heinz bodies; bite cells G6PD deficiency (HMP shunt) No NADPH โ†’ no GSH โ†’ peroxide denatures Hb
Chronic hemolysis, normal-shaped RBCs, no Heinz bodies, โ†“ ATP PK deficiency (EMP) ATP-poor cells lose cation control, become rigid
Cyanosis, normal PaOโ‚‚, chocolate-brown blood; improves with vitamin C Methemoglobinemia (cytochrome b5 reductase deficiency) Feยณโบ Hb not reduced back to Feยฒโบ
Brothers (siblings) with lifelong cyanosis, normal heart and lungs Hereditary (autosomal recessive) MetHb reductase deficiency Inherited enzyme defect, not acquired
Right-shifted Oโ‚‚ curve at altitude or in chronic anemia โ†‘ 2,3-BPG Rapoport-Luebering activity rises to unload Oโ‚‚
Spherocytes, โ†‘ MCHC, โ†‘ osmotic fragility, family history Hereditary spherocytosis (vertical defect) Lost membrane vesicles โ†’ โ†“ surface area
Elliptocytes/ovalocytes, family history Hereditary elliptocytosis (horizontal defect) Spectrin dimerโ€“dimer or spectrinโ€“4.1 bonds weak
Hemoglobinuria, cytopenias, thrombosis; CD55/CD59-negative cells PNH Acquired PIGA mutation โ†’ no GPI anchor
Rh-null phenotype with abnormal RBC shape RhAG absence RhAG required for Rh antigen expression
Target cells / acanthocytes in liver disease Membrane cholesterol imbalance Lipid exchange with abnormal plasma
Very high Kโบ in a hemolyzed sample In-vitro hemolysis artifact RBC Kโบ is ~25ร— plasma

Commonly Confused Pairs

Feature A B
NADPH vs NADH HMP (G6PD) โ†’ glutathione โ†’ peroxide detox EMP (G3PD) โ†’ MetHb reductase โ†’ Feยณโบ โ†’ Feยฒโบ
HMP vs MetHb reductase Prevents oxidation Reverses oxidation of heme iron
Integral vs peripheral Glycophorin A, band 3, Rh, Glut-1 (cross the bilayer) Spectrin, ankyrin, actin, 4.1, 4.2 (under the bilayer)
Vertical vs horizontal Spectrinโ€“ankyrinโ€“band 3โ€“4.2 โ†’ HS Spectrinโ€“spectrin, spectrinโ€“actinโ€“4.1 โ†’ HE
Aerobic vs anaerobic glycolysis (Rodak terms) Aerobic = HMP (oxidative) Anaerobic = EMP (ATP)
Cholesterol โ†‘ vs โ†“ Stronger, less elastic More elastic, less tensile strength
CD55 vs CD59 DAF โ€” accelerates decay of C3/C5 convertases MIRL โ€” blocks membrane attack complex
Coomassie vs PAS on SDS-PAGE Stains most proteins Stains glycophorins (carbohydrate-rich)
Colloid osmotic hemolysis vs Gรกrdos dehydration Naโบ/water in โ†’ swelling โ†’ lysis Caยฒโบ โ†’ Kโบ/water out โ†’ dense, dehydrated cell

Timeline & Kinetics Table

Marker / Event Onset Peak Duration Best Exam Use
Loss of ribosomes & mitochondria After marrow release โ€” 24โ€“48 h Why mature RBCs rely on glycolysis only
HMP activity under oxidant stress Immediate Up to ~30ร— baseline While stress persists Reserve capacity; fails in G6PD deficiency
Hemolysis after oxidant drug in G6PD deficiency* ~1โ€“3 days โ€” Often self-limited as old cells are cleared Why testing during crisis can be falsely normal
Methemoglobin formation Continuous (~2%/day) 20โ€“40% if unreduced โ€” Size of the daily reducing workload
Lipid lateral redistribution Within ~100 ms of stress โ€” โ€” How the membrane deforms at a capillary
Enzyme decline โ†’ senescence Throughout life span โ€” ~120 days Why RBCs die

4 ยท Diagnostic Flowcharts

Flowchart 1 โ€” Hereditary hemolytic anemia: enzyme or membrane?

Chronic/episodic hemolysis (โ†‘ retic, โ†‘ indirect bili, โ†“ Hp), DAT negative โ†’ Look at the smear
Spherocytes, โ†‘ MCHC, family history โ†’ Membrane โ€” vertical defect โ†’ HS
Elliptocytes/ovalocytes โ†’ Membrane โ€” horizontal defect โ†’ HE
Normal shape; hemolysis triggered by drug/infection; Heinz bodies, bite cells โ†’ Enzyme โ€” HMP โ†’ G6PD deficiency
Normal shape; chronic hemolysis; no oxidant trigger โ†’ Enzyme โ€” EMP โ†’ PK deficiency (nonspherocytic)
โš ๏ธ Common mistake: calling every hereditary hemolysis ‘spherocytosis’. Enzyme defects typically give nonspherocytic hemolytic anemia.

Flowchart 2 โ€” Cyanosis with normal lungs and heart

Blue skin, PaOโ‚‚ normal, SpOโ‚‚ low, brown blood โ†’ Measure MetHb by co-oximetry
โ†‘ MetHb + new oxidant drug/chemical โ†’ Acquired methemoglobinemia
โ†‘ MetHb since birth, affected sibling โ†’ Hereditary cytochrome b5 reductase deficiency
Responds to vitamin C (reducing agent) โ†’ Confirms reducible MetHb
Cyanosis not reversible, abnormal spectrum โ†’ Consider sulfhemoglobin or Hb M variant*
โš ๏ธ Common mistake: choosing the HMP shunt (NADPH) as the pathway that reduces MetHb. The main pathway uses NADH via cytochrome b5 reductase.

Flowchart 3 โ€” Timing: when to test enzymes after hemolysis

Suspected G6PD or PK deficiency during acute hemolysis โ†’ Note retic count and transfusion history
High retic or recent transfusion โ†’ Enzyme level may be falsely normal (young/donor cells)*
Screen normal but strong clinical suspicion โ†’ Repeat ~2โ€“3 months after recovery / transfusion*
Screen low (no fluorescence) โ†’ Confirm with quantitative assay โ†’ deficiency
โš ๏ธ Common mistake: trusting a normal G6PD result drawn in the middle of a hemolytic crisis.

Flowchart 4 โ€” Method selection for a suspected membrane disorder*

Spherocytes on smear, DAT negative โ†’ Rule out immune hemolysis first (DAT)
Need a screening test for HS โ†’ Osmotic fragility (ยฑ incubated) or EMA binding by flow
Need to identify the defective protein โ†’ SDS-PAGE of membrane proteins (reference lab) / molecular testing
Hemoglobinuria + cytopenias โ†’ Not a structural membrane defect โ†’ PNH flow (CD55/CD59, FLAER)
โš ๏ธ Common mistake: diagnosing HS from osmotic fragility alone when spherocytes could be immune (warm AIHA) โ€” the DAT separates them.

Flowchart 5 โ€” Why is this cell swelling and lysing?

Glucose deprivation or glycolytic enzyme defect โ†’ โ†“ ATP
โ†“ ATP โ†’ Naโบ/Kโบ and Caยฒโบ pumps fail (they normally use ~15% of ATP)
Naโบ and Caยฒโบ enter, water follows โ†’ Cell swells โ†’ sphere โ†’ loses deformability
Spheres trapped in spleen or rupture โ†’ Colloid osmotic hemolysis
โš ๏ธ Common mistake: thinking RBCs need ATP to carry oxygen. They need it to survive (pumps, shape), not for Oโ‚‚ binding.

5 ยท 25 High-Yield Facts

# Fact
1 Mature RBCs lack mitochondria and depend entirely on anaerobic glycolysis for ATP.
2 Glucose enters the RBC through Glut-1 without energy expenditure or insulin.
3 Oxygen and COโ‚‚ transport by hemoglobin does not require energy.
4 The Embden-Meyerhof pathway consumes 2 ATP and generates 4, for a net gain of 2 ATP per glucose.
5 Hexokinase and phosphofructokinase are the ATP-consuming, rate-limiting steps of glycolysis.
6 Pyruvate kinase is activated allosterically by fructose-1,6-bisphosphate.
7 Glycolytic enzyme deficiencies cause hereditary nonspherocytic hemolytic anemia.
8 About 5โ€“10% of glucose-6-phosphate enters the HMP shunt at rest, rising up to 30-fold under oxidative stress.
9 G6PD is the only source of NADPH in the RBC.
10 NADPH keeps glutathione reduced so glutathione peroxidase can detoxify hydrogen peroxide.
11 G6PD deficiency is the most common inherited RBC enzyme deficiency worldwide.
12 Methemoglobin contains ferric (Feยณโบ) iron and cannot carry oxygen.
13 Cytochrome b5 reductase uses NADH from glycolysis and provides over 65% of methemoglobin-reducing capacity.
14 The Rapoport-Luebering pathway forms 2,3-BPG, which lowers hemoglobinโ€“oxygen affinity and increases Oโ‚‚ delivery.
15 Diverting 1,3-BPG to 2,3-BPG sacrifices 2 ATP.
16 The RBC membrane is about 52% protein, 40% lipid, and 8% carbohydrate.
17 The RBC has about 40% more surface area than a sphere of equal volume, allowing deformation.
18 MCHC above 36% increases viscosity and reduces deformability.
19 Phosphatidylcholine and sphingomyelin are in the outer leaflet; phosphatidylserine and phosphatidylethanolamine are in the inner leaflet.
20 Exposure of phosphatidylserine on the outer leaflet signals macrophages to destroy the RBC.
21 Band 3 is the most abundant transmembrane protein and exchanges chloride for bicarbonate.
22 Glycophorin A carries MN, glycophorin B carries Ss, and glycophorin C carries Gerbich antigens.
23 Vertical linkage defects (band 3, ankyrin, protein 4.2, spectrin) cause hereditary spherocytosis; horizontal defects cause hereditary elliptocytosis.
24 Naโบ/Kโบ and Caยฒโบ pumps keep RBC Kโบ high and Naโบ and Caยฒโบ low; ATP failure causes colloid osmotic hemolysis.
25 An acquired PIGA mutation prevents GPI anchor synthesis, leading to CD55/CD59 loss and paroxysmal nocturnal hemoglobinuria.
โญ EXAM TIP โ€” The single most testable pattern

Match product to pathway to disease: ATP โ†’ EMP โ†’ PK deficiency ยท NADPH โ†’ HMP โ†’ G6PD deficiency (Heinz bodies) ยท NADH โ†’ MetHb reductase โ†’ methemoglobinemia (cyanosis) ยท 2,3-BPG โ†’ Rapoport-Luebering โ†’ Oโ‚‚ affinity. For the membrane: vertical = spherocytes, horizontal = elliptocytes.

Quick Interpretation Table

Lab Finding Clinical Interpretation
G6PD fluorescent spot test: no fluorescence G6PD deficiency (no NADPH produced)
Heinz bodies on supravital stain + bite cells Oxidant hemolysis (HMP failure)
โ†‘ MetHb on co-oximetry, cyanosis, normal PaOโ‚‚ Methemoglobinemia
Spherocytes + โ†‘ osmotic fragility + DAT negative Hereditary spherocytosis (vertical defect)
Elliptocytes > 25% of RBCs, family history* Hereditary elliptocytosis (horizontal defect)
RBCs/granulocytes lacking CD55 and CD59 by flow PNH (GPI anchor deficiency)
Falsely high Kโบ with visible hemolysis In-vitro hemolysis โ€” recollect

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

Glycolysis (EMP)

  • ~90% glucose ยท net 2 ATP (โˆ’2 +4)
  • Spend: hexokinase, PFK
  • Earn: PGK, PK
  • PK โ† F1,6-BP
  • Defect โ†’ nonspherocytic HA (PK)

HMP Shunt

  • 5โ€“10% ยท up to 30ร— under stress
  • G6PD โ†’ NADPH (only source)
  • NADPH โ†’ GSH โ†’ peroxide โ†’ Hโ‚‚O
  • Defect โ†’ Heinz bodies, bite cells
  • Spot test: no fluorescence

MetHb & 2,3-BPG

  • b5 reductase + NADH โ†’ Feยฒโบ (>65%)
  • MetHb 2%/day ยท 20โ€“40% if unreduced
  • Cyanosis ยท brown blood ยท vitamin C
  • BPG mutase โ†’ 2,3-BPG โ†’ right shift
  • Costs 2 ATP ยท โ†‘ at altitude/anemia

Deformability Numbers

  • ~90 fL ยท 140 ยตmยฒ ยท +40% surface
  • Stretch 2.5ร— ยท pores ~2โ€“3 ยตm
  • MCHC 32โ€“36% ยท >36% = rigid
  • 52% protein ยท 40% lipid ยท 8% CHO

Lipids

  • Outer: PC, SM ยท Inner: PS, PE
  • PS out = eat me (senescent, SCD, thal)
  • Cholesterol โ‰ˆ 1:1 phospholipid, both leaflets
  • โ†‘ cholesterol: strong, less elastic
  • Glycolipids: ABH, Lewis

Transmembrane Proteins

  • Band 3: Clโป/HCOโ‚ƒโป, most abundant
  • GPA MN ยท GPB Ss ยท GPC Gerbich
  • RhAG โ†’ Rh-null ยท Kidd = urea
  • Glut-1 glucose ยท aquaporin 1 water
  • Duffy = chemokine receptor

Skeleton

  • Bands 1/2 = ฮฑ/ฮฒ spectrin ยท 2.1 ankyrin
  • 4.1 junction ยท 4.2 ankyrin helper ยท 5 actin
  • Vertical: spectrinโ€“ankyrinโ€“band 3โ€“4.2 โ†’ HS
  • Horizontal: spectrinโ€“spectrin, spectrinโ€“actinโ€“4.1 โ†’ HE
  • Low band # = high MW

Osmotic Balance

  • Naโบ in:out 1:12 ยท Kโบ 25:1
  • Caยฒโบ 5โ€“10 ยตmol/L (calmodulin)
  • Na/K pump 3 out / 2 in
  • Pumps = 15% of ATP
  • โ†“ ATP โ†’ colloid osmotic hemolysis

Confusing Pairs

  • NADPH (HMP) โ‰  NADH (MetHb reductase)
  • Prevent oxidation (HMP) โ‰  reverse (b5R)
  • Integral (GPA) โ‰  peripheral (ankyrin)
  • CD55 DAF โ‰  CD59 MIRL
  • Coomassie proteins โ‰  PAS glycophorins

Associations

  • PIGA โ†’ no GPI โ†’ PNH
  • Sulfa/primaquine/fava โ†’ G6PD
  • Blue brothers โ†’ b5R deficiency
  • Liver disease โ†’ target cells
  • Sickle โ†’ Caยฒโบ โ†’ dehydration

9 ยท Final Memory Map

  • โš™๏ธ RBC METABOLISM & MEMBRANE
    • Energy Needs
      • No mitochondria โ†’ anaerobic glycolysis only
      • Glut-1 glucose entry (no insulin)
      • Oโ‚‚ transport needs no energy
    • Embden-Meyerhof
      • Net 2 ATP ยท hexokinase/PFK spend, PGK/PK earn
      • PK deficiency โ†’ nonspherocytic HA
    • Shunts
      • HMP: G6PD โ†’ NADPH โ†’ GSH โ†’ Heinz bodies if deficient
      • MetHb reductase: NADH + b5 reductase โ†’ Feยฒโบ; cyanosis if deficient
      • Rapoport-Luebering: 2,3-BPG โ†’ Oโ‚‚ release, costs 2 ATP
    • Membrane Lipids
      • 52/40/8 ยท cholesterol strength vs elasticity
      • Outer PC/SM ยท inner PS/PE ยท PS out = eat me
      • Glycolipids ABH/Lewis ยท target cells in liver disease
    • Membrane Proteins
      • Transmembrane: band 3, glycophorins, Rh/RhAG, Kell, Duffy, Kidd, Glut-1
      • Ankyrin complex + 4.1 complex = vertical
      • GPI: CD55/CD59 ยท PIGA โ†’ PNH
    • Skeleton
      • ฮฑ/ฮฒ spectrin tetramers ยท hexagonal lattice
      • Vertical defect โ†’ HS ยท Horizontal defect โ†’ HE
      • SDS-PAGE: Coomassie / PAS
    • Osmotic Balance
      • Water + anions pass ยท cations blocked
      • Naโบ 1:12 ยท Kโบ 25:1 ยท pumps use 15% ATP
      • ATP loss โ†’ colloid osmotic hemolysis ยท Caยฒโบ โ†’ dehydration



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