Hematology Week 4

Hemoglobin structure

Globular protein with 2 pairs of unlike globin chains and 4 heme groups; each chain contains one heme.

Heme

Protoporphyrin IX ring with central ferrous iron (Fe2+); each heme can reversibly bind one O2.

Hb oxygen capacity

One hemoglobin molecule can carry up to four O2 molecules.

Primary Hb structure

The amino acid sequence of the globin chains.

Secondary Hb structure

Arrangement of the chain into helices and nonhelical regions.

Tertiary Hb structure

Three-dimensional folding of an individual globin chain.

Quaternary Hb structure

Tetramer formed from four globin subunits, each containing heme.

Adult HbA

Predominant adult hemoglobin; composed of α2β2.

HbA1c

Glycated HbA in which glucose attaches to the N-terminal valine of the β chain.

Start of heme synthesis

Mitochondria of erythrocyte precursors.

First heme-synthesis reaction

Glycine + succinyl-CoA → ALA, catalyzed by ALA synthase.

Rate-limiting enzyme of heme synthesis

ALA synthase catalyzes the key initial reaction producing ALA.

ALA synthase regulation

Heme inhibits transcription of the ALA synthase gene.

Iron delivery to developing RBCs

Transferrin transports Fe3+; Fe3+ is reduced in mitochondria and joins protoporphyrin IX.

Globin gene locations

α and ζ genes are on chromosome 16; ε, γ, δ, β genes are on chromosome 11.

Chromosome 16 globin genes

α1, α2, and ζ.

Chromosome 11 globin genes

ε, Gγ, Aγ, δ, and β.

Gower 1

Embryonic Hb composed of ζ2ε2.

Gower 2

Embryonic Hb composed of α2ε2.

Portland Hb

Embryonic Hb composed of ζ2γ2.

HbF

Fetal Hb composed of α2γ2.

HbA2

Adult Hb composed of α2δ2.

Normal adult Hb percentages

HbA1 >95%; HbA2 <3.5%; HbF 1–2%.

Hb function

Binds O2 in lungs, transports O2, and unloads O2 efficiently to tissues.

P50

PO2 at which hemoglobin is 50% saturated.

O2 dissociation curve

Sigmoidal curve representing cooperativity.

Right shift

Decreases Hb oxygen affinity, so O2 is released more easily.

Right-shift conditions

↑CO2, ↑H+, ↑acidity, ↑2,3-BPG, and ↑temperature.

Left-shift conditions

↓CO2, ↓PO2, ↓acidity, ↓2,3-BPG, ↓temperature, and fetal Hb.

T state

Tense, deoxygenated Hb conformation stabilized by 2,3-BPG.

R state

Relaxed, fully oxygenated Hb conformation.

2,3-BPG effect

Stabilizes deoxygenated Hb, decreases O2 affinity, and promotes O2 release.

HbF vs HbA

HbF has γ instead of β chains and has higher oxygen affinity than HbA.

Bohr effect and chloride shift

The lecture identifies both as mechanisms involved in carbon dioxide transport.

Ferrous iron importance

Heme iron must remain Fe2+ for functional oxygen binding.

Methemoglobin

Hb with oxidized ferric iron (Fe3+); cannot bind oxygen normally.

MetHb normal level

About 1% of total hemoglobin.

MetHb >30%

Can cause cyanosis and hypoxia.

MetHb >50%

Can cause coma and death.

MetHb appearance

Blood has a characteristic chocolate-brown color.

Sulfhemoglobin

Irreversible oxidation by sulfur-containing drugs/chemicals; sulfur becomes incorporated into heme.

Carboxyhemoglobin

Hb carrying CO instead of O2.

Hb affinity for CO

Hb affinity for CO is 240 times greater than for O2.

COHb in smokers

Smokers can have increased carboxyhemoglobin.

RBC average concentration

About 5 million RBCs per microliter of blood.

RBC average volume

About 90 fL.

RBC biconcavity

Allows deformation and supports optimal gas exchange.

RBC component recycling

Globin, iron, cytoplasmic proteins, and phospholipids are recycled; protoporphyrin is excreted as bilirubin.

Why RBCs use anaerobic glycolysis

RBCs lack mitochondria and therefore rely on anaerobic glycolysis for energy.

Embden-Meyerhof pathway

Anaerobic glycolysis in which glucose is catabolized to pyruvate.

EMP ATP yield

Consumes 2 ATP and generates 4 ATP for a net gain of 2 ATP.

Three RBC shunts

HMP pathway, methemoglobin reductase pathway, and Rapoport-Luebering pathway.

HMP pathway goal

Produces reducing substances, especially glutathione, to protect RBCs from oxidant damage.

NADPH in HMP

NADPH reduces GSSG to GSH through glutathione reductase.

GSH function

Reduced glutathione helps reduce peroxide and protects RBCs from oxidant damage.

Glutathione peroxidase

Uses GSH to reduce peroxide to water.

Glutathione reductase

Uses NADPH to reduce GSSG to GSH.

G6PD

Glucose-6-phosphate dehydrogenase participates in NADPH production in the HMP pathway.

MetHb reductase pathway goal

Maintains heme iron in the functional reduced ferrous (Fe2+) state.

Why methemoglobin is dysfunctional

Its iron is ferric (Fe3+) and cannot bind oxygen.

NADH metHb reduction

NADH from glycolysis reduces metHb using NADH methemoglobin reductase/cytochrome b5 reductase.

NADPH metHb reduction

NADPH from HMP can reduce metHb more efficiently with NADPH methemoglobin reductase.

Rapoport-Luebering shunt goal

Regulates oxygen delivery by controlling 2,3-BPG production.

2,3-BPG alternative names

2,3-bisphosphoglycerate, 2,3-diphosphoglycerate, or 2,3-DPG.

2,3-BPG and oxygen delivery

Decreases Hb O2 affinity and enhances oxygen delivery to tissues.

ATP cost of Rapoport-Luebering shunt

Using the shunt results in loss of 2 ATP molecules.

Why shunt use is limited

Its use must be limited because it causes loss of ATP production.

Main RBC metabolic pathway

Glycolytic pathway; accounts for about 90% of RBC glucose consumption.

HMP overall function

Provides NADPH and glutathione that protect functional Hb from oxidation.

MetHb reductase overall function

Protects Hb from oxidation using NADH from glycolysis and metHb reductase.

Rapoport-Luebering overall function

Controls 2,3-BPG production and therefore affects Hb oxygen affinity.

RBC deformability

Ability of an RBC to stretch and pass through narrow vessels and splenic pores.

RBC surface-to-volume effect

Excess surface area relative to volume increases stretching potential and deformability.

RBC aging and deformability

Deformability decreases as membrane surface area is lost during aging.

Loss of RBC membrane surface

Increases cellular viscosity and promotes damage in narrow capillaries or splenic pores.

Poorly deformable RBC fate

RBCs unable to pass through splenic pores are removed by splenic macrophages.

RBC membrane composition

About 8% carbohydrates, 52% proteins, and 40% lipids.

RBC membrane lipids

Lipid portion is approximately equal parts cholesterol and phospholipids.

Membrane lipid bilayer

Cholesterol and phospholipids form a bilayer; balance supports deformability and strength.

Increased membrane cholesterol

Causes loss of elasticity.

Membrane lipid imbalance

Can produce abnormal RBC morphologies such as spur cells and codocytes.

Glycolipid rafts

Located on the external membrane half and support carbohydrate side chains.

Transmembrane RBC proteins

Integral proteins that provide structural integrity to the RBC membrane.

Ankyrin/4.1 complex

Membrane protein-associated complex involved in RBC membrane structure.

Spectrin

Cytoskeletal protein; α- and β-spectrin provide lateral membrane stability.

RBC cytoskeleton proteins

Includes α/β-spectrin, ankyrin, protein 4.1, actin, adducin, tropomodulin, and dematin.

RBC membrane permeability

Impermeable to Na+, K+, Ca2+; permeable to water, HCO3−, and Cl−.

Aquaporin 1

Allows water to flow freely into the RBC in response to internal osmotic changes.

Na+/K+ ATPase

Maintains Na+ and K+ concentration gradients across the RBC membrane.

RBC sodium distribution

Na+ is low in RBC cytoplasm and high in plasma.

RBC potassium distribution

K+ is high in RBC cytoplasm and low in plasma.

Ca2+ ATPase

Regulates intracellular calcium concentration.

RBC calcium distribution

Ca2+ is maintained at low concentration in the RBC cytoplasm.

ATP cost of cation pumps

Cation pumps consume about 15% of RBC ATP production.