HEMA 311: Hgb Metabolism

Hemoglobin (Hgb)

• 95% of RBC cytoplasmic content

• Molecular weight of 64k Da

• Transport oxygen and nitric oxide (NO)

• Contributes to acid-base balance (by binding and releasing hydrogen ions)

X-Ray Crystallography

Hgb structure was first described using?

Methemoglobin (MetHgb)

Other term for oxidized Hgb

Heme Composition

A central atom of divalent ferrous iron with a ring consisting of: (atoms)

• Carbon (C),

• Hydrogen (H), and

• Nitrogen (N)

Ferroprotoporphyrin IX

Other term for Heme (Ferro = Iron) (Protoporphyrin IX = Heme ring)

Protoporphyrin IX

Other term for the heme ring (C + H + N)

Globin Chains

Consists of two identical pairs of unlike polypeptide chains

Hgb Primary Structure

Refers to amino acid sequence of polypeptide chains

Hgb Secondary Structure

Refers to chain arrangements in helices and nonhelices

Hgb Tertiary Structure

Refers to arrangement of helices into a pretzel-like configuration

E7 and F8

Between what helix group of the globin chain is a heme group suspended in?

Proximal Histidine

Other term for F8

Distal Histidine

Other term for E7 and is only a residue

Hgb Quaternary Structure

Tetramer; describes the complete Hgb molecule

Adult Hgb A

• 2 α-globin chains (α1 and α2)

• 2 β-globin chains (β1 and β2)

  • important for stability of Hgb quaternary structure

Glycation

Post-translational modification formed by non-enzymatic binding of various sugars to globin chain amino groups

HbA1c

Most characterized glycated Hgb; glucose attaches to N-terminal valine of β-chain

• Normal value of 4-6%

Pronormoblast stage

Erythrocyte precursor where heme and globin biosynthesis begins

Reticulocyte / Polychromatic Erythrocyte

Erythrocyte precursor where heme and globin biosynthesis still occurs but eventually end

Ferrochelatase

Heme synthase that catabolizes ferrous Iron + protoporphyrin IX to make heme

Transferrin

Plasma protein that carries iron in ferric state to developing RBCs

• Brought into endosome for acidification to release iron into mitochondria

Mitochondria

Organelle that reduces ferric iron to ferrous iron and combines it with protoporphyrin IX (to make heme)

Cytoplasm

Environment that contains organelles where heme is joined to globin chains

Normal Hemoglobin

.

1.34 mL

Volume of oxygen per gram of Hgb

Partial Pressure of Oxygen (PO2)

Defined in terms of amount of oxygen needed to saturate 50% of Hgb (P50 value)

P50 = 27 mmHg

PO2 Normal Value

P50 = >27 mmHg

Right shift in hemoglobin-oxygen dissociation curve and occurs in:

• increased 2,3-BPG

• acidosis (lowered pH) = Bohr effect (curve due to change in pH)

• increased temp

• increased partial pressure of CO2 (PCO2)

P50 = <27 mmHg

Left shift in hemoglobin-oxygen dissociation curve and occurs in:

• decreased 2,3-BPG

• alkalosis (lowered pH) = Bohr effect (curve due to change in pH)

• decreased temp

• decreased partial pressure of CO2 (PCO2)

• fetal Hgb

• Myglobin

Rhabdomylosis

Myoglobin is released into plasma due to damage to the muscles

Nitric Oxide (NO)

A vasodilator that attaches to cysteine on the globin chain, forming S-nitrosoHgb

• Vasoconstriction: NO transported by free Hgb away from vessel wall (Hgb outside RBC)

• Vasodilation: NO transported by Hgb towards vessel wall

Dyshemoglobins

Dysfunctional Hgbs that are unable to transport oxygen

Methemoglobinemia

Increase in methemoglobinemia

Toxic Methemoglobinemia

Acquired form of MetHgbemia that occurs in normal individuals after exposure to exogenous oxidant

1. Nitrites

2. Primaquine

3. Dapsone

4. Benzocaine

   • <25% = Asymptomatic

   • 30% = Cyanosis or Hypoxia symptoms

   • >50% = Coma or Death

Chocolate Brown

Color of blood in MetHgb

Hemiglobin

Other name of MetHgb

Hereditary Methemoglobinemia

1. NADH-CYB5R3: gene mutation involved that causes enzyme defects in hereditary MetHgbemia (autosomal recessive pattern) (Type 1)

2. Globin Gene Mutation: α- , β- , and γ-globin genes are mutated, producing structurally abnormal polypeptide chain that favors oxidized ferric iron that produces MetHgb called “M hemoglobin / Hb M” (autosomal dominant pattern) (Type 2)

Sulfhemoglobin (HgbS)

Formed by addition of sulfur atom to pyrrole ring of heme

Pyrrole Ring of Heme

5-membered heterocyclic ring that involves the nitrogen (N) that binds to iron

Porphyrin Ring

4 pyrrole rings joined together by methine bridges (=CH-)

Greenish

Color of blood with HgbS (due to C. perfringens / C. welchii “old name lang niya”)

Mauve-Lavender

Color of blood in sulfhemoglobinemia

Carboxyhemoglobin (COHb)

Results from combination of carbon monoxide (CO) with heme iron

• causes left shift in hemoglobin-oxygen dissociation curve

• toxic effects appear at blood levels of 20-30% COHb

Carbon Monoxide (CO)

Termed as the “silent killer” because it is an odorless and colorless gas

Carbon Monoxide Poisoning

Diagnosis is made if COHb levels is:

• >3% in nonsmokers

• >10% in smokers

Cyanmethemoglobin Method

Golden standard for Hgb assay