Chem 233 Lecture 16 – Modulators of Hemoglobin Oxygen-Binding

Question-and-answer flashcards covering the Bohr effect, BPG modulation, fetal hemoglobin, abnormal Hb variants (including sickle-cell), and alternate oxygen transport proteins.

What is the Bohr effect?

A phenomenon in which increasing H⁺ concentration (lower pH) shifts hemoglobin’s O₂-binding curve to the right, decreasing O₂ affinity.

How does increasing acidity (lower pH) affect hemoglobin’s O₂-binding curve?

It shifts the curve to the right, meaning hemoglobin releases O₂ more readily.

Which metabolic by-products most commonly lower blood pH?

Carbon dioxide (CO₂) and lactic acid produced by active tissues, especially muscle.

Write the hydration reaction of CO₂ that produces protons in blood.

CO₂ + H₂O ⇌ HCO₃⁻ + H⁺

Give the chemical equation that illustrates proton-induced O₂ release from hemoglobin.

Hb–O₂ + H⁺ ⇌ Hb–H⁺ + O₂

Where do “Bohr protons” bind on hemoglobin?

Primarily at the C-terminal His146 of the β-chains in the T-state (interacting with Asp94).

How does CO₂ directly bind to hemoglobin and affect O₂ affinity?

CO₂ forms carbamates at the N-termini of the globin chains, increasing acidity and promoting O₂ release (Bohr effect).

What is the effect of chloride ions (Cl⁻) on hemoglobin O₂ binding?

Cl⁻ binds preferentially to the T-state and helps shift the O₂-binding curve to the right, facilitating O₂ release.

Name three physiological factors that shift hemoglobin’s O₂-binding curve to the right.

Increased H⁺ (lower pH), elevated CO₂, and higher Cl⁻ concentration.

What does the acronym BPG stand for, and what is notable about its charge?

2,3-Bisphosphoglycerate; it carries five negative charges at physiological pH.

Where does BPG bind on hemoglobin?

In the central cavity between the four subunits, but only when hemoglobin is in the T-state (deoxy form).

Does BPG bind to the R-state of hemoglobin?

No; BPG binds exclusively to the T-state.

How does increased BPG concentration affect hemoglobin’s O₂ affinity and P₅₀?

It decreases affinity and increases the P₅₀ value, making it easier for Hb to release O₂.

Why do BPG levels rise at high altitude?

Lower ambient O₂ increases BPG synthesis, raising P₅₀ so hemoglobin can unload O₂ more effectively to tissues.

What subunit change in fetal hemoglobin (HbF) reduces BPG binding?

His143β is replaced by Ser in the γ chain of HbF.

What is the composition of fetal hemoglobin?

HbF = α₂γ₂.

Why does fetal hemoglobin have higher O₂ affinity than adult hemoglobin?

Reduced BPG binding (due to the His→Ser substitution) allows HbF to retain higher O₂ affinity under physiological conditions.

What is the combined effect of low pH, high CO₂, and elevated BPG on the O₂-binding curve?

A substantial rightward shift, dramatically lowering hemoglobin’s O₂ affinity.

What is the normal adult hemoglobin called?

Hemoglobin A (HbA).

Which single amino-acid substitution produces sickle-cell hemoglobin (HbS)?

Glu6β → Val.

Under which state does HbS polymerize into fibers?

The deoxy (T-state) form of hemoglobin.

Why do HbS fibers form only when hemoglobin is deoxygenated?

Val6β fits into a hydrophobic pocket on another deoxy Hb tetramer; the pocket is absent in the R-state.

How does the sickle-cell trait provide resistance to malaria?

Slight intracellular acidification caused by Plasmodium leads to partial sickling, increased membrane leakiness, K⁺ loss, and parasite death in heterozygotes.

Differentiate sickle-cell disease from the sickle-cell trait.

Disease = homozygous HbS (severe symptoms); trait = heterozygous HbA/HbS (usually mild, malaria resistant).

Name an abnormal hemoglobin that weakens heme binding.

Hb Hammersmith (Phe42β → Ser) or Hb Bristol (Val67β → Asp).

What effect does the Hb Yakima mutation have on O₂ release?

Asp99β → His destabilizes the T-state, raising O₂ affinity and reducing O₂ delivery (P₅₀ ≈ 12 torr).

Describe the functional consequence of the Hb Kansas mutation.

Asn102β → Thr destabilizes the R-state, giving low O₂ affinity (P₅₀ ≈ 70 torr) and reduced cooperativity.

What defines methemoglobins (HbM variants)?

They contain Fe³⁺ (oxidized iron) instead of Fe²⁺ and cannot bind O₂.

Give an example of an HbM variant and its mutation.

Hb Milwaukee (Val67β → Glu) stabilizes Fe³⁺, producing methemoglobin.

Which metal ion binds O₂ in hemocyanin?

Copper (Cu⁺/Cu²⁺).

In which organisms is hemocyanin commonly found?

Mollusks such as squid and octopus, and some arthropods.

Which non-heme iron protein transports or senses O₂ in some marine invertebrates?

Hemerythrin.

What are chlorocruorin and erythrocruorin?

Large, multi-subunit heme-based O₂-binding proteins found in certain worms and insects (often >100 subunits).