Hemoglobin and Myoglobin Structure, Function, and Allostery

Vocabulary flashcards detailing key terminology, structural features, conformational states, and physiological properties of hemoglobin and myoglobin.

Hemoglobin (Hb)

A tetramultimer protein composed of four polypeptide subunits that functions in oxygen transport by binding $O_2$ in the lungs, carrying it to tissues, and releasing it for cellular use.

Myoglobin (Mb)

A monomeric protein containing a single heme group that functions in oxygen storage in muscle tissue, releasing $O_2$ during muscle exertion or fight-or-flight responses.

Prosthetic Group

A non-protein molecule that is tightly bound to a protein and essential to that protein's biological function.

Heme

A prosthetic group containing a central $Fe^{2+}$ ion that coordinates with oxygen.

Ferrous Iron

The $Fe^{2+}$ or $Fe(II)$ oxidation state of iron with a $2+$ charge found at the center of heme that binds oxygen.

$O_2$ Binding Curve

A graphical plot of partial pressure of oxygen on the x-axis versus percent $O_2$ saturation on the y-axis, illustrating how protein saturation changes as oxygen concentration rises.

$$P_{50}$$

The partial pressure of oxygen ($pO_2$) at which hemoglobin or myoglobin achieves $50\%$ saturation with $O_2$.

Hyperbolic Binding Curve

An oxygen binding curve exhibiting high binding affinity at low partial pressures followed by leveling off, characteristic of monomeric myoglobin.

Sigmoidal Binding Curve

An S-shaped oxygen binding curve where binding is slow initially, accelerates as more oxygen binds, and levels off near saturation, characteristic of multimeric hemoglobin.

Dimer

A molecular structure composed of two subunits.

Hemoglobin Subunit Composition

A quaternary tetramultimer formed by a dimer of $\alpha\beta$ dimers, consisting of two $\alpha$-chains ($141$ residues each) and two $\beta$-chains ($146$ residues each).

T (Tense) Conformation

The deoxy state of hemoglobin characterized by low $O_2$ binding affinity that readily releases oxygen.

R (Relaxed) Conformation

The oxy state of hemoglobin characterized by high $O_2$ binding affinity that binds oxygen easily and holds it tightly.

Cooperativity

The process in hemoglobin where initial oxygen binding triggers conformational changes in adjacent subunits toward the R state, progressively enhancing subsequent $O_2$ binding.

His F8

The 8th amino acid residue located on the F helix of the globin protein subunit that serves as the 5th coordinating ligand to the central $Fe^{2+}$ ion.

Metmyoglobin

A modified form of myoglobin containing oxidized ferric iron ($Fe^{3+}$) that is unable to bind oxygen.

Bohr Effect

The reduction of hemoglobin's oxygen binding affinity caused by allosteric binding of $H^+$ protons, which promotes oxygen release in tissues.

Carbamate

The chemical group formed when $CO_2$ binds covalently to the N-terminus of hemoglobin polypeptide chains, stabilizing the deoxy T state.

Allostery

The regulation of protein function or shape triggered by the binding of an effector molecule at a regulatory site other than the active site.

2,3-Bisphosphoglycerate (2,3-BPG)

A negatively charged allosteric effector that binds the central cavity of the hemoglobin tetramer to stabilize the T state, generating hemoglobin's sigmoidal binding curve.

Fetal Hemoglobin (Hb F)

A tetrameric hemoglobin variant ($\alpha_2\gamma_2$) possessing lower affinity for 2,3-BPG and higher affinity for oxygen, enabling oxygen transfer from maternal Hb A at the placenta.

Adult Hemoglobin (Hb A)

The primary hemoglobin form found in adults, composed of two $\alpha$ chains and two $\beta$ chains.

Hb S

The mutant hemoglobin associated with sickle-cell anemia, generated by the substitution of valine for glutamate at position 6 on both $\beta$-chains.

Hydroxyurea

A pharmacological treatment for sickle-cell anemia that activates the Hb F gene, boosting fetal hemoglobin production to prevent sickling.

Anemia

Any physiological condition characterized by an insufficient quantity of healthy red blood cells or functional hemoglobin in the body.