Hemoproteins: Hemoglobin and Myoglobin Lecture Review

These flashcards cover the structural, functional, and biochemical properties of hemoproteins, focusing on the differences between myoglobin and hemoglobin, their oxygen-binding mechanisms, and clinical applications like pulse oximetry.

How are hemoproteins defined?

Conjugated proteins formed by a specific globular protein portion (globin) and a prosthetic group that is the same for all hemoproteins (the heme group).

Where is hemoglobin localized and what is its primary function?

It is localized in the red blood cell and its function is $O_2$ transport.

Where is myoglobin localized and what is its primary function?

It is localized in cardiac and skeletal muscle tissue where it acts as a deposit and reserve of $O_2$.

On which chromosomes are the genes for Myoglobin (Mb), Cytoglobin (CYGB), and Neuroglobin (NGB) located?

Myoglobin is on Chromosome $22$, Cytoglobin is on Chromosome $17$, and Neuroglobin is on Chromosome $14$.

What is hematology?

The study of blood and blood disorders.

What is hematocrit (packed cell volume)?

The volume of red blood cells determined by centrifuging heparinized blood in a capillary tube.

What is the chemical name of the prosthetic group in myoglobin and hemoglobin?

Protoporphyrin IX (heme).

What is the role of the globin part in Myoglobin (Mb) and Hemoglobin (Hb) regarding the iron ion?

To slow down the inevitable oxidation of the ferrous iron ($Fe^{2+})$ to ferric iron ($Fe^{3+})$.

What are the specific distances and angles for the oxy form of Myoglobin?

Proximal $His - Fe = 2.17 \text{ Å}$; $Fe - O_2 = 1.83 \text{ Å}$; Distal $His - O_2 = 2.76 \text{ Å}$; $Fe - O - O \text{ angle} = 115.6^{\circ}$.

Why is the role of distal His E7 physiologically important regarding Carbon Monoxide (CO)?

It H-bonds to $O_2$ but precludes the linear binding of CO, which selectively diminishes CO binding to the heme. This is important because CO is a low-level by-product of cellular metabolism (heme oxygenase).

What are the colors associated with different states of globins?

Deoxy- is purple, Oxy- is bright red, and Metha- (where iron is $Fe^{3+})$ is brown.

How does a pulse oximeter distinguish between oxygenated and deoxygenated hemoglobin?

It compares the absorption of red light (approx. $660 \text{ nm}$, absorbed more by deoxyHb) and infrared light (approx. $940 \text{ nm}$, absorbed more by oxyHb).

What is the secondary structure composition of Myoglobin (Mb)?

It has an ellipsoidal shape consisting of $8 \text{ α-helices}$ (A through H) connected by short loops, with approx. $80\%$ of the residues ($121/153$) in $α-helices$.

In Myoglobin, which residue acts as the 5th ferrous ion ligand and what is it called?

$His\text{ F8 } (93)$, which is called the proximal histidine.

What does the $P_{50}$ value indicate in the context of oxygen binding?

It is a measure of the relative binding affinity for $O_2$; it represents the oxygen partial pressure required for $50\%$ $O_2$ saturation.

How do the dissociation curves of Myoglobin and Hemoglobin differ effectively?

Myoglobin has a hyperbolic curve (indicating higher affinity and storage function), while Hemoglobin has a sigmoidal curve (indicating cooperative binding and transport function).

What is the quaternary structure of adult Hemoglobin?

It is a tetramer ($64 \text{ kDa}$) composed of two types of polypeptides: two $α$ chains and two $β$ chains ($α_2β_2$).

Which interface in the Hemoglobin tetramer is dynamic or mobile?

The $α_1-β_2$ (or $α_2-β_1$) interface, which includes approx. $19$ residues found in the C helix and the FG loop.

What quaternary change occurs in Hemoglobin upon oxygenation?

The $α_1-β_1$ dimer rotates $15^{\circ}$ with respect to the $α_2-β_2$ dimer, and the solvent channel narrows.

Contrast the T (tense) state and R (relaxed) state of Hemoglobin.

The T state (deoxy) has low $O_2$ affinity and the $Fe^{2+}$ is displaced $0.6 \text{ Å}$ out of the heme plane. The R state (oxy) has high $O_2$ affinity and the $Fe^{2+}$ is coplanar with the planar heme.

What are the three core steps of the Perutz stereochemical mechanism?

1. $Fe$ moves into the heme plane upon binding $O_2$. 2. $Fe$ drags $His\text{ F8}$ causing a $1 \text{ Å}$ lateral displacement of Helix F. 3. Displacement of $α_1β_2$ and $α_2β_1$ interfaces occurs with the breaking of ionic bonds and a rotation of $15^{\circ}$.

In the Hill equation, what do the values of the Hill coefficient ($n$) signify?

$n = 1$: non-cooperative; $n > 1$: positive cooperativity (sigmoid); $n < 1$: negative cooperativity (biphasic).

What is the difference between the MWC (concerted) model and the KNF (sequential) model of allostery?

In the MWC model, the entire tetramer shifts between T and R states simultaneously (symmetry is preserved). In the KNF model, ligand binding at one subunit promotes a conformational change in adjacent subunits individually.

What is a homotropic allosteric effect?

An effect where the allosteric effector and the ligand are the same molecule (e.g., $O_2$ binding to Hemoglobin).

What specific salt bridges stabilize the T state in Hemoglobin at the β-termini?

An inter-subunit salt bridge between $His146$ (C-terminal $-COO^-$) and $Lys40$ of the nearby $α\text{-globin}$, and an intra-subunit interaction between the protonated $His146$ side chain and $Asp94$.