Biochemistry Chapter 5

The heme prosthetic group:

• consists of protoporphyrin and an iron (II) ion.

• is found only in myoglobin and hemoglobin.

• contains a single iron atom in its ferrous (Fe2+) state that has a total of 4 coordination bonds.

• is only found bound to protein.

Answer explanation

The heme prosthetic group contains a single iron atom in its ferrous (Fe2+) state, which forms four coordination bonds with surrounding atoms, allowing it to effectively bind oxygen.

Myoglobin:

• has quaternary structure.

• contains a pocket for binding heme that is made up of largely the C and D helices.

• contains multiple binding sites for O2.

• consists mostly of α helices.

Answer explanation

Myoglobin primarily consists of α helices, which are crucial for its structure and function. Unlike hemoglobin, it does not have a quaternary structure and has a single binding site for O2, making the correct choice about its helices.

Protein A has a binding site for ligand L with a Ka of 105 M⁻¹. Protein B has a binding site for ligand L with a Ka of 108 M⁻¹. Which protein has the higher affinity for ligand L?

• Protein A

• Protein B

• It is not possible to determine based only on Ka values.

• Both have the same affinity.

Answer explanation

Protein B has a higher Ka value (10^8 M⁻¹) compared to Protein A (10^5 M⁻¹), indicating that Protein B has a greater affinity for ligand L. Higher Ka values correspond to stronger binding.

Protein A has a Ka = 6.0 μM⁻¹ for binding ligand L, and protein B has a Kd = 4.0 μM for binding ligand L. Based on this information, which statement is true?

• Protein B binds L with higher affinity.

• Protein A is half-saturated with L when [L] is 6.0 μM⁻¹.

• The Ka of protein B for L is 0.25 μM⁻¹.

• When [L] = 1 μM, Y = 0.17 for protein A.

Answer explanation

The dissociation constant Kd is the inverse of the association constant Ka. For protein B, Kd = 4.0 μM implies Ka = 1/Kd = 0.25 μM⁻¹. Thus, the correct statement is that the Ka of protein B for L is 0.25 μM⁻¹.

When the partial pressure of oxygen is equal to the P50 of myoglobin, what is the value of Y?

• 0.0

• 0.25

• 0.50

• 0.75

• 1.0

Answer explanation

When the partial pressure of oxygen equals the P50 of myoglobin, it indicates that myoglobin is 50% saturated with oxygen. Therefore, the value of Y, which represents saturation, is 0.50.

Oxygen carried by whole blood in animals is bound and transported by hemoglobin in what type of cell?

• hemocytoblasts

• erythrocytes

• white blood cells

• stem cells

• neurons

Answer explanation

Oxygen is primarily transported in the blood by erythrocytes, also known as red blood cells, which contain hemoglobin. Hemoglobin binds to oxygen, allowing efficient transport throughout the body.

Tags

NGSS.HS-LS1-7

The α and β subunits of hemoglobin:

• each contain 4 heme prosthetic groups.

• contain several stretches of identical amino acids.

• are both tetramers.

• have high structural similarity to each other.

• differ in structure and sequence.

Answer explanation

The α and β subunits of hemoglobin share a high degree of structural similarity, as they are both derived from similar genes and have comparable folding patterns, despite some differences in sequence.

Tags

NGSS.HS-LS1-1

The T → R transition is triggered by:

• Ion pairs forming between residues and O2 at the α1β2 (and α2β1) interface.

• The structures of the individual subunits changing.

• O2 causing the heme to assume a more planar conformation, shifting the position of His F8 and the F helix.

• αβ subunit pairs sliding past each other and rotating, narrowing the pocket between the α subunits.

Answer explanation

The T → R transition is primarily triggered by O2 binding, which causes the heme to become more planar. This change shifts His F8 and the F helix, facilitating the transition and altering the protein's conformation.

Leghemoglobin is an oxygen-binding protein in root nodules that contain bacteria and fix atmospheric nitrogen. Which statement is true if leghemoglobin is like myoglobin and not hemoglobin?

• There are four oxygen-binding sites.

• The oxygen-binding curve is hyperbolic.

• There is cooperative oxygen binding.

• The oxygen-binding curve is sigmoidal.

• Oxygen binding changes the heme configuration from T to R.

Answer explanation

Leghemoglobin, like myoglobin, has a hyperbolic oxygen-binding curve due to its single binding site, unlike hemoglobin which exhibits cooperative binding and a sigmoidal curve. Thus, the correct statement is that the oxygen-binding curve is hyperbolic.

Which molecule is a homotropic modulator of oxygen binding to hemoglobin?

• oxygen

• H+

• carbon dioxide

• hemoglobin

• carbon monoxide

Answer explanation

Oxygen is a homotropic modulator of hemoglobin, meaning its binding enhances the binding of additional oxygen molecules. This cooperative binding is crucial for efficient oxygen transport in the body.