Question 1

Which of the following best explains why the heme prosthetic group is essential for myoglobin's ability to bind oxygen?

Accepted Answer

It contains a central Fe atom that can reversibly bind O$_2$, enabling oxygen transport and storage.

Question 2

What is the oxidation state of the iron (Fe) atom in the heme group of myoglobin when it is capable of binding oxygen?

Accepted Answer

$\mathrm{Fe}^{2+}$

Question 3

Which of the following statements about the interaction of Fe$_{2+}$ in the heme group is correct?

Accepted Answer

Fe$_{2+}$ forms five covalent bonds with the porphyrin ring and one with a proximal histidine residue.

Question 4

Carbon monoxide (CO) is dangerous because it binds to the heme group with greater affinity than oxygen. What structural feature of the heme group allows CO to outcompete O$_2$ for binding?

Accepted Answer

CO binds linearly to Fe$_{2+}$ above the plane of the heme, which is energetically favored.

Question 5

When O$_2$ binds to the heme group, what conformational change occurs in the Fe atom?

Accepted Answer

Fe atom becomes smaller and shifts into the plane of the heme.

Question 6

Which amino acid residue is directly involved in stabilizing the O$_2$ bound to the heme group in myoglobin?

Accepted Answer

Distal histidine

Question 7

The distal histidine in myoglobin serves several functions. Which of the following is NOT one of its roles?

Accepted Answer

Directly coordinating with the Fe atom in the heme group

Question 8

What is the effect of O$_2$ binding to the heme group on the overall conformation of hemoglobin?

Accepted Answer

It causes hemoglobin to shift from the T state to the R state.

Question 9

Which of the following best describes the structure of the heme group?

Accepted Answer

A hydrophobic pocket formed by nonpolar amino acids surrounding a central Fe atom complexed with a planar porphyrin ring.

Question 10

Which of the following scenarios would most likely result in the formation of free radicals that can damage cells?

Accepted Answer

Unbound free Fe$^{2+}$ reacts with O$_2$ outside the heme group.

Question 11

Which of the following is a direct consequence of Fe$^{2+}$ shifting into the plane of the heme upon O$_2$ binding?

Accepted Answer

It triggers a conformational change in the hemoglobin subunit, promoting cooperative binding.

Question 12

Which of the following bonds is NOT formed by the Fe atom in the heme group?

Accepted Answer

Bond with the distal histidine

Question 13

A patient is exposed to high levels of carbon monoxide. Which molecular interaction explains why CO poisoning is so dangerous?

Accepted Answer

CO binds to the Fe atom in the heme group with much higher affinity than O$_2$, preventing oxygen transport.

Question 14

Which of the following best describes the role of the proximal histidine in the heme group?

Accepted Answer

It forms a covalent bond with the Fe atom, anchoring the heme to the protein.

Question 15

Which of the following changes occurs in hemoglobin when O$_2$ binds to one subunit?

Accepted Answer

The protein undergoes a conformational change that increases the affinity of the remaining subunits for O$_2$.

Question 16

Which of the following is a direct effect of the distal histidine on CO binding to the heme group?

Accepted Answer

It reduces the ability of CO to bind to the heme group by steric hindrance.

Question 17

Which of the following best explains the cooperative binding of O$_2$ in hemoglobin?

Accepted Answer

Binding of O$_2$ to one subunit increases the affinity of the remaining subunits for O$_2$ due to conformational changes.

Question 18

Which of the following is true regarding the heme group in myoglobin and hemoglobin?

Accepted Answer

Both proteins use a heme group with Fe$^{2+}$ coordinated to a planar porphyrin ring and a proximal histidine.

Heme Prosthetic Group and Its Role in Protein Function

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