Fetal Hemoglobin - Online Tutor, Practice Problems & Exam Prep

Fetal hemoglobin (FHB) differs structurally from adult hemoglobin, featuring 2 alpha and 2 gamma subunits instead of 2 beta subunits. This structural variation results in a lower affinity for 2,3-bisphosphoglycerate (BPG), enhancing FHB's oxygen affinity. Consequently, oxygen flows from maternal hemoglobin to fetal hemoglobin, facilitating oxygen transfer essential for fetal development. The dissociation constant (K_d) for FHB is lower than that of adult hemoglobin, indicating a stronger affinity for oxygen, which is crucial for effective oxygenation during gestation.

Fetal Hemoglobin

In this video, we're going to talk about fetal hemoglobin. It turns out that the human fetus actually has structurally different hemoglobin than the adult hemoglobin that we've been talking about all along. The reason for this is because it's important for the human fetus to be able to obtain oxygen from its mother's blood. Fetal hemoglobin, which we're going to abbreviate as FHB moving forward, actually has 2 gamma subunits, rather than 2 beta subunits like the adult hemoglobin has. Fetal hemoglobin structure includes 2 alpha subunits and 2 gamma subunits, whereas adult hemoglobin has 2 alpha and 2 beta subunits. The presence of gamma subunits instead of beta subunits allows fetal hemoglobin to have a lowered affinity for BPG. BPG is a negative heterotropic allosteric inhibitor, and a low affinity for an inhibitor correlates with even greater or stronger oxygen affinity. Consequently, fetal hemoglobin exhibits a stronger oxygen affinity than adult hemoglobin, enabling oxygen to flow from the maternal oxyhemoglobin, HBO₂, to the fetal deoxyhemoglobin, FHB, allowing fetal hemoglobin to obtain oxygen.

Looking at our oxygen saturation curve, notice the y-axis represents fractional saturation, theta (θ), and the x-axis shows the partial pressure of oxygen in units of torrs. We have two different curves here: a green curve for fetal hemoglobin and a red curve for adult hemoglobin. In summary, fetal hemoglobin possesses the same 2 alpha subunits but, instead of beta subunits, it has gamma subunits. This structural variation results in fetal hemoglobin having a stronger oxygen affinity, due to having a lowered affinity for BPG. This permits a flow of oxygen from adult hemoglobin to fetal hemoglobin, facilitating the acquisition of oxygen by the fetus from its mother's blood.

In our example, we're asked according to the dissociation constant Kd in the plot below, which hemoglobin has a stronger affinity for oxygen? We observe that for the green curve, the Kd is lower than the Kd for adult hemoglobin. A lower Kd corresponds with a stronger affinity. Therefore, the hemoglobin with the stronger affinity for oxygen is fetal hemoglobin, and 'b' here is the correct answer for this example problem. This completes the example and the lesson on fetal hemoglobin, and we will be able to get some practice as we move forward in our course. I'll see you guys in our next video.

Fetal hemoglobin binds oxygen with a ___________ affinity than adult hemoglobin, because it lacks the binding site for ______, which is an allosteric ____________ of oxygen binding to adult hemoglobin.

• A. Lower ; BPG ; inhibitor.
• B. Higher ; H+ ; inhibitor.
• C. Higher ; BPG ; activator.
• D. Higher ; BPG ; inhibitor.
• E. Lower ; CO ; inhibitor.

Here’s what students ask on this topic:

What is the structural difference between fetal hemoglobin and adult hemoglobin?

Fetal hemoglobin (FHB) differs from adult hemoglobin in its subunit composition. FHB consists of 2 alpha (α) and 2 gamma (γ) subunits, whereas adult hemoglobin (HbA) is composed of 2 alpha (α) and 2 beta (β) subunits. This structural difference is crucial for the fetus as it allows FHB to have a higher affinity for oxygen compared to adult hemoglobin. The presence of gamma subunits in FHB reduces its affinity for 2,3-bisphosphoglycerate (BPG), a molecule that decreases oxygen affinity in hemoglobin. As a result, FHB can effectively extract oxygen from the maternal blood, ensuring proper oxygenation for fetal development.

Why does fetal hemoglobin have a higher affinity for oxygen than adult hemoglobin?

Fetal hemoglobin (FHB) has a higher affinity for oxygen than adult hemoglobin (HbA) due to its structural composition. FHB contains 2 alpha (α) and 2 gamma (γ) subunits, while HbA contains 2 alpha (α) and 2 beta (β) subunits. The gamma subunits in FHB result in a lower affinity for 2,3-bisphosphoglycerate (BPG), a molecule that binds to hemoglobin and decreases its oxygen affinity. With reduced BPG binding, FHB can bind oxygen more tightly, facilitating the transfer of oxygen from the maternal blood to the fetal blood. This higher oxygen affinity is essential for the fetus to receive adequate oxygen for growth and development.

How does the oxygen dissociation curve of fetal hemoglobin compare to that of adult hemoglobin?

The oxygen dissociation curve of fetal hemoglobin (FHB) is shifted to the left compared to that of adult hemoglobin (HbA). This leftward shift indicates that FHB has a higher affinity for oxygen. On the oxygen dissociation curve, the y-axis represents fractional saturation (θ or Y), and the x-axis represents the partial pressure of oxygen (pO₂) in units of torr. The lower dissociation constant (K_d) for FHB compared to HbA means that FHB binds oxygen more readily at lower oxygen pressures. This characteristic is crucial for the efficient transfer of oxygen from the maternal blood to the fetal blood, ensuring proper oxygenation during fetal development.

What role does 2,3-bisphosphoglycerate (BPG) play in the function of fetal hemoglobin?

2,3-Bisphosphoglycerate (BPG) is a negative heterotropic allosteric inhibitor that binds to hemoglobin and decreases its affinity for oxygen. In fetal hemoglobin (FHB), the presence of gamma (γ) subunits instead of beta (β) subunits results in a lower affinity for BPG. This reduced BPG binding allows FHB to maintain a higher affinity for oxygen compared to adult hemoglobin (HbA). The higher oxygen affinity of FHB is essential for the efficient transfer of oxygen from the maternal blood to the fetal blood, ensuring that the fetus receives adequate oxygen for growth and development. Thus, BPG plays a crucial role in modulating the oxygen-binding properties of hemoglobin.

Why is the lower dissociation constant (K_d) of fetal hemoglobin important for fetal development?

The lower dissociation constant (K_d) of fetal hemoglobin (FHB) is important for fetal development because it indicates a stronger affinity for oxygen. A lower K_d means that FHB can bind oxygen more tightly and efficiently at lower oxygen pressures. This high oxygen affinity is crucial for the transfer of oxygen from the maternal blood to the fetal blood. During gestation, the fetus relies on this efficient oxygen transfer to receive the necessary oxygen for growth and development. The structural composition of FHB, with its 2 alpha (α) and 2 gamma (γ) subunits, facilitates this high oxygen affinity by reducing its affinity for 2,3-bisphosphoglycerate (BPG), an inhibitor that decreases oxygen binding in hemoglobin.