Problem 93 - Chemistry An Atoms First Approach

Question

Oxygen is carried from the lungs to tissues by the protein hemoglobin in red blood cells. Sickle cell anemia is a disease resulting from abnormal hemoglobin molecules in which a valine is substituted for a single glutamic acid in normal hemoglobin. How might this substitution affect the structure of hemoglobin?

Step-by-Step Solution

Answer

The substitution of valine for glutamic acid in hemoglobin causes significant changes in the protein's structure, as valine is hydrophobic while glutamic acid is hydrophilic. This may alter the local secondary structure due to changes in hydrogen bonding and hydrophobic interactions. Additionally, it can disrupt the stability of the quaternary structure due to altered protein-protein interactions. As a result, abnormal aggregates called "sickle fibers" form, causing red blood cells to become rigid and sickle-shaped, ultimately leading to sickle cell anemia.

Step 1: Understand the structure of hemoglobin

Hemoglobin is a complex protein composed of four polypeptide chains - two alpha chains and two beta chains. Each chain contains an iron-containing heme group that binds to oxygen. The overall structure and folding of these polypeptide chains are critical for the protein's ability to carry and release oxygen in the body.

Step 2: Identify the difference between valine and glutamic acid

Valine and glutamic acid are both amino acids, the building blocks of proteins. However, they have different properties that contribute to protein structure and function. Valine is a hydrophobic (water-repelling) amino acid with a nonpolar side chain, whereas glutamic acid is a hydrophilic (water-attracting) amino acid with a polar side chain and a negative charge. These differences in properties can affect how the amino acids interact with nearby amino acids as well as with the surrounding environment.

Step 3: Determine the consequences of the amino acid substitution

The substitution of valine for glutamic acid in sickle cell hemoglobin (also known as hemoglobin S) could lead to several possible effects on the structure of the protein: 1. Alteration of the local protein structure: Since valine is hydrophobic and glutamic acid is hydrophilic, the substitution might cause changes in the local secondary structure (e.g., alpha helices or beta sheets) due to changes in hydrogen bonding and hydrophobic interactions. 2. Disruption of protein-protein interactions: The altered local structure might affect the way the four polypeptide chains in hemoglobin interact with one another, potentially disrupting or weakening the stability of the overall quaternary structure. 3. Formation of abnormal aggregates: Hemoglobin S is known to form abnormal aggregates called "sickle fibers" under certain conditions, such as low oxygen levels. These fibers cause the red blood cells to become rigid and take on a sickle shape, which can lead to various negative outcomes in the affected individual.

Step 4: Summarize the impacts of the substitution on the structure of hemoglobin

In conclusion, the substitution of a single amino acid, valine for glutamic acid, in the hemoglobin protein can have significant effects on the structure and function of the protein. The changes in local structure and protein-protein interactions can lead to abnormal aggregation and the formation of sickle fibers, ultimately causing sickle cell anemia – a serious genetic disorder.