# Significance of Amino Acid Sequence The sequence of amino acids in a polypeptide chain constitutes the *primary structure* of a protein. This specific sequence is critical as it dictates the protein's final three-dimensional shape and, consequently, its biological function. The sequence is genetically determined by the sequence of nucleotides in DNA. A minor change, such as a *point mutation* (a change in a single nucleotide), can alter the amino acid sequence, leading to a dysfunctional protein and potentially severe diseases. Frederick Sanger was the first scientist to determine the amino acid sequence of a protein molecule, specifically **Insulin**. He concluded that proteins have a specific chemical characteristic due to their precise sequence of amino acids. ### Normal Hemoglobin (HbA) * **Function**: An iron-rich protein in red blood cells that carries oxygen from the lungs to the rest of the body. * **Structure**: Composed of four polypeptide chains: * Two alpha (α) chains, each with 141 amino acids. * Two beta (β) chains, each with 146 amino acids. * **Total Amino Acids**: A single hemoglobin molecule contains 574 amino acids. * **Associated Red Blood Cells**: Normal red blood cells are disc-shaped, flexible, and move easily through blood vessels. ### Sickle Cell Anemia and Sickle Hemoglobin (HbS) Sickle cell anemia is a hereditary disease that serves as a classic example of how a single amino acid change can have drastic consequences. * **Cause**: A point mutation in the gene that codes for the beta-chain of hemoglobin. * **Molecular Change**: This mutation causes a single amino acid substitution at the sixth position of the beta-chain: **Glutamic acid** is replaced by **Valine**. * **Resulting Protein**: The abnormal hemoglobin is called sickle hemoglobin or HbS. * **Effect on Red Blood Cells**: HbS causes red blood cells to deform into a *sickle* or *crescent* shape. These cells are stiff, sticky, and fragile. * **Physiological Consequences**: * Sickle cells can block blood flow in small blood vessels. * This blockage can lead to severe pain, tissue and organ damage, and other serious complications. | Feature | Normal Hemoglobin (HbA) | Sickle Hemoglobin (HbS) | | :--- | :--- | :--- | | **Associated Condition** | Healthy individual | Sickle Cell Anemia | | **Beta-Chain Amino Acid at Position 6** | Glutamic Acid | Valine | | **Red Blood Cell Shape** | Biconcave disc, flexible | Sickle or crescent, rigid | | **Oxygen Transport** | Efficient | Impaired | | **Blood Flow** | Smooth | Prone to blockages | ### Functional Importance The primary structure is crucial for the protein's proper folding into its functional three-dimensional shape. This illustrates the profound impact that a small change at the molecular level can have on the overall health and physiology of an organism. For more on how these proteins are synthesized, see .