Lecture 8 Protein-4 PDF

Faculty of Medicine Academic Year: 2024-2025 Year: 1 Semester: 1 Module: Human Body Function (HBF) 102  Protein chemistry IV Globular protein structure function relationship By: Shahenda Mahgoub Associate Professor Department: Biochemistry & Molecular Biology 11/18/2...

Faculty of Medicine Academic Year: 2024-2025 Year: 1 Semester: 1 Module: Human Body Function (HBF) 102  Protein chemistry IV Globular protein structure function relationship By: Shahenda Mahgoub Associate Professor Department: Biochemistry & Molecular Biology 11/18/2024 22  Objectives ✓ Identify general function of globular protein ✓ Identify hemoglobin structure ✓ Identify structure of myoglobin ✓ Describe differences between fibrous and globular protein 3 HBF - 102 3  Introduction Globular proteins are round structures. They have a round, spherical form because the hydrophobic parts of the protein fold inwards while the hydrophilic parts are arranged around the external surface. Globular proteins are water soluble. As only the hydrophilic components of the protein are on the outer surface. HBF - 102 44  Globular proteins have tertiary or quaternary structure of protein. The combination of two or more polypeptide subunits held together by non-covalent gives an oligomer (quaternary structure). Each constituent peptide chain is called monomer or subunit.  Function of Globular Proteins How does the structure of globular proteins affect their function? The compact, spherical shape of globular proteins allows them to easily bind with other molecules and perform specific functions, such as: ▪ catalysing reactions in enzymes. ▪ transmitting signals in hormones. ▪ recognizing and neutralizing foreign invaders in antibodies. 11/18/2024 HBF - 102 6  Function of Globular Proteins 1.Enzymes. All enzymes are globular proteins as their round shape can be altered appropriately to fit their target with high specificity. e.g., Digestive enzymes such as amylase, pepsin, and lipase which break down starch, protein, and fats, respectively. 11/18/2024 HBF - 102 7  Function of Globular Proteins 2. Transport proteins. Due to their soluble nature, globular proteins function as transport proteins as they can cross cell membranes. e.g., Hemoglobin, which transports oxygen. 11/18/2024 HBF - 102 8  Function of Globular Proteins 3. Messenger proteins. ▪ Their solubility makes globular proteins suitable as messenger proteins, known as hormones. ▪ They regulate the body’s metabolic processes. e.g., Insulin, which regulates blood sugar levels. 11/18/2024 HBF - 102 9  Hemoglobin ▪ Hemoglobin is a quaternary protein, made up of 4 tertiary globular subunits. ▪ Each globular unit is covalently bonded to haem. ▪ Haem is not a protein therefore it is called a “prosthetic group”. ▪ Haem contains iron to which oxygen binds. 11/18/2024 HBF - 102 10  Hemoglobin ▪ Hb molecule contains 4 heme groups & carries 4 O2molecules. ▪ The iron in hemoglobin must be maintained in the reduced state for the transport of oxygen. ▪ Oxidation of the iron of hemoglobin from the Fe2+ to the Fe3+ state results in the formation of met-hemoglobin. 11/18/2024 HBF - 102 11  Hemoglobin ▪ Hemoglobin is considered a “conjugated protein”. Because hemoglobin is a protein that is associated with non-protein structures, it is called a conjugated protein. ▪ Hemoglobin A (HBA1) the major Hb in adults (90%) contains Two α chains, and two β chains. 11/18/2024 HBF - 102 12 Fetal Hemoglobin (HbF) ▪ Major hemoglobin found in the fetus during gestation. ▪ Tetramer with two alpha and two gamma subunits. ▪ Transfers O2 from maternal to fetal circulation across placenta. 11/18/2024 HBF - 102 13 HbA1C ▪ HbA is slowly and non-enzymatically glycosylated. ▪ Glycosylation depends on plasma glucose levels ▪ HbA1C levels are high in patients with diabetes mellitus  Sickle cell anemia One of the most important hemoglobinopathies of genetic origin. ▪It occurs when an individual is homozygous for hemoglobin S in which there is a mutation in the β subunits. ▪There is hemolytic anemia. Thalassemia ▪ Autosomal recessive blood disease ▪ There are defects in the synthesis of either the alpha or beta chains. ▪ It causes the formation of abnormal hemoglobin molecules, thus causing anemia.  Myoglobin ▪Myoglobin is a tertiary protein, made up of one tertiary globular subunit (monomer). ▪Its structure is similar to Hemoglobin. ▪It is a haemoprotein present in heart and skeletal muscle. 11/18/2024 HBF - 102 17  Myoglobin ▪Myoglobin can bind only one molecule of oxygen, because it contains only one heme group. ▪It functions both as a reservoir for oxygen, and as an oxygen carrier that increases the rate of transport of oxygen within the muscle cell. 11/18/2024 HBF - 102 18  Differences between fibrous and globular protein 11/18/2024 Helwan Special Medical Program 19 Based on shape and size Fibrous proteins: When the axial ratio of length: width of a protein molecule is more than 10. Fibrous proteins are long, thin proteins. Globular protein: When the axial ratio of length: width of a protein molecule is less than 10. Globular proteins have a compact, spherical shape. Based on water solubility & Function. Fibrous proteins: They are insoluble in water. They provide structural support in tissues such as hair, nails, and tendons. Globular protein: They are usually soluble in water. They play important roles in cellular processes such as enzymes, hormones, and antibodies. 11/18/2024 HBF - 102 21  Interactive Question Why is it called a globular protein? A globular protein is called so because of its spherical or globular shape. The term “globular” refers to the overall shape of the protein, which is roughly spherical, and the term “protein” refers to the fact that it is a biomolecule made up of amino acids. 11/18/2024 HBF - 102 22 22  Summary In summary, while both globular and fibrous proteins are important biomolecules with specific functions in the body, their shape, amino acid sequence, solubility, and function are different. Common examples of globular proteins include enzymes, such as lactase and catalase, hormones, such as insulin and testosterone, and antibodies, such as immunoglobulin G. 11/18/2024 HBF - 102 23 23  References Ferrier, D. R. (2014). Lippincott’s illustrated reviews. USA: Lippincott Williams & Wilkins. 11/18/2024 HBF - 102 24 24