Globular Proteins

Globular Proteins Respiratory Block | 1 Lecture ReemSallam; MD, MSc, PhD Clinical Biochemistry Unit Department of Pathology

Overview What are globular proteins? Types and functions of globular proteins Hemoglobin (major globular protein) Myoglobin a, b-globulins g-globulins (immunoglobulins) Diseases associated with globular proteins

Globular proteins • Amino acid chains fold into shapes that resemble spheres are called globular proteins • This type of folding increases solubility of proteins in water • Polar groups on the protein’s surface • Hydrophobic groups in the interior • Fibrous proteins are mainly insoluble structural proteins

Globular proteins Hemoglobin: oxygen transport function Myoglobin: oxygen storage/supply function in heart and muscle a1, a2, b-globulins: various functions g-globulins (immunoglobulins: immune function) Enzymes: catalysis of biochemical reactions

Hemoglobin • A major globular protein in humans • Composed of four polypeptide chains: • Two a and two b chains • Contains two dimers of ab subunits • Held together by non-covalent interactions • Each chain is a subunit with a heme group in the center that carries oxygen • A Hb molecule contains 4 heme groups and carries 4 molecules of O2

Types of Hb

HbA structure Oxygen binding to hemoglobin

Hemoglobin function • Carries oxygen from the lungs to tissues • Carries carbon dioxide from tissues back to the lungs • Normal level (g/dL): • Males: 14-16 • Females: 13-15

Types of hemoglobin Fetal hemoglobin (HbF): Major hemoglobin found in the fetus and newborn Tetramer with two a and two g chains Higher affinity for O2 than HBA Transfers O2 from maternal to fetal circulation across placenta

Types of hemoglobin HbA2: Appears ~12 weeks after birth Constitutes ~2% of total Hb Composed of two a and two d globin chains

Types of hemoglobin HbA1c: HbA undergoes non-enzymatic glycosylation Glycosylation depends on plasma glucose levels HbA1c levels are high in patients with diabetes mellitus

Abnormal Hbs Unable to transport O2 due to abnormal structure Carboxy-Hb: CO replaces O2 and binds 200X tighter than O2(in smokers) Met-Hb: Contains oxidized Fe3+ (~2%) that cannot carry O2 Sulf-HB: Forms due to high sulfur levels in blood (irreversible reaction)

Hemoglobinopathies • Disorders of hemoglobin caused by: • Synthesis of structurally abnormal Hb • Synthesis of insufficient quantities of normal Hb • Combination of both

Hemoglobinopathies Sickle cell (HbS) disease • Caused by a single mutation in b-globin gene • Glutamic acid at position 6 in HbA is replaced by valine • The mutant HbS contains bs chain • The shape of RBCs become sickled • Causes sickle cell anemia

Hemoglobinopathies Hemoglobin C disease: • Caused by a single mutation in b-globin gene • Glutamic acid at position 6 in HbA is replaced by lysine • Causes a mild form of hemolytic anemia

Hemoglobinopathies Methemoglobinemia: Caused by oxidation of Hb to ferric (Fe3+) state Methemoglobin cannot bind oxygen Caused by certain drugs, reactive oxygen species and NADH-cytochrome b5 reductase deficiency Chocolate cyanosis: brownish-blue color of the skin and blood

Hemoglobinopathies Thalassemia: • Defective synthesis of either a or b-globin chain due to gene mutation • a-thalassemia: • Synthesis of a-globin chain is decreased or absent • Causes mild to moderate hemolytic anemia • b-thalassemia: • Synthesis of b-globin chain is decreased or absent • Causes severe anemia • Patients need regular blood transfusions

Myoglobin • A globular hemeprotein in heart and muscle • Stores and supplies oxygen to the heart and muscle only • Contains a single polypeptide chain forming a single subunit with eight a-helix structures • The interior of the subunit is composed of nonpolar amino acids

Page 328 Structure of myoglobin

Myoglobin The charged amino acids are located on the surface The heme group is present at the center of the molecule Myoglobin gives red color to skeletal muscles Supplies oxygen during aerobic exercise

Myoglobin in disease Myoglobinuria: Myoglobin is excreted in urine due to muscle damage (rhabdomyolysis) May cause acute renal failure Specific marker for muscle injury Less specific marker for heart attack

Immunoglobulins Defensive proteins produced by the B-cells of the immune system Types of g-globulins Y-shaped structure with 2 heavy and 2 light polypeptide chains Neutralize bacteria and viruses Types: IgA, IgD, IgE, IgG, IgM

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Globular Proteins