Structure-Function of Globular Proteins

Structure-Function Relationship in Specific Proteins

• Globular proteins, such as hemoglobin and myoglobin, have a compact, roughly spherical shape.
• Globular proteins are water-soluble, readily dissolving in blood.
• Globular proteins have diverse functions, including acting as enzymes, transporting substances, and regulating processes.
• Examples of globular proteins are enzymes, transport proteins, motor proteins, regulatory proteins, and immunoglobulins.
• Globular proteins have a size range from 2000Å x 5Å (B conformation) to 900Å x 11Å (a helix).

Myoglobin and Hemoglobin

• Myoglobin is a heme protein found in muscle tissue.
• Its function is to store oxygen and supply for intense respiration.
• Myoglobin has a tertiary structure.
• Hemoglobin is a heme protein found in red blood cells (erythrocytes).
• Its function is to transport oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs.
• Hemoglobin has a quaternary structure.
• Both myoglobin and hemoglobin have heme prosthetic groups that contain iron in its ferrous (Fe²⁺) state.

Heme Group Structure

• The heme group's complex organic ring structure is a protoporphyrin.
• Iron at the center of the heme group forms six coordination bonds.
• Four bonds are in the same plane as the porphyrin ring
• Two bonds are perpendicular to the plane of the porphyrin ring.

Globular Proteins - Important Residues

• The heme group is located within a hydrophobic pocket in the apoprotein.
• This protects the iron from oxidation.
• The pocket also prevents carbon monoxide from binding to the heme group.
• In both myoglobin and hemoglobin, specific amino acid residues are crucial for oxygen binding.

Hemoglobin (Continued)

• Hemoglobin is an oligomeric protein with a quaternary structure.
• It consists of four polypeptide chains (2 α subunits and 2 β subunits).
• Each subunit has a bound heme prosthetic group.
• Subunits are arranged in pairs.
• Their arrangement in the protein relies on noncovalent interactions.

Allosteric Interactions (Hemoglobin)

• Hemoglobin is an allosteric protein, meaning interactions at one site influence binding at another.
• Oxygen binding to one subunit facilitates oxygen binding to adjacent subunits.
• Hemoglobin exhibits two main conformational states (T-state and R-state).
• The binding of oxygen causes a conformational shift (T → R).
• The change in shape increases oxygen binding affinity in hemoglobin.

Factors Affecting Oxygen Binding (Hemoglobin)

• The concentration of protons (H⁺) and carbon dioxide (CO₂) in peripheral tissues affects hemoglobin's oxygen affinity.
• High H⁺ and CO₂ levels reduce oxygen affinity.
• This allows oxygen release to the tissues as needed.
• 2,3-bisphosphoglycerate (BPG) is another factor influencing oxygen affinity.
• Higher BPG concentration decreases oxygen affinity in hemoglobin.

Fibrous Proteins (General Description)

• Fibrous proteins are insoluble in water due to their high concentration of hydrophobic amino acids on their surfaces.
• They play important structural functions, often providing strength and flexibility to tissues.
• They form complex supramolecular structures with similar polypeptide chains packed together.
• Common examples include collagen, myosin, and actin.

Collagen

• Collagen is an important structural fibrous protein.
• It provides strength to connective tissues like tendons, cartilage, and bone.
• Collagen has a highly organized, repetitive amino acid sequence (Gly-X-Y).
• Contains large amounts of Glycine, Proline, and Hydroxyproline amino acids.
• These particular amino acids are important components of the left-handed coiled structure in collagen.
• Collagen fibers are also stabilised by cross-links, which are covalent bonds resulting in differing degrees of flexibility or rigidity.

Myosin

• Myosin is a fibrous protein that is a component in muscle cells.
• It contains multiple subunits, including 2 heavy chains and 2 light chains.
• The carboxyl terminus of the polypeptide chains forms a long tail, with the amino terminus forming a head.
• The head forms cross bridges with actin in muscle contraction.

Actin

• Actin is a fibrous protein that also plays a role in muscle contraction.
• It's a component of the thin filaments in muscle.
• Actin is composed of G-actin monomers in a right-handed polymerized structure called F-actin.
• Actin has a high affinity for G-actin monomers with specific binding sites for myosin.

Explore the intricate structure-function relationship in globular proteins, focusing on myoglobin and hemoglobin. This quiz delves into their unique shapes, water solubility, and essential biological roles. Understand the significance of these proteins in oxygen transport and storage.