Types of Proteins: Globular Proteins

Types of Proteins

Globular Proteins

• Characterized by a spherical shape and water solubility.
• Hydrophilic polar R groups face outward, facilitating hydrogen bonding with water, while hydrophobic R groups point inward.
• Exhibit tertiary structure with a specific 3D shape, often resulting in metabolic activity; some have quaternary structure.
• Sensitive to environmental changes; slight alterations in shape can lead to loss of function.
• Form colloids in solution rather than true solutions due to large molecular size, suspending in cytoplasm.
• Essential for maintaining molecular arrangement within cells.
• Examples include hemoglobin, myoglobin, insulin, antibodies, and enzymes.

Protein Structure Formation

• Peptide bonds form between the amino group of one amino acid and the carboxyl group of another.
• The primary structure consists of the sequence of amino acids in the polypeptide chain.
• R group position, orientation, and bond types (e.g., disulfide and hydrogen bonds) influence protein folding.
• Folding leads to secondary and tertiary structures, creating functional active sites.
• Hydrophilic amino acids orient outward while hydrophobic ones orient inward, affecting solubility.
• Enzymes are a specific example of globular proteins with distinct tertiary structures and active sites.

Fibrous Proteins

• Have a simpler structure lacking tertiary formation, with polypeptides aligned parallel for stability.
• Form long strands of polypeptide chains interlinked by cross-links, enhancing resilience to temperature and pH changes.
• Generally water insoluble and consist of repetitive amino acid sequences.
• Serve a structural role due to their toughness and strength.
• Examples include keratin (in hair, nails, and skin) and collagen (in artery walls).

Collagen Adaptations and Functions

• An insoluble fibrous protein found in tendons, cartilage, bones, teeth, and blood vessel walls.
• Composed of three polypeptide chains in a helical structure, each approximately 300 nm long and 1000 amino acids in length.
• Glycine, the smallest amino acid, occurs at every third position, aiding structural integrity.
• Three helical chains form a triple helix, reinforced by numerous hydrogen bonds and covalent cross-links.
• Staggered arrangement of parallel molecules prevents weak spots along fibrils.
• Groups of fibrils align to create strong bundles called fibers, granting high tensile strength to collagen.

Conjugated Proteins

• Comprise protein molecules associated with prosthetic groups not derived from amino acids.
• Hemoglobin is a key example, featuring a globular structure with four polypeptide chains (2 alpha and 2 beta).
• Each chain contains a heme group, allowing a single hemoglobin molecule to carry four oxygen molecules.
• Lipoproteins, formed by conjoining proteins and lipids (e.g., triglycerides), are crucial for cholesterol transport in blood.
• Low-Density Lipoproteins (LDL) carry cholesterol to cells, while excess LDL can lead to artery narrowing.
• High-Density Lipoproteins (HDL) transport excess cholesterol back to the liver for breakdown and removal, comprising more protein than LDL, making them denser.

Glycoproteins

• Characterized by the presence of carbohydrate prosthetic groups, enhancing water retention and resistance to enzymatic digestion.
• Examples include mucus and synovial fluid, essential for reducing friction and protecting protein walls from digestive enzymes, particularly in the stomach.

Explore the fascinating world of globular proteins, which are essential for various metabolic functions. This quiz covers their structures, solubility, and why their spherical shape is crucial for their activity. Test your knowledge of how amino acids contribute to their properties!