Globular and Fibrous Proteins Overview

Globular and Fibrous Proteins

• Globular proteins typically have a spherical shape due to tightly folded polypeptide chains.
• Hydrophobic amino acid groups are usually found on the inside of the protein, while hydrophilic groups are on the outside. This makes many globular proteins water-soluble.
• Examples include transport proteins (like hemoglobin and myoglobin), enzymes (like lipase and DNA polymerase), and hormones (like estrogen and insulin).
• Hemoglobin is a reddish-purple oxygen-carrying pigment found in red blood cells. It's made up of 4 polypeptide chains (2 alpha and 2 beta).
• The hydrophobic parts of the chains point inwards, while the hydrophilic parts point outwards. This helps maintain the protein's solubility in water.
• Hemoglobin contains a haem group with an iron ion (Fe2+) at its center.
• The iron combines with oxygen when oxygen concentration is high, releasing oxygen when oxygen concentration is low.
• One hemoglobin molecule can carry 4 oxygen molecules.
• The colour of hemoglobin is bright red when combined with oxygen, and purplish if not.

Fibrous Proteins

• Fibrous proteins generally have a long, narrow shape and are usually composed of long strands.
• They're primarily involved in providing structural support and strength.
• They often have little to no tertiary structure.
• Fibrous proteins usually have long parallel polypeptide chains, stabilized by cross-links that form fibers or sheets.
• Examples include collagen, keratin, and elastin.
• Collagen is a major structural protein found in connective tissues (such as skin, tendons, cartilage, bones, teeth, and walls of blood vessels).
• Collagen is a triple-helix made up of 3 polypeptide chains.
• Each chain consists of ~1000 amino acids.
• Every third amino acid in collagen is glycine to allow tight packing and forming a strong cable.
• Collagen has a repeating amino acid sequence, which makes it less sensitive to changes in pH or temperature, etc.
• Collagen molecules interact with each other to form fibers.
• Collagen has a high tensile strength, about one quarter that of mild steel.

Differences Between Fibrous and Globular Proteins

Feature: Fibrous ,Globular

Shape: Long and narrow ,Round or spherical

Purpose: Structural, Functional

Acid Sequence: Repetitive, Irregular

Durability: Less sensitive to changes in pH, temperature, etc. , More sensitive to changes in pH, temperature, etc.

Examples: Collagen, keratin, elastin Enzymes, hemoglobin, insulin

Solubility: Generally insoluble in water, Generally soluble in water

Incorrect Statement About Collagen

• Collagen has polypeptide chains primarily in an alpha-helix conformation. (This is false). Collagen has a triple-helix structure.