Protein Structure Overview

Protein Structure Summary

• Proteins are complex molecules crucial for various bodily functions
• Amino acids are the building blocks of proteins
• Protein structure is hierarchical, progressing from primary to quaternary
• Primary structure: The linear sequence of amino acids
• Secondary structure: Localized folding patterns (e.g., alpha-helices, beta-sheets) stabilized by hydrogen bonds. Alpha-helices are right-handed and form a coiled structure; beta-sheets are formed by hydrogen bonding between side chains and typically have a pleated appearance
• Tertiary structure: The overall 3D shape of a polypeptide chain, stabilised by interactions between R-groups of amino acids (including the hydrophobic effect, hydrogen bonds, and ionic bonds, disulfide bonds).
• Quaternary structure: The arrangement of multiple polypeptide chains in a protein with more than one chain.
• Fibrous proteins have elongated structures (e.g., keratin, collagen), while globular proteins have compact, spherical shapes (e.g., myoglobin, hemoglobin).
• Fibrous proteins have high tensile strength and are often structural components, like tendons, cartilage, hair or silk fibres
• Globular proteins often carry out functions like transport, catalysis (enzymes), and signaling (hormones)
• Myoglobin and hemoglobin are examples of globular proteins involved in carrying oxygen
• Protein folding and stability are dependent on various interactions between amino acid side chains
• Denaturation occurs when a protein loses its native structure, often due to changes in temperature, pH, or salt concentration; loss of function follows loss of shape

Myoglobin Structure

• Myoglobin is a small protein with 153 amino acid residues; it is a common example of a globular protein.
• It is found in muscle tissue
• Its main role is to store oxygen
• The molecule is extremely compact
• A significant portion, about 75%, of the polypeptide consists of alpha helices

Hemoglobin Structure

• Hemoglobin serves as the oxygen-transport protein in red blood cells; it's a large, globular protein, consisting of 4 polypeptide chains.
• Each chain is very similar to myoglobin, but it carries 4 polypeptide chains instead of just one; 2 alpha and 2 beta chains
• Contains a prosthetic group called heme, which includes an iron atom
• The heme group is crucial for oxygen binding
• Hemoglobin has two main conformations: T state (low oxygen affinity) and R state (high oxygen affinity).
• Small changes in the structure of the protein influence the affinity of oxygen

Characteristics of A-Keratin

• Keratin is predominantly a fibrous protein
• A-keratin is rich in hydrophobic amino acids, making it insoluble in water
• A-keratin lacks proline, which prevents helix breaking
• Disulfide bonds contribute to the stabilization of keratin structure
• Disulfide bonds can be broken (e.g. by reducing agents) and reformed (by oxidizing agents), leading to changes in the structure and function of the hair
• This process is used to generate permanent hairstyles

Collagen Structure

• Collagen is a highly abundant fibrous protein
• Its basic unit is a collagen polypeptide that forms a left-handed helix
• Collagen molecules assemble into a triple helix structure
• The triple helix is stabilized by hydrogen bonds, and the protein is rich in glycine, proline, and hydroxyproline residues.
• Proline and hydroxyproline residues are key to the formation of the triple helix structure
• The structure of collagen is critical in forming strong structural tissues (e.g., tendons, skin, cartilage)