Protein Structure

Primary Structure

  • Proteins are made up of polypeptide chains, which are amino acids joined together with peptide bonds. The unique sequence of amino acids that make up a protein or polypeptide chain is called the Primary Structure.
Primary Structure: The unique sequence of amino acids that makes up a protein or polypeptide chain.
  • Peptide bonds are created by enzyme catalysed condensation reactions and broken down by enzyme catalysed hydrolysis reactions. Breaking down proteins is important in many areas of the body, not merely in digestion. For example, in hormone regulation, cells that are targeted by hormones contain enzymes to break down those hormones. This stops their effects from being permanent and allows them to be controlled.

Secondary Structure

  • After synthesis, polypeptide chains are folded or pleated into different shapes, called their Secondary Structure. Two common examples of secondary structures are Alpha Helices and Beta Pleated Sheets. Secondary structure is held together by many Hydrogen bonds, overall giving the shape great stability.
Secondary Structure: The way in which the primary structure of a polypeptide chain folds.

Tertiary Structure

  • The final 3D structure of a protein is its Tertiary Structure, which pertains to the shaping of the secondary structure. This may involve coiling or pleating, often with straight chains of amino acids in between.
Tertiary Structure: The final 3D structure of a protein, entailing the shaping of a secondary structure.
  • Tertiary structure is held together by four different bonds and interactions:
    • Disulphide Bonds - Where two Cysteine amino acids are found together, a strong double bond (S=S) is formed between the Sulphur atoms within the Cysteine monomers.
    • Ionic Bonds - If two oppositely charged ‘R’ groups (+ve and -ve) are found close to each other, and ionic bond forms between them.
    • Hydrogen Bonds - Your typical everyday Hydrogen bonds.
    • Hydrophobic and Hydrophilic Interactions - Some amino acids may be hydrophobic while others are hydrophilic. In a water based environment, a globular protein will orientate itself such that it’s hydrophobic parts are towards its centre and its hydrophilic parts are towards its edges

  • Tertiary structure can be broken by the action of heat. Increasing the kinetic energy of protein with a tertiary structure makes it vibrate more, and so the bonds that maintain its shape (which are mainly weak, non-covalent bonds) will be more likely to break. When a protein loses its shape in this way it is said to be Denatured. Even when cool the protein will not (or is highly unlikely to) form its original complex shape.

  • Proteins with a 3D structure fall into two main types:
    • Globular - These tend to form ball-like structures where hydrophobic parts are towards the centre and hydrophilic are towards the edges, which makes them water soluble. They usually have metabolic roles, for example: enzymes in all organisms, plasma proteins and antibodies in mammals.
    • Fibrous - They proteins form long fibres and mostly consist of repeated sequences of amino acids which are insoluble in water. They usually have structural roles, such as: Collagen in bone and cartilage, Keratin in fingernails and hair.

Quaternary Structure

  • Some proteins are made up of multiple polypeptide chains, sometimes with an inorganic component (for example, a haem group in haemoglogin) called a Prosthetic Group. These proteins will only be able to function if all subunits are present.
Quaternary Structure: The structure formed when two or more polypeptide chains join together, sometimes with an inorganic component, to form a protein.

Haemoglobin and Collagen

  • Haemoglobin is a water soluble globular protein which is composed of two α polypeptide chains, two β polypeptide chains and an inorganic prosthetic haem group. Its function is to carry oxygen around in the blood, and it is facilitated in doing so by the presence of the haem group which contains a \(\text{Fe}^{2+}\) ion, onto which the oxygen molecules can bind.

  • Collagen is a fibrous protein consisting of three polypeptide chains wound around each other. Each of the three chains is a coil itself. Hydrogen bonds form between these coils, which are around 1000 amino acids in length, which gives the structure strength. This is important given collagen’s role, as structural protein. This strength is increased by the fact that collagen molecules form further chains with other collagen molecules and form Covalent Cross Links with each other, which are staggered along the molecules to further increase stability. Collagen molecules wrapped around each other form Collagen Fibrils which themselves form Collagen Fibres.

  • Collagen has many functions:
    • Form the structure of bones
    • Makes up cartilage and connective tissue
    • Prevents blood that is being pumped at high pressure from bursting the walls of arteries
    • Is the main component of tendons, which connect skeletal muscles to bones
  • Haemoglobin may be compared with Collagen as such:
    • Basic Shape - Haemoglobin is globular while Collagen is fibrous
    • Solubility - Haemoglobin is soluble in water while Collagen is insoluble
    • Amino Acid Constituents - Haemoglobin contains a wide range of amino acids while Collagen has 35% of it primary structure made up of Glycine
    • Prosthetic Group - Haemoglobin contains a haem prosthetic group while Collagen doesn’t possess a prosthetic group
    • Tertiary Structure - Much of the Haemoglobin molecule is wound into α helices while much of the Collagen molecule is made up of left handed helix structures
Written by Sam Adam-Day.