Biomolecules: Protein Structure

An introduction to the four main groups of biomolecules (proteins, nucleic acids, lipids and carbohydrates) is presented here.  Today we will discuss proteins, specifically protein structure.

Protein structure is organized into four different levels.  Another way to say this, is that proteins have four levels of structure.  Let’s start out with the most basic level: primary structure.

Primary Structure of Proteins

The primary structure of a protein is simply its amino acid sequence.  If you recall, amino acids are the subunits that make up proteins.  This level of structure is similar to letters that are strung together to create a word or beads on a string

Secondary Structure of Proteins

Secondary structure is when intermolecular forces, specifically hydrogen bonds between the amine and carboxyl groups of amino acids, cause the primary sequence to bend or twist.  A bend is called a β-pleated sheet and looks similar to a paper fan.  Twists are called alpha helices and look like an old telephone cord, like the one your mom might have had in the kitchen.

Tertiary Structure of Proteins

When the primary sequence and secondary sequences bend and fold upon themselves, we call this the tertiary structure.  This third level of structure is caused by intermolecular forces (such as hydrophobic interactions or hydrogen bonding), ionic bonding and covalent disulfide bridges.  Unlike in the secondary structure where the structure is formed from interactions between the polypeptide backbone (the amine – carboxyl groups), these interactions in the tertiary structure are driven by the characteristics of the R-groups.  Tertiary structure can be visualized again by your kitchen telephone cord.  Imagine the alpha helix of the cord twisted, so that now we have a twisted alpha helix.  This is the tertiary structure of a polypeptide.

Quaternary Structure of Proteins

The final level of protein structure is quaternary structure.  Quaternary structure when multiple polypeptides interact with each other to fold.  This is similar to maybe a box of computer cables tangled in a box in your closet or multiple telephone cords, each twisted upon the alpha helices and then all tangled together.  The types of forces that make up quaternary structure are the same that are found at the tertiary level: intermolecular forces such as hydrophobic interactions or hydrogen bonding, ionic bonding and covalent disulfide bridges.  In the drawing below, each color represents individual polypeptide chains.  This protein is made up of 4 different polypeptide chains, red, green, black and blue.

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