Seventh Lesson: Overview of Proteins

Seventh Lesson: Overview of Protein Folding

From the previous lesson we have learned that through the combination of both Transcription and Translation, DNA is converted into proteins.  Remember that proteins are made up of amino acid building blocks that are attached through peptide bonds (a special type of covalent bond). Proteins become functional as the amino acids naturally fold into the right native structure as they emerge from the ribosome.  Only when a protein is folded correctly is it functional.  Why?  Think about it this way.  Many proteins work through a lock & key mechanism in which something binds and the protein converts it into something else.  A lock has to be the right shape to fit its particular key.  If a lock is not the right shape (a protein that is improperly folded), it will not work correctly.

There are four levels of protein complexity that range from simple to the most complex: primary, secondary, tertiary, and quaternary.  All proteins have some form of a primary, secondary, and tertiary folding, but not every protein has quaternary structure.

The primary structure is simple.  If I were to read to you each amino acid in a series of connected amino acids to you in order, you would have the primary structure.  It is like reading the sentences in a book in order.  See, I told you it was simple, right?

The secondary structure is a little bit more complex than the primary structure.  In the secondary structure the linear backbone of the protein begins interacting and taking shape.  Amino acids will fold to form two different but common structures: either α-helices or β-strands.  α-helices are helical structures formed by the amino acid backbone interacting together (something that looks like a screw).  β-strands are structures that occur when the backbone folds on top of itself and interacts with hydrogen bonds.  (The picture below helps a lot to visualize it.)

The tertiary structure is the last universal protein structure.  The tertiary structure, by definition, is the 3D arrangement of all the amino acids of the proteins.  In other words, it is the 3D image of the entire protein.  This is the most complex and most common image we see of most proteins today.  Most tertiary structure images are found on very powerful computers that are designed to predict protein folding.

The term protein is very misleading; many proteins have multiple but distinct chains of amino acids that fold together to form a bigger structure.  Think of it this way: it is like teeing a bow. A bow can be constructed from one piece of string, but more complex bows use multiple pieces of string to form the structure.  When this happens, the protein is said to have a quaternary structure.  Not ever protein is comprised of many different individual chains of amino acids, so some natural proteins only have a tertiary structure. (Many molecular biologist use the term polypeptide to identify a protein that is comprised of only a single chain of amino acids and not more than one.)  In the next lesson I will be discussing the concept of DNA packaging.  This involves taking an object that is 3 meters long and stuffing it into a space of that is only fractions of a hair thick!

Here is are some Youtube videos discussing protein folding