Tertiary Structure of Protein – Part 1

Last time, we saw that the second level of protein structure is called the secondary structure.

But it doesn’t stop here.

For example, the secondary structure can fold in on itself again, to form this messy shape

Which is the third level of protein structure, called the tertiary structure.

And this time, the shape we get is caused by the side groups of the amino acids in the polypeptide chain.

So, to understand how polypeptide chains can form such funky shapes, we need to look at three different types of bonds that are possible between those side groups.

First off, take these two amino acids.

If we look at their side groups…

… we see that one amino acid has an oxygen with a partial negative charge

… and the other amino acid has a hydrogen with a partial positive charge,

So, these two amino acids form…

These two amino acids form a hydrogen bond.

And it’s the same for these two amino acids…

And these two amino acids.

So, side groups of any two amino acids can form hydrogen bonds between each other.

Second, take these two amino acids.

If we look at their side groups…

… we see that one amino acid has an oxygen with a negative charge…

… and the other amino acid has a group with a positive charge.

So, these two amino acids form…

These two amino acids form an ionic bond.

And it’s the same for these two amino acids…

… and these two amino acids.

So, side groups of any two amino acids can form ionic bonds between each other. 

Third and finally, these two amino acids are both called cysteine

And every cysteine has a sulfhydryl group [SH group] at the end of its side chain.

Now, these two sulfhydryl groups react [split off both H atoms]… to form this bond [S-S]… and a molecule of H2  [combine both Hs to H2]

We call this bond a disulfide bond, or disulfide bridge

Because it forms a bridge between the two sulfur atoms of the cysteine molecules.

And because a disulfide bridge is a covalent bond, we represent it with a straight line, like this [highlight bond between S – S].

Finally, disulfide bridges can only form between two cysteine amino acids.

So, now, identify the type of bond between the following pairs of amino acids…

This is an ionic bond…

This is a hydrogen bond…

And, this is a disulfide bridge.

To sum up…

The third level of protein structure is called the…

The third level of protein structure is called the tertiary structure.

Which is caused by bonds between…

The tertiary structure is caused by bonds between the side groups of amino acids in the polypeptide chain.

And, these side groups form…

Side groups form hydrogen bonds, ionic bonds, and disulfide bridges.