Animated Solution for Chemistry - Biomolecules: The number of $C=O$ groups present in a tripeptide Asp-Glu-Lys is ........... .

Biochemistry often feels like a puzzle where molecules are the pieces. In this problem, we are tasked with finding the total number of carbonyl ($C=O$) groups in a specific tripeptide: Asp-Glu-Lys.

To solve this, we need to understand the anatomy of a tripeptide. A tripeptide is formed by linking three amino acids together via condensation reactions. When they link, they form peptide bonds. Each peptide bond is an amide linkage ($-C(=O)-NH-$), which inherently contains one $C=O$ group.

But that's not the whole story! We must also meticulously inspect the side chains (R-groups) of each specific amino acid, as well as the free ends of the peptide chain.

Let's start with the first amino acid in our sequence: Aspartic Acid (Asp). Because it is at the beginning of the chain, its amino group ($-N{H}_2$) forms the N-terminus.

Now, look closely at its side chain. Aspartic acid is an acidic amino acid, meaning its side chain contains a carboxylic acid group: $-C{H}_2-COOH$. This side chain provides our very first $C=O$ group.

As Aspartic acid links to the next amino acid, Glutamic Acid (Glu), it forms our first peptide bond. This amide linkage naturally contains our second $C=O$ group.

Moving on to Glutamic Acid itself, we find another acidic side chain. Glu is very similar to Asp, but it has an extra methylene group: $-C{H}_2-C{H}_2-COOH$. This carboxylic acid group in the side chain gives us our third $C=O$ group.

Next, Glutamic acid links to the final amino acid, Lysine (Lys). This forms our second peptide bond, which contributes our fourth $C=O$ group.

Finally, let's examine Lysine. Lysine is a basic amino acid. Its side chain is a long aliphatic chain terminating in an amino group: $-(C{H}_2{)}_4-N{H}_2$. Since there are no oxygen atoms here, the side chain contributes zero carbonyl groups.

However, we must not forget the backbone! Because Lysine is the last amino acid in our tripeptide, it forms the C-terminus. This means its main-chain carboxyl group remains unreacted and free as $-COOH$. This provides our fifth and final $C=O$ group.

Let's tally them all up to be absolutely sure: 1. Aspartic acid side chain: 1 2. First peptide bond (Asp-Glu): 1 3. Glutamic acid side chain: 1 4. Second peptide bond (Glu-Lys): 1 5. Lysine C-terminus: 1

Adding these together, we get a total of 5 $C=O$ groups. Mastering the structures of the 20 standard amino acids is the ultimate key to unlocking these types of questions with confidence!