Question

Draw a structure for each tetrapeptide. a. Ser-Ala-Leu-Cys b. Gln-Met-Cys-Gly c. Gly-Cys-Met-Gln

Short answer

Draw Ser-Ala-Leu-Cys, Gln-Met-Cys-Gly, and Gly-Cys-Met-Gln tetrapeptides, starting from the N-terminus on the left and ending with the C-terminus on the right, ensuring proper peptide bonds and side chains.

Step-by-step solution

Identify Amino Acids

Firstly, identify the amino acids and their three-letter codes. Ser is Serine, Ala is Alanine, Leu is Leucine, Cys is Cysteine, Gln is Glutamine, Met is Methionine, and Gly is Glycine.

Draw the First Tetrapeptide - Ser-Ala-Leu-Cys

Begin with the N-terminus (Ser) on the left and end with the C-terminus (Cys) on the right. Connect the amino acids in the order given by peptide bonds. Each amino acid's central carbon (alpha carbon) is connected to an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group). Remember that during the formation of the peptide bond, a water molecule is released as the carboxyl group of one amino acid and the amino group of the next amino acid link.

Draw the Second Tetrapeptide - Gln-Met-Cys-Gly

Again, start with the N-terminus (Gln) and end with the C-terminus (Gly). Follow the same peptide-bonding process as before, ensuring that the correct R groups for Glutamine, Methionine, Cysteine, and Glycine are attached to their respective alpha carbons.

Draw the Third Tetrapeptide - Gly-Cys-Met-Gln

Begin with Glycine at the N-terminus and end with Glutamine at the C-terminus, creating peptide bonds between each pair of amino acids. Make sure the R groups for Glycine, Cysteine, Methionine, and Glutamine are correctly depicted for each amino acid.

Review Peptide Bond Formation

Ensure you have correctly illustrated the loss of water (dehydration synthesis) that occurs when forming each peptide bond, and that the correct orientation of the amino and carboxyl groups is displayed in your tetrapeptides.

Key concepts

Amino Acids and Peptide Bonds

Amino acids are the building blocks of proteins, linked together by peptide bonds to form polypeptide chains. Each amino acid consists of a central alpha carbon to which an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a distinctive side chain (known as an R group) are attached. The chemistry of the R group gives each of the 20 amino acids its unique properties.

Peptide bonds are covalent bonds formed between the amino group of one amino acid and the carboxyl group of another. The formation of a peptide bond is a dehydration synthesis reaction, where a molecule of water is released. This bond is what links the amino acids, and it occurs between the carbon atom of the carboxyl group and the nitrogen atom of the amino group. Importantly, when students are illustrating peptide bonds in a tetrapeptide, they must ensure that each bond correctly shows this connection. This careful linking creates the primary structure of proteins, which ultimately folds into higher-level structures responsible for protein function.

N-terminus and C-terminus Orientation

Proteins have directionality, and understanding N-terminus and C-terminus orientation is crucial for anyone studying protein structure. The N-terminus is the end of the protein chain where the free amino group is located, and it is always written and depicted at the leftmost side of the structure. Similarly, the C-terminus is the end with the free carboxyl group, positioned at the rightmost side of the chain.

When drawing or interpreting the structure of peptides, such as tetrapeptides, it's imperative to start with the amino terminal (N-terminal) amino acid and conclude with the carboxyl terminal (C-terminal) amino acid. Incorrect orientation would lead to a fundamental misunderstanding of how proteins are synthesized and function. This directional paving, from N-terminus to C-terminus, also plays a role in how the protein will eventually fold and maintain its bioactive conformation.

Dehydration Synthesis in Peptides

The process of forming a peptide bond through dehydration synthesis is an essential concept in biochemistry. Dehydration suggests the loss of water, and in the context of peptide bonds, it refers to the elimination of a water molecule as a direct result of creating the bond between two amino acids. During this reaction, the hydroxyl group (OH) from the carboxyl end of one amino acid and the hydrogen (H) from the amino group of another are removed, combining to form water (H2O), and consequentially, a new bond is established.

This dehydration synthesis is a condensation reaction, happening multiple times during the formation of polypeptides from amino acids. Each peptide bond within the tetrapeptide is the outcome of such a synthesis. Students need to illustrate this by showing water molecules being released in their tetrapeptide structure drawings. Recognizing this reaction furthers the understanding of the energy investment and the intricacy of protein biosynthesis.