Question

Draw a structural formula of these tripeptides. Mark each peptide bond, the \(N\)-terminal amino acid, and the G-terminal amino acid. (a) Phe-Val-Asn (b) Leu-Val-Gln

Short answer

Question: Draw the structural formula of the tripeptides (a) Phe-Val-Asn and (b) Leu-Val-Gln, clearly marking the peptide bonds, N-terminal amino acid, and C-terminal amino acid. Answer: For tripeptide (a) Phe-Val-Asn: - Structural formula: NH_2-CH(CH_2C_6H_5)-CO-NH-CH(CH_3)_2-CH_2-CO-NH-CH(CONH_2)-CH_2-COOH - Peptide bonds: CO-NH (between Phe and Val) and CO-NH (between Val and Asn) - N-terminal amino acid: Phenylalanine (Phe) - C-terminal amino acid: Asparagine (Asn) For tripeptide (b) Leu-Val-Gln: - Structural formula: NH_2-CH(CH_3)_2-CH_2-CH_2-CO-NH-CH(CH_3)_2-CH_2-CO-NH-CH(CH_2CONH_2)-CH_2-COOH - Peptide bonds: CO-NH (between Leu and Val) and CO-NH (between Val and Gln) - N-terminal amino acid: Leucine (Leu) - C-terminal amino acid: Glutamine (Gln)

Step-by-step solution

1. Identify the amino acids in the tripeptide

In each tripeptide, three amino acids are abbreviated with their corresponding three-letter codes. For (a) Phe-Val-Asn, the amino acids are Phenylalanine (Phe), Valine (Val), and Asparagine (Asn). For (b) Leu-Val-Gln, the amino acids are Leucine (Leu), Valine (Val), and Glutamine (Gln).

2. Draw the structural formula of each amino acid

To begin, draw the structural formula of each specific amino acid present in the tripeptides, focusing on the amino group (NH_2), the carboxyl group (COOH), and the side chain (R) specific to each amino acid. Phenylalanine (Phe): NH_2-CH(CH_2C_6H_5)-COOH Valine (Val): NH_2-CH(CH_3)_2-CH_2-COOH Asparagine (Asn): NH_2-CH(CONH_2)-CH_2-COOH Leucine (Leu): NH_2-CH(CH_3)_2-CH_2-CH_2-COOH Glutamine (Gln): NH_2-CH(CH_2CONH_2)-CH_2-COOH

3. Connect the amino acids with peptide bonds

In both tripeptides, the amino acids will be connected via peptide bonds. Peptide bonds are formed between the carboxyl group (COOH) of one amino acid and the amino group (NH_2) of the adjacent amino acid, releasing a water molecule (H_2O) in the process. For tripeptide (a) Phe-Val-Asn, we can form the following structure: NH_2-CH(CH_2C_6H_5)-CO-NH-CH(CH_3)_2-CH_2-CO-NH-CH(CONH_2)-CH_2-COOH For tripeptide (b) Leu-Val-Gln, we can form the following structure: NH_2-CH(CH_3)_2-CH_2-CH_2-CO-NH-CH(CH_3)_2-CH_2-CO-NH-CH(CH_2CONH_2)-CH_2-COOH

4. Mark the peptide bonds, N-terminal amino acid, and C-terminal amino acid

In each tripeptide, the peptide bonds are located between the CO group of one amino acid and the NH group of the next amino acid. The N-terminal amino acid is the first amino acid in the tripeptide with a free NH_2 group, and the C-terminal amino acid is the last amino acid with a free COOH group. For tripeptide (a) Phe-Val-Asn, mark the following: - Peptide bonds: CO-NH (between Phe and Val) and CO-NH (between Val and Asn) - N-terminal amino acid: Phenylalanine (Phe) - C-terminal amino acid: Asparagine (Asn) For tripeptide (b) Leu-Val-Gln, mark the following: - Peptide bonds: CO-NH (between Leu and Val) and CO-NH (between Val and Gln) - N-terminal amino acid: Leucine (Leu) - C-terminal amino acid: Glutamine (Gln)

Key concepts

Peptide Bond

Peptide bonds are the backbone of protein structures, acting as the glue that connects individual amino acids into long chains called polypeptides. Their formation is a result of a dehydration synthesis reaction, where the carboxyl group (COOH) of one amino acid reacts with the amino group (NH₂) of another, releasing a water molecule (H₂O). In structural terms, the peptide bond can be depicted as -CO-NH-. This bond is strong and has a partial double-bond character, restricting rotation and thus influencing the overall shape of the protein. It's essential to understand that each peptide bond formation consumes energy and is vital for the 3D shape, which ultimately determines the protein's function.

Going back to our tripeptide examples, the peptide bonds form the links between the individual amino acids which are detailed in the step-by-step solution, giving the tripeptide its initial structure before it folds into a more complex shape.

N-terminal Amino Acid

In any given protein or peptide chain, the N-terminal amino acid is the start of the chain, represented by the very first amino acid with a free amino group (NH₂). This is significant because it indicates the direction in which the chain is synthesized and read, as proteins are biologically synthesized from the N-terminus to the C-terminus. This terminus often plays a key role in protein targeting and can undergo various modifications after protein synthesis that can alter protein function or location.

In our exercise examples, our N-terminal amino acids are Phenylalanine (Phe) for tripeptide (a) and Leucine (Leu) for tripeptide (b). Recognizing the N-terminus is a crucial step for scientists when sequencing proteins and understanding their properties.

C-terminal Amino Acid

The C-terminal amino acid is found at the other end of a peptide or protein chain, characterized by a free carboxyl group (COOH). It is the last amino acid added to the growing peptide chain during protein synthesis. The C-terminus can affect where the protein ultimately resides within the cell and can be involved in regulating protein stability.

In the given solutions, the C-terminal amino acids are Asparagine (Asn) for tripeptide a and Glutamine (Gln) for tripeptide b. Identifying the C-terminal amino acid sets the stage for understanding the complete structure and functionality of the protein or peptide.

Amino Acid Structure

Amino acids, the building blocks of proteins, consist of a central carbon atom, also known as the alpha (α) carbon, bonded to four different groups: an amino group (NH₂), a carboxyl group (COOH), a hydrogen atom, and a unique side chain represented by the symbol R. The R group varies among different amino acids and determines the chemical nature of each amino acid, influencing protein structure and function.

The structural formula of amino acids provided in the exercise is foundational for biochemistry and molecular biology. It is through the diverse side chains that amino acids can interact with one another and with other molecules, creating the incredibly complex and versatile structures that perform a myriad of functions within living organisms.