Question

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

Short answer

Question: Draw the structural formulas of the tripeptides Phe-Val-Asn and Leu-Val-Gln, and identify the N-terminal amino acid, C-terminal amino acid, and peptide bonds for each tripeptide. Answer: In Phe-Val-Asn, the N-terminal amino acid is Phe (Phenylalanine), the C-terminal amino acid is Asn (Asparagine), and the peptide bonds are formed between Phe and Val, and Val and Asn. In Leu-Val-Gln, the N-terminal amino acid is Leu (Leucine), the C-terminal amino acid is Gln (Glutamine), and the peptide bonds are formed between Leu and Val, and Val and Gln.

Step-by-step solution

Draw the structural formula for Phe-Val-Asn

Start with the first amino acid, Phe (Phenylalanine). Draw its amino group (NH2), alpha carbon (with a side chain of benzyl group -CH2-C6H5), and carboxyl group (COOH). Then, draw the second amino acid, Val (Valine), and connect its amino group (NH2) to the carboxyl group (COOH) of Phe via a peptide bond (C-N). Ensure Val's side chain (-CH(CH3)2) is connected to its alpha carbon. Next, draw the third amino acid, Asn (Asparagine), and connect its amino group (NH2) to the carboxyl group (COOH) of Val via a peptide bond (C-N). Asn's side chain is (-CH2-CONH2).

Identify the N-terminal and C-terminal amino acids for Phe-Val-Asn

The N-terminal amino acid is the first amino acid in the chain, which has a free amino group (NH2) that is not involved in any peptide bond. In this case, the N-terminal amino acid is Phe (Phenylalanine). On the other hand, the C-terminal amino acid is the last amino acid in the chain, which has a free carboxyl group (COOH) that is not involved in any peptide bond. In this case, the C-terminal amino acid is Asn (Asparagine).

Draw the structural formula for Leu-Val-Gln

Begin with the first amino acid, Leu (Leucine). Draw its amino group (NH2), alpha carbon (with a side chain of -CH2-CH(CH3)2), and carboxyl group (COOH). Then, draw the second amino acid, Val (Valine), and connect its amino group (NH2) to the carboxyl group (COOH) of Leu via a peptide bond (C-N). Ensure Val's side chain (-CH(CH3)2) is connected to its alpha carbon. Next, draw the third amino acid, Gln (Glutamine), and connect its amino group (NH2) to the carboxyl group (COOH) of Val via a peptide bond (C-N). Gln's side chain is (-CH2-CH2-CONH2).

Identify the N-terminal and C-terminal amino acids for Leu-Val-Gln

The N-terminal amino acid is the first amino acid in the chain, which has a free amino group (NH2) that is not involved in any peptide bond. In this case, the N-terminal amino acid is Leu (Leucine). On the other hand, the C-terminal amino acid is the last amino acid in the chain, which has a free carboxyl group (COOH) that is not involved in any peptide bond. In this case, the C-terminal amino acid is Gln (Glutamine).

Key concepts

Peptide Bond

Picture a string of pearls gracefully draped. That string is much like a peptide bond, a special kind of chemical bond that is essential for life as we know it. It acts like a hook, linking amino acids together to form peptides, which are the short chains, and proteins, which are the longer chains.

When two amino acids join forces, they release a molecule of water in a reaction known as a condensation reaction. This creates the peptide bond, which is a strong covalent bond between the nitrogen atom of one amino acid and the carbon atom of another. Precisely, it connects the carboxyl group (COOH) of one amino acid to the amino group (NH2) of the next. The formula of this bond is quite concise, represented by -C(=O)-NH-. Imagine each bond as connecting the pearls, representing amino acids, into a unique and functional necklace, essential for various bodily functions.

N-terminal Amino Acid

When reading a book, the first page holds the key to starting the story just right. In the language of proteins, the N-terminal amino acid serves a similar role - it's the beginning of all protein sequences. 'N-terminal' stands for nitrogen-terminal. This term refers to the end of a peptide or protein that has a free amino group (-NH2); an end that's not linked to another amino acid by a peptide bond.

Identifying the N-terminal Amino Acid

In the tripeptide sequence Phe-Val-Asn, for instance, the hero initiating the chain is Phenylalanine (Phe), with its unbonded -NH2 group ready to lead the way. This is the case for any peptide; the N-terminus is always the alpha-amino of the very first amino acid.

C-terminal Amino Acid

Every story must have an ending, and in the linear narrative of peptides, that ending is demarked by the C-terminal amino acid. The 'C-terminal' - carbon-terminal, to be exact - is the very last amino acid in the chain. It has a free carboxyl group (-COOH) not involved in a peptide bond.

Locating the C-terminal in Peptides

Consider the tripeptide Leu-Val-Gln. Here, Glutamine (Gln) completes the trilogy, its -COOH group unattached, finalizing the peptide sequence. It's essentially the punctuation at the end of our peptide sentence, signaling the end of the construct, ready for whatever biological role it's set to perform.

Amino Acid Structure

Amino acids are the building blocks of life, essentially the legos that construct our body’s protein structures. At their core, they all share a general structure: a central carbon atom, also known as the alpha carbon, flanked by four different groups. These groups are an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (H), and a unique side chain or R-group which decides the identity and properties of the amino acid.

Diversity Among Amino Acids

It's the R-group's structure that distinguishes one amino acid from another, leading to a variety of functions and shapes. In tripeptides like Phe-Val-Asn and Leu-Val-Gln, these side chains interact, but all hold the same fundamental structure that enables them to form the beautiful complexity that we see in proteins.