Problem 166 - Objective Biology for NEET

Question

Assertion: The first amino acid in primary structure protein is called as N-terminal amino acid. Reason: \(\mathrm{NH}_{2}\) group of first amino acid is free, not bound to form peptide bond.

Step-by-Step Solution

Answer

The first amino acid in a primary structure protein is indeed called the N-terminal amino acid because its amine (\(\mathrm{NH}_{2}\)) group is not bound to form a peptide bond.

Step 1: Understanding the Terms

The primary structure of a protein is its sequence of amino acids. These amino acids are joined together by peptide bonds. Each amino acid has two functional groups: the amine group (\(\mathrm{NH}_{2}\)) and the carboxyl group (\(\mathrm{COOH}\)).

Step 2: Confirming the Assertion

The first amino acid of a protein sequence is known as the 'N-terminal' amino acid because at this end of the protein, the amine (or nitrogen-terminus, hence the 'N' in 'N-terminal') group is free and not involved in forming a peptide bond.

Step 3: Examining the Reason

The amine group (\(\mathrm{NH}_{2}\)) of the first amino acid is unbound, which is why it is known as the N-terminal amino acid. It does not form a peptide bond because peptide bonds are formed between the amine group of one amino acid and the carboxyl group of the next amino acid in the sequence. Therefore, as there would be no preceding amino acid for the first one, its amine group remains free.

Key Concepts

N-terminal amino acid

In the structure of a protein, the N-terminal amino acid holds a special position. It is the first amino acid in the protein chain and is characterized by its free amine group (\( \mathrm{NH}_2 \)). This amino group is not part of any peptide bond. Why is this significant? Because it helps give proteins directionality. Proteins have two ends: the N-terminal, starting end, and the C-terminal, ending end.

• The N-terminal end has a free amine group (\( \mathrm{NH}_2 \)).
• The C-terminal end has a free carboxyl group (\( \mathrm{COOH} \)).

Understanding these ends is crucial, as they dictate the sequence in which amino acids are added during protein synthesis. The unique freedom of the N-terminal amino group also plays a role in protein identification and interactions within cells.

Peptide bonds

Peptide bonds are the links that hold amino acids together in proteins. Each peptide bond forms when the amine group (\( \mathrm{NH}_2 \)) of one amino acid reacts with the carboxyl group (\( \mathrm{COOH} \)) of the next. This reaction results in a covalent bond between them, releasing a molecule of water—an example of a dehydration reaction.

• Peptide bonds are strong and not easily broken.
• They give proteins their primary structure.

The primary structure of a protein is vital as it determines the protein's unique characteristics and functions. When long chains of amino acids are linked together by peptide bonds, they form complex three-dimensional structures that perform various roles in the body.

Amino acid functional groups

Amino acids, the building blocks of proteins, contain two essential functional groups: the amine group (\( \mathrm{NH}_2 \)) and the carboxyl group (\( \mathrm{COOH} \)). Each plays a specific role in forming proteins:

• The amine group is a key component in forming peptide bonds, linking one amino acid to another.
• The carboxyl group also participates in creating peptide bonds, acting as the binding site for the subsequent amino acid.

These functional groups are essential for the construction of proteins. Beyond peptide bond formation, they contribute to the chemical properties that affect protein folding and function. By rearranging these functional groups, amino acids can create proteins with varied and unique roles in biological systems.