Question

Indicate whether each statement is true or false. (a) Tryptophan is an aromatic amino acid. (b) Lysine is positively charged at pH 7. (c) Asparagine has two amide bonds.

Short answer

True for (a) Tryptophan is an aromatic amino acid, true for (b) Lysine is positively charged at pH 7, and false for (c) Asparagine has two amide bonds (it has only one in its side chain).

Step-by-step solution

Statement (a): Tryptophan is an aromatic amino acid.

Tryptophan is one of the 20 standard amino acids. It has an indole side chain, which is an aromatic ring. Based on its chemical structure, we can say that Tryptophan is indeed an aromatic amino acid. Therefore, this statement is true.

Statement (b): Lysine is positively charged at pH 7.

Lysine is an amino acid with an aliphatic side chain. The R-group (side chain) of Lysine contains a primary amino group (-NH2). At pH 7, which is neutral pH, this amino group can accept a proton to form a positively charged ammonium ion (-NH3+). Lysine itself has a pKa value of 9.74 for its side chain, meaning that a significant portion of Lysine molecules will have positively charged side chains at pH 7. Therefore, this statement is true.

Statement (c): Asparagine has two amide bonds.

Asparagine is an amino acid with a polar, uncharged side chain. Its side chain has an amide functional group, meaning it has one amide bond. It is important to note that every amino acid contains an amide bond between the amino group and the carboxyl group, which is the main peptide bond. However, we are only considering the properties of the side chain in this statement. Since Asparagine has only one amide bond in its side chain, this statement is false.

Key concepts

Aromatic Amino Acids

Aromatic amino acids are a special group of amino acids that contain aromatic rings in their side chains. These rings are stable, planar structures that allow electrons to move freely across the ring, which we call "aromaticity." This property gives them unique characteristics.
Tryptophan, phenylalanine, and tyrosine are the primary aromatic amino acids in proteins. Tryptophan, for example, has an indole ring, making it an aromatic amino acid.

• Tryptophan: Contains an indole side chain.
• Phenylalanine: Has a benzyl side chain.
• Tyrosine: Similar to phenylalanine but with a hydroxyl group on the benzene ring.

These aromatic rings are crucial for protein folding, protein-protein interactions, and are often involved in the binding of molecules like hormones or neurotransmitters.

Ionization and pH

Ionization and pH are important concepts in understanding amino acid behavior in different environments. At different pH levels, the ionization state of amino acids changes, affecting their charge and thus their properties.
The pH is a measure of acidity or alkalinity. At a pH of 7, which is considered neutral (like pure water), some amino acids can carry charges. Lysine, for instance, has an aliphatic side chain with a primary amino group (-NH2) that can gain a proton at neutral pH, becoming positively charged (-NH3+).
This behavior is crucial for protein function, as the charge can dictate how proteins interact with other molecules or with each other. For instance, although the pKa value is different for different side chains, knowing this helps predict how an amino acid like Lysine will behave at any pH.

• Lysine is often positively charged at physiological pH (around 7) due to its side chain ionization.
• This affects protein solubility, activity, and interaction with other biomolecules.

Chemical Structure of Amino Acids

The chemical structure of amino acids includes a central carbon atom bonded to four different groups: a hydrogen atom, a carboxyl group (-COOH), an amino group (-NH2), and a side chain (R group) specific to each amino acid. This specific arrangement makes them building blocks of proteins.
Each amino acid's unique side chain determines its characteristics and role within proteins. For example, Asparagine, which has a polar, uncharged side chain, contains one amide bond in its side chain, distinguishing it from amino acids with multiple amide bonds or those without.

• The amide bond, or peptide bond, links amino acids together in polypeptides.
• Amino acids can be charged, polar, or nonpolar, depending on the properties of their side chains, influencing their role in proteins.

Understanding the structural specifics of amino acids helps in grasping how they form complex proteins, participate in cellular functions, and interact with other molecules.