What type of amino acid is phenylalanine? A) Polar neutral B) Nonpolar neutral C) Acidic D) Basic
The correct answer and explanation is:
Correct Answer: B) Nonpolar neutral
Explanation (300 words):
Phenylalanine is classified as a nonpolar neutral amino acid. This classification is based on the chemical structure and properties of its side chain (R group).
Phenylalanine has the following structure:
- An amino group (–NH₂)
- A carboxyl group (–COOH)
- A hydrogen atom
- A side chain consisting of a benzyl group (a benzene ring attached to a –CH₂– group)
The benzyl side chain is hydrophobic (repels water) and lacks any groups that can donate or accept hydrogen bonds or carry a charge under physiological conditions. Therefore, phenylalanine is nonpolar, meaning it does not interact well with water, and it is also considered neutral, because it has no ionizable groups on the side chain at physiological pH (~7.4).
Categories of Amino Acids:
Amino acids are typically classified based on the properties of their side chains:
- Nonpolar neutral: These amino acids have hydrophobic side chains and do not carry a charge (e.g., leucine, isoleucine, phenylalanine).
- Polar neutral: These have side chains that can form hydrogen bonds but do not carry a charge (e.g., serine, threonine).
- Acidic: These amino acids have negatively charged side chains at physiological pH due to the presence of carboxylic acid groups (e.g., aspartic acid, glutamic acid).
- Basic: These have positively charged side chains at physiological pH (e.g., lysine, arginine).
Biological Role:
Phenylalanine is essential in the human diet because the body cannot synthesize it. It is also a precursor to tyrosine, dopamine, norepinephrine, and epinephrine, making it important in neurotransmitter biosynthesis.
In summary, due to its hydrophobic benzyl side chain and lack of charge, phenylalanine is best classified as a nonpolar neutral amino acid.