Amino acids contain amino (–NH₂) and carboxyl (–COOH) functional groups. Depending upon the relative position of amino group with respect to carboxyl group, the amino acids can be classified as α, β, γ, δ and so on. Only α-amino acids are obtained on hydrolysis of proteins. They may contain other functional groups also. All α-amino acids have trivial names, which usually reflect the property of that compound or its source. Glycine is so named since it has sweet taste (in Greek glykos means sweet) and tyrosine was first obtained from cheese (in Greek, tyros means cheese.) Amino acids are generally represented by a three letter symbol, sometimes one letter symbol is also used. Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Equal number of amino and carboxyl groups makes it neutral; more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic. The amino acids, which can be synthesized in the body, are known as nonessential amino acids. On the other hand, those which cannot be synthesized in the body and must be obtained through diet, are known as essential amino acids.

Which of the following statements about amino acids is correct?

Amino acids are classified according to the structures and properties of their side chains

The correct answer is option 2. Amino acids are classified according to the structures and properties of their side chains.

Among the statements provided about amino acids, the correct one is: Amino acids are classified according to the structures and properties of their side chains.

Amino acids are organic compounds that contain both an amino group \((-NH_2)\) and a carboxyl group \((-COOH)\), attached to a central carbon atom (the \(\alpha \)-carbon). The diversity among amino acids primarily arises from the nature of the side chain (\(R\) group) attached to this α-carbon. These side chains vary significantly in size, shape, charge, and hydrophobicity, which influences the chemical properties and biological roles of each amino acid.

Classification Based on Side Chains:

Non-Polar (Hydrophobic) Amino Acids:

These amino acids have side chains that are predominantly hydrocarbon-based, lacking polar functional groups. Examples include glycine, alanine, valine, leucine, isoleucine, phenylalanine, tryptophan, and methionine. They tend to cluster together in the interior of proteins, away from aqueous environments.

Polar (Hydrophilic) Amino Acids:

Polar amino acids have side chains that contain functional groups such as hydroxyl \((-OH)\), amino \((-NH_2)\), or carbonyl groups. Examples include serine, threonine, cysteine, tyrosine, asparagine, and glutamine. These amino acids can form hydrogen bonds with water molecules and other polar groups.

Positively Charged (Basic) Amino Acids:

These amino acids have side chains with basic groups that can accept protons \((H^+)\). Examples include lysine, arginine, and histidine. They often interact with negatively charged molecules or groups in proteins and other biomolecules.

Negatively Charged (Acidic) Amino Acids:

Acidic amino acids have side chains that contain carboxyl groups \((-COOH)\), which can donate protons \((H^+)\). Examples include aspartic acid and glutamic acid. They typically interact with positively charged molecules or groups.

Importance of Classification:

Structural and Functional Diversity: The classification of amino acids based on their side chains helps in understanding their roles in protein structure, function, and interactions with other molecules.

Biochemical Significance: Different amino acids contribute distinct chemical properties to proteins, influencing their stability, folding, catalytic activity, and binding specificity.

Protein Design and Engineering: Knowledge of amino acid properties is essential for protein engineering and designing novel biomolecules with specific functions or properties.

Conclusion: Understanding how amino acids are classified based on the structures and properties of their side chains provides a foundational framework in biochemistry and molecular biology. This classification system facilitates the study of protein structure and function, enzymology, and the underlying mechanisms of biological processes.