Which of the following amino acids are neutral by nature? (1) Lysine (2) Aspartic acid (3) Proline (4) Leucine (5) Serine (a) \(1,3,4\) (b) \(3,4,5\) (c) 4,5 (d) \(1,3,4,5\)

Amino acids are the organic compounds that combine to form proteins, which are essential for many functions in the body. One of the key properties to understand is their side chain, which can determine an amino acid's nature and role. Side chains vary in size, composition, and electric charge, giving the amino acid unique properties such as hydrophobicity or hydrophilicity, and neutrality, acidity, or basicity.

At a specific pH level, notably the human body’s physiological pH of about 7.4, amino acids can exist in a charged or uncharged state. Amino acids with uncharged side chains at this pH do not release or attract hydrogen ions and are considered neutral. This neutrality plays an important role in protein structure and function, affecting how proteins fold, interact with other molecules, and perform their biological roles.

Amino acids can be classified into two distinct groups based on the nature of their side chains at physiological pH – acidic and basic.

Acidic Amino Acids

Acidic amino acids, such as Aspartic acid and Glutamic acid, have side chains that contain carboxyl groups, which can donate an extra hydrogen ion and hence possess a negative charge at a pH of 7.4.

Basic Amino Acids

Basic amino acids like Lysine, Arginine, and Histidine have side chains with amino groups that can accept hydrogen ions, giving them a positive charge under the same conditions. This distinction is crucial as it affects how amino acids interact with each other and with different molecules within the cells, influencing everything from enzyme catalysis to signal transduction.

Classification of amino acids doesn't only revolve around their charge properties but also includes other attributes such as polarity and the ability to be involved in specific interactions.

• Nonpolar amino acids tend to be hydrophobic and are often found in the interior of proteins to avoid water. Examples are Proline and Leucine, which are neutral and nonpolar.
• Polar amino acids, which can be neutral such as Serine, are often found on the protein's exterior surface interacting with the aqueous environment.
• Lastly, the special amino acids with unique roles, like Cysteine with its ability to form disulfide bonds, and Glycine, which confers flexibility due to its small size, need to be recognized as well.

This classification helps in understanding not only the structure and stability of proteins but also their interactions with other molecules, which is essential to biochemical pathways.