Question

Acidic amino acids have two -COOH groups and one \(-\mathrm{NH}_{2}\) group per molecule. Select the pair that consists of acidic amino acids. (a) Aspartic acid, glutamic acid (b) Lysine, arginine (c) Glycine, alanine (d) Both (a) and (b)

Short answer

The correct pair of acidic amino acids is selection (a) Aspartic acid and glutamic acid.

Step-by-step solution

Identify Acidic Amino Acids

Acidic amino acids are characterized by their two carboxyl groups (-COOH) and one amino group (-NH2). These groups are what give them their acidic properties.

Analyze the Given Options

Examine each pair of amino acids provided in the options to determine if they have the two -COOH groups and one -NH2 group that define acidic amino acids.

Determine the Correct Pair

Option (a) Aspartic acid and glutamic acid, both have two -COOH groups and one -NH2 group. Option (b) Lysine and arginine, are both basic amino acids with more -NH2 groups. Option (c) Glycine and alanine are neutral amino acids. Therefore, option (a) is the correct pair of acidic amino acids.

Key concepts

Aspartic Acid

Aspartic acid, also known by its symbol Asp or D, plays a crucial role in the synthesis of other amino acids and in the urea cycle, the biochemical pathway that detoxifies ammonia by converting it into urea.

From a structural perspective, aspartic acid features a side chain with a second carboxylic acid group, which is responsible for the molecule's acidic properties. This side chain is what gives aspartic acid a negative charge at physiological pH, making it highly polar and reactive in chemical processes within the body. Due to its acidic characteristics, aspartic acid is often found in the active sites of enzymes, interacting with positively charged molecules and catalyzing essential biochemical reactions.

It's important for students to remember that when looking for acidic amino acids, they should check for the presence of more than one carboxyl (-COOH) group in their side chains. This feature is pivotal in classifying amino acids like aspartic acid as acidic.

Glutamic Acid

Glutamic acid, represented as Glu or E, is another acidic amino acid with an additional carboxylic acid group on its side chain, similar to aspartic acid, but with a longer carbon chain. This extra -COOH group is what imparts the acidity to glutamic acid and makes it a key player in cellular metabolism, particularly in the brain where it functions as an important neurotransmitter.

Glutamic acid has a major role in the central nervous system. It acts as an excitatory neurotransmitter and is involved in cognitive functions like learning and memory. Furthermore, because of its ability to donate a -COOH group, it is essential in the process known as transamination, which is critical for producing other non-essential amino acids and for carrying nitrogen in the bloodstream.

Students should be aware that the properties of glutamic acid are influenced by its acidity, and that under physiological conditions, it is typically found in its deprotonated form, glutamate. This form is very effective in binding and transporting cations across cell membranes due to its negative charge.

Amino Acids Properties

Amino acids, the building blocks of proteins, are organic compounds that share a basic structure consisting of a central carbon atom, an amino group (-NH2), a carboxyl group (-COOH), and a unique side chain (R group) that determines the properties and classification of each amino acid.

The properties of amino acids can be divided into four main categories: nonpolar (hydrophobic), polar (hydrophilic), acidic, and basic. Acidic amino acids, such as aspartic and glutamic acid, have an additional -COOH group, which makes them negatively charged at physiological pH, whereas basic amino acids possess an extra -NH2 group, giving them a positive charge.

The side chains of amino acids affect not only their charge but also their ability to form bonds, their solubility in water, and their reactivity. For instance, hydrophobic amino acids tend to cluster inside proteins, away from water, while hydrophilic amino acids are often found on the surface of proteins, where they can interact with the aqueous environment. Understanding these properties is essential for recognizing how amino acids contribute to the function and structure of proteins.