Question

Give two examples of each of the following, along with their structure: (a) Neutral amino acids (b) Acidic amino acids (c) Basic amino acids (d) Semi-essential amino acids (e) Sulfur containing amino acids (f) \(\alpha\) -imino acids

Short answer

Examples include: Neutral - Ser, Ala; Acidic - Asp, Glu; Basic - Lys, Arg; Semi-essential - Arg, His; Sulfur-containing - Cys, Met; -Imino acids - Pro, Hydroxyproline.

Step-by-step solution

Understanding Amino Acids

Amino acids are organic compounds that combine to form proteins. They consist of a basic amino group (-NH₂), an acidic carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group) that defines the type of amino acid. These side chains determine the properties and classification of amino acids.

Identifying Neutral Amino Acids

Neutral amino acids are those with side chains that do not impart a net electrical charge to the entire molecule at physiological pH. Examples include serine (Ser), which has a hydroxymethyl group as its side chain, and alanine (Ala) with a simple methyl group side chain.

Identifying Acidic Amino Acids

Acidic amino acids have side chains that contain carboxylic acid functional groups, which give them a net negative charge at physiological pH. Examples include aspartic acid (Asp) and glutamic acid (Glu). Both have carboxyl groups in their side chains, with glutamic acid having one extra methylene group compared to aspartic acid.

Identifying Basic Amino Acids

Basic amino acids have side chains that contain amino groups, which give them a net positive charge at physiological pH. Examples include lysine (Lys) with an aliphatic amino group, and arginine (Arg) that has a guanidino group as part of its side chain.

Identifying Semi-Essential Amino Acids

Semi-essential amino acids are those that are not synthesized in sufficient amounts by the human body during certain conditions. Examples include arginine (Arg) and histidine (His), which may be necessary in the diet of children and certain adults.

Identifying Sulfur Containing Amino Acids

Sulfur-containing amino acids have side chains that include sulfur atoms. Examples include cysteine (Cys), with a thiol (-SH) group, and methionine (Met), which has a thioether group in its side chain.

Identifying 𝜶-Imino Acids

-Imino acids have a structure where the amino group is replaced by an imino group (-NH), making them secondary amines. Proline (Pro) and hydroxyproline are examples, where proline forms a cyclic structure by bonding with its side chain, and hydroxyproline is a hydroxylated derivative of proline.

Key concepts

Neutral Amino Acids

Neutral amino acids are characterized by having side chains that don't contribute a distinct net electrical charge to the molecule at physiological pH. This means that, while they do possess a basic amino group and an acidic carboxyl group, their side chains are neither overly acidic nor basic, which allows them to remain electrically neutral under normal conditions.
One common example of a neutral amino acid is serine, which has a hydroxymethyl group in its side chain. These hydroxyl groups can partake in hydrogen bonding, which plays a crucial role in protein structure stabilization. Another example is alanine. Its side chain consists of a simple methyl group, and it is considered a key player in both protein structure because of its simplicity and in the metabolism of glucose. Neutral amino acids serve as fundamental building blocks in protein synthesis and are involved in various metabolic pathways.

Acidic Amino Acids

Acidic amino acids are unique because their side chains contain carboxylic acid groups. These groups give the amino acid a net negative charge under physiological conditions, which makes them essential for the chemical behavior of proteins.
Examples of acidic amino acids include aspartic acid and glutamic acid. Aspartic acid's structure contains one carboxyl group in its side chain, whereas glutamic acid has an extra methylene group compared to aspartic acid, lending it a longer side chain. This structural difference is significant because it influences how these acids interact with other molecules, including metal ions and receptors. Acidic amino acids are vital in processes such as neurotransmission and enzyme activity, acting as key players in various biochemical interactions.

Basic Amino Acids

Basic amino acids are classified by their side chains, which are basic due to the presence of additional amino groups. These groups confer a net positive charge to the molecule at physiological pH.
Lysine, for instance, contains an aliphatic amino group in its side chain, resulting in strong hydrogen bonding capabilities. This makes lysine extremely influential in strengthening protein structures. Arginine contains a guanidino group, which is even more potent as a basic compound. This property enables arginine to interact dynamically within the cell, playing a role in protein synthesis and the urea cycle. These basic amino acids are vital for multiple physiological processes such as signal transduction and nitrogen balance.

Semi-Essential Amino Acids

Semi-essential amino acids are a special category that needs closer attention as they are not always required from the diet. Their need varies with factors like age and health conditions, which is why they are termed 'semi-essential.'
Arginine and histidine are great examples of semi-essential amino acids. Arginine is vital during periods of rapid growth, such as childhood, because it's crucial for proper growth and immune function. Histidine, meanwhile, is not only a building block for proteins but also an important precursor for histamine, a compound involved in immune responses and gastric secretions. These amino acids highlight the body's adaptability and how dietary needs can change over a person's lifetime.

Sulfur-Containing Amino Acids

Sulfur-containing amino acids are notable for having sulfur atoms in their structure, which influences their chemical behavior significantly. These amino acids are crucial in various biochemical roles including catalysis and maintaining structural integrity.
Cysteine and methionine are two prime examples. Cysteine contains a thiol group, which is pivotal in forming disulfide bonds, a type of covalent bond that stabilizes protein structures. Methionine, with its thioether group, is essential in protein synthesis and methylation processes. The presence of sulfur in these amino acids is critical in detoxification pathways and the regulation of redox potential within cells, showcasing the diversity and necessity of sulfur-containing amino acids in both structure and function.