What is the Difference Between Aliphatic and Aromatic Amino Acids

The key difference between aliphatic and aromatic amino acids is that aliphatic amino acids have no cyclic structures with alternating double-bond characteristics, whereas aromatic amino acids have cyclic structures with alternating double-bond characteristics.

Amino acids are the building units of protein molecules and polypeptides. Therefore, amino acids can play an important role in metabolic pathways, gene expression, and cell signaling pathways. The basic functional groups in a simple amino acid are the amine group, carboxyl group, hydrogen atom, and a side chain attached to a central carbon atom. There are 20 amino acid molecules that humans need. Among these 20 amino acids, 11 are made in the body, while the other 9 are consumed from dietary sources. Amino acids can be categorized mainly into two groups; they are aliphatic and aromatic amino acids, depending on the chemical structure of these chemical compounds.

CONTENTS

1. Overview and Key Difference
2. What are Aliphatic Amino Acids
3. What are Aromatic Amino Acids
4. Aliphatic vs Aromatic Amino Acids in Tabular Form
5. Summary – Aliphatic vs Aromatic Amino Acids 

What is Aliphatic Amino Acid?

Aliphatic amino acids are amino acids consisting of aliphatic side chains functional groups. These compounds are nonpolar and hydrophobic amino acids. Typically, aliphatic amino acids can be found within protein molecules, with two exceptions alanine and glycine, found either inside or outside of a protein molecule. Some examples of aliphatic amino acids include alanine, isoleucine, leucine, proline, and valine. Sometimes, methionine is also considered an aliphatic amino acid, but it contains a sulfur atom in the side chain that makes it fairly non-reactive, like true aliphatic amino acids.

In aliphatic amino acids, when the number of carbon atoms on the side chain increases, hydrophobicity increases. Since aliphatic amino acid molecules have an equal charge distribution across the molecule, these compounds do not react strongly in the presence of other molecules because there is no distinct positive or negative charge.

What is Aromatic Amino Acid?

Aromatic amino acids are amino acids consisting of aromatic side-chain functional groups. These amino acids include phenylalanine, tyrosine, and tryptophan. These compounds have no to little charge, and they vary between hydrophobic and not hydrophobic nature, e.g. phenylalanine and tyrosine, respectively.

Aromatic amino acids refer to the attachment of a highly stable aromatic ring that does not easily react with other compounds or chemical elements. Except for aryl compounds, aromatic compounds can be found in the human body. E.g. each and every nucleotide in our DNA and RNA contain aromatic molecules.

Sometimes, histidine amino acid is incorrectly grouped in aromatic amino acid groups. The amino groups of this compound can be aromatic-like but are reactive with a weak positive charge and hydrophilic characteristics.

What is the Difference Between Aliphatic and Aromatic Amino Acids?

Aliphatic and aromatic amino acids are biochemical compounds having the basic amino acid functional groups with some important side chains. The key difference between aliphatic and aromatic amino acids is that aliphatic amino acids have no cyclic structures with alternating double-bond characteristics, whereas aromatic amino acids have cyclic structures with alternating double-bond characteristics. Alanine, isoleucine, leucine, proline, and valine are some examples of aliphatic amino acids, whereas tyrosine, phenylalanine, and tryptophan are some examples of aromatic amino acids.

Below is a summary of the difference between aliphatic and aromatic amino acids in tabular form for side-by-side comparison.

Summary – Aliphatic vs Aromatic Amino Acids

Aliphatic amino acids are amino acids consisting of aliphatic side chains functional groups, while aromatic amino acids are amino acids consisting of aromatic side-chain functional groups. The key difference between aliphatic and aromatic amino acids is that aliphatic amino acids have no cyclic structures with alternating double-bond characteristics, whereas aromatic amino acids have cyclic structures with alternating double-bond characteristics.

Reference:

1. Helmenstine, Anne Marie. “Aliphatic Amino Acid Definition.” ThoughtCo, Aug. 26, 2020.

Image Courtesy:

1. “Valine (amino acid)” (CC0) via Free SVG
2. “Aromatic amino acid side-chains” By Athel Cornish-Bowden – Own work (CC BY-SA 4.0) via Commons Wikimedia