Amino Acid Side Chains: Different Types and Functions

You might be familiar with amino acid, but do you know what are amino acid side chains? Amino acids with side chains are used to build proteins, allowing for the expression of certain chemical characteristics.

What are amino acid side chains?

Small molecules known as amino acids serve as the foundation for proteins. Proteins carry out numerous crucial chemical reactions as well as provide structural support for the cell. Twenty different smaller, simpler amino acid combinations make up the molecules that make up each protein. Long sequences of amino acids called protein molecules are folded into a three-dimensional structure.

Chemically speaking, an amino acid is a molecule made up of a carbon atom called the carbon and two groups of amino acids: a carboxylic acid group and an amine group. The R group, a particular side chain, is also joined to the carbon in each of the 20 amino acids. Shapes, sizes, charges, and reactivities of the R groups vary. As a result, amino acids can be categorized based on the characteristics of their side chains.

Serine, threonine, and asparagine are a few amino acids that have polar side chains that are soluble in water. Isoleucine, phenylalanine, and valine are examples of hydrophobic amino acids, which avoid contact with water. The side chain of the amino acid cysteine is chemically reactive and capable of forming bonds with other cysteines.

Additionally, amino acids can be acidic or basic, such as glutamic acid or lysine. Each protein can fold into a certain three-dimensional structure and carry out biological tasks thanks to interactions between its side chains and the amino acid sequence.

Why are amino acid side chains so important?

Only twenty different amino acids, each with a distinct side chain, make up proteins. Different chemistries can be found in the side chains of amino acids. The majority of amino acids have side chains that are nonpolar. While some amino acids have polar but uncharged side chains, others have side chains with positive or negative charges.

Because amino acid side chains can form bonds with one another to retain a length of protein in a certain shape or conformation, their chemistry is essential to protein structure. Ionic connections can develop between charged amino acid side chains, and hydrogen bonds can form between polar amino acids. Weak van der Waals interactions mediate interactions between hydrophobic side chains.

These side chains create a large number of noncovalent bonds. Actually, due to their unique side chains, cysteines are the only amino acids that can create covalent connections. Because of side chain interactions, a protein's amino acid sequence and position determine where its bends and folds take place.

Side chain options

Each amino acid is unique because each side chain is unique. The biological characteristics of each side chain, such as its polarity, charge, or 3D structure, when linked together in various orders, enable the creation of an almost unlimited variety of unique proteins and activities. Amino acids are frequently divided into groups according to the biochemistry of their side chains, which can be as follows:

Non-polar and neutral

A chemical is non-polar if it is uncharged. This condition develops either because the positively and negatively charged atoms are equally distributed and cancel each other out, or because the atoms have no charge at all. Non-polar amino acids are those that have side chains made of benzene rings or uncharged hydrocarbon groups. Neutral side chains don't have any acidic or basic characteristics. The majority of amino acids are neutral, however alanine, glycine, isoleucine, leucine, methionine, phenylalanine, proline, and valine are non-polar and neutral amino acids.

Polar and neutral

Polar amino acids, on the other hand, have side chains that are either net positively charged or net negatively charged. Depending on the particular chemistry of the side chain, the degree of polarity varies. For instance, because its side chain is shorter and lacks a ring structure, serine is more polar than either threonine or tyrosine. This is because serine is less non-polar than the other two amino acids. Asparagine, cysteine, glutamine, and tryptophan are polar and chemically neutral amino acids in addition to serine, threonine, and tyrosine.

Polar and acidic

Since each amino acid has a basic amino group and an acidic carboxyl group, all amino acids actually exhibit acid-base characteristics. From an acid-base perspective, however, these groups balance each other out, therefore the amino acid's side chain is what ultimately determines whether it is basic or acidic. Acidic amino acids have a side chain that behaves like an acid and can ionize by giving an extra hydrogen atom to the environment. Aspartic acid and glutamic acid are the only two acidic amino acids, and both of these molecules are polar.

Polar and basic

The final three amino acids are polar and basic: arginine, histidine, and lysine. Beyond what is present in the amino acid's primary structure, each of its side chains consists of an extra amino group. As a result, they act as fundamental molecules that can take an atom of hydrogen from their surroundings. Since they are more water soluble due to their polarity than the eight non-polar amino acids, they and other charged amino acids are often found on the outside of proteins in aqueous solutions.