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Amino Acid Side Chains: Function & Examples

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  • 0:03 The Structure of Amino Acids
  • 2:00 Nonpolar Amino Acid Chains
  • 3:51 Polar Amino Acid Side Chains
  • 5:02 Basic Amino Acid Side Chains
  • 5:37 Acidic Amino Acid Side Chains
  • 6:00 Lesson Summary
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Instructor: Christie Spadafora

Christie has a B.S. and an M.S. in Biology. She teaches life and chemical science courses at college, high school, and middle school levels in MA.

Amino acids are the building blocks of proteins and vary based on their side chains. This lesson will discuss the side chains of various amino acids and how they are biochemically important.

The Structure of Amino Acids

It would be pretty hard to take a count of all of the English words you know. The English language has over one million words, but they are all made using the same 26 letters. Organisms use a similar mechanism to make all of their proteins. Although there are numerous existing amino acids, most organisms rely on the same 20 amino acids, strung together in different orders, to make thousands of different proteins. Although each amino acid is different, they are always composed of five parts.

Each amino acid has one carbon atom in the center, called the central carbon atom. A carbon atom is capable of making four covalent bonds, and each central carbon atom of an amino acid has the other four parts of the amino acid bound to it.

Each central carbon atom always has a one hydrogen atom, one carboxylic acid, and one amino group bound to it. In fact, the groups carboxylic acid and amino are where the term amino acid came from.

This leaves one additional bond around the central carbon. Each amino acid is bound to a unique chemical group at this position called its side chain. It is this side chain that makes each amino acid different, giving each amino acid a unique set of chemical properties. The side chain is often abbreviated as an R group and denoted with the letter R for short.

Side Chain Options

As each side chain is unique, each amino acid is unique. It is the biochemical properties of each side chain, such as the 3D shape, polarity, or the charge of the chain, that, when strung together in different orders, allows for an essentially infinite number of different proteins and functions to be created. Often times, amino acids are grouped based on the biochemistry of their side chains which can be nonpolar, polar, basic, and acidic. Let's take a closer look at each of these four groups.

Nonpolar Amino Acid Chains

When electrons are shared evenly among atoms bound together, these bonds are nonpolar. Nonpolar molecules don't like water (which is a polar molecule), so you'll find a lot of nonpolar bonds in things like oil, gasoline, and butter. Nine of the 20 common amino acids have nonpolar side chains. Let's look at the first five:

These five amino acids have simple, nonpolar side chains made of nothing but hydrogen and carbon. Notice that leucine and isoleucine differ only by the placement of one methyl group.

The other four nonpolar amino acids have more complex side chains, as shown here:

Amino Acids

Phenylalanine (the amino acid alanine, plus a phenyl group) and tryptophan are two of the only three aromatic (or benzene-containing) amino acids. These rings absorb ultraviolet light with a wavelength of 280 nanometers (nm). Because of these rings, a quick scan under UV light at 280 nm can give a biochemist a decent estimate of protein concentration. Notice that proline has a special ring - its side chain connects to the amino acid's amino group to form the ring. Because of this special bend, proline causes a kink whenever present within a protein, allowing proteins to fold in special 3D ways. Although it does not contain a ring, methionine is also special, as it is one of only two amino acids that contain sulfur.

Amino acids with nonpolar side chains are used to build parts of proteins that won't come into contact with water. For example, some proteins pass across the cell membrane, meaning that much of the protein is embedded within the nonpolar part of the cell membrane. These proteins are made of many nonpolar amino acids.

Polar Amino Acid Side Chains

Sometimes, atoms within a bond do not share electrons equally. These bonds are called polar, and six amino acids have polar side chains. In all but one of these side chains, oxygen is the element that is hogging the electrons, causing the polarity.

Most of the polar amino acid side chains are fairly straightforward but contain an electronegative (electron-loving) element like oxygen that causes polarity. However, there are two polar amino acids worth noting. Cysteine, like methionine, contains sulfur. -SH groups at the end of two cysteine side chains are able to bind together, forming a disulfide bridge. These covalent bonds allow special protein structures to form, and are responsible for unique shapes, like curly hair found on some humans. Tyrosine is also worth noting, as it has the third and final aromatic side chain.

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