In this tutorial, you will learn what proteins are and how they are useful to life. You will also learn about the building block of all proteins, the amino acid.
Proteins are Essential
From small, single celled organisms like Escherichia coli to large and complex organisms like human, proteins are an essential building block of life. For example the protein, collagen, makes up a major part of our bones, skin, muscles, tendons, and cartilage. In each of our cells, proteins help carry out the necessary functions of life, such as DNA replication, and catalyzing important metabolic reactions.
Amino Acids Make up Proteins
Proteins are incredibly diverse both their shapes (structure) and what they do (function). For example, the image below shows a part of protein involved in blood pressure regulation, ACE2 protease, and the second image is a protein that regulates how we grow, FGF23. The second protein is much smaller, and has a different shape altogether.
So, what do they all have in common? These two proteins, like all proteins, consist of the same building blocks, amino acids, of which 20 are “canonical.” You can learn about all 20 with this useful chart. The human body regularly synthesizes these amino acids. Selenocysteine, a rarer one, also exists in the human body, but does not qualify as a canonical amino acid. These amino acids string together in unique combinations to create unique molecules referred to as polypeptide chains.
The Amino Acid
While the “beads” in the figure above represent amino acids as they exist in a polypeptide chain, a more accurate representation is given below. Each amino acid resembles the molecule shown below, with a carboxyl group (O=C-OH), an amine (-NH2), and an R group. The R group, or side chain, is special in that it can represent a number of different structures, and each amino acid contains a different R group. In the section below, “The Amino Acids,” look for each amino acid and see if you can find the carboxyl, amine, and R (variable) group.
The Amino Acids
These amino acids all have differing properties, such as charge and polarity, which ultimately contribute to the characteristics of the protein they make up. They do this through forming hydrogen or ionic bonds. These interactions can be broken down in certain conditions, resulting in the collapse of the protein structure, or denaturation.
Here is a complete list of the canonical amino acids (plus selenocysteine), their structures, and their properties. Along with their names, for example “Arginine,” amino acids are given a three letter code, like “Arg,” and a one letter representation, like “R.” Although these are the most common, researchers also synthesize other amino acids to build polypeptides with unique properties.
How are They Held Together?
Amino acids connect to one another by special covalent bonds, called peptide bonds. These bonds form when a carboxyl group of one amino acid (O=C-OH) reacts with an amine group (-NH2) of another. This reaction is called a dehydration reaction, because water is released in the process. The entire process is shown below.
Inversely, the breaking apart of a peptide bond is called hydrolysis, because water is added in the process that breaks the two amino acids apart.