Proteins perform a vast array of functions within living systems, from building structures to catalyzing reactions. These complex molecules are assembled from smaller, repeating units, much like Lego bricks combine to form an intricate model. In proteins, these fundamental building blocks are known as amino acids, serving as the protein monomers. Each amino acid links together to create long chains, which then fold into three-dimensional structures that dictate their roles in the body.
The Structure of an Amino Acid
Each amino acid shares a common structure, centered around an alpha (α) carbon atom. This central carbon is bonded to four different groups. One attachment is an amino group (-NH2). On the opposite side, the alpha carbon connects to a carboxyl group (-COOH).
A single hydrogen atom also attaches to the central carbon, completing three of its four bonds. The fourth bond is occupied by a component called the “R-group” or “side chain”. This R-group is the distinguishing feature among the 20 common amino acids found in proteins, varying greatly in chemical composition, size, and electrical charge. Some R-groups are simple, like glycine’s single hydrogen atom, while others are larger and more complex, such as tryptophan’s ring structure. The diverse properties of these R-groups enable proteins to carry out their biological tasks.
Building a Polypeptide Chain
Amino acids connect to one another through a chemical linkage called a peptide bond. This bond arises from a reaction between the carboxyl group of one amino acid and the amino group of an adjacent amino acid. During this process, a hydroxyl group (-OH) is removed from the carboxyl end of one amino acid, and a hydrogen atom (H) is removed from the amino group of the other.
The removed hydrogen and hydroxyl atoms combine to form a molecule of water (H2O). This type of reaction, where a water molecule is removed to create a larger molecule, is known as dehydration synthesis or a condensation reaction. As this process repeats, amino acids are added, forming a continuous linear chain. The resulting polymer, a long string of amino acids linked by peptide bonds, is called a polypeptide.
How Monomer Sequence Determines Function
The specific order in which amino acids are arranged within a polypeptide chain acts as a blueprint for the protein’s final three-dimensional shape. This linear arrangement of amino acids is referred to as the protein’s primary structure. The properties of each amino acid’s R-group, such as whether it is hydrophobic (water-fearing) or hydrophilic (water-loving), dictate how the polypeptide chain will fold.
Hydrophobic R-groups tend to cluster together in the protein’s interior, away from water, while hydrophilic R-groups orient towards the exterior. These interactions, alongside others like hydrogen bonds and electrostatic forces, guide the polypeptide to twist and bend into a precise and stable three-dimensional conformation. This intricate folding results in the protein’s tertiary structure, the shape required for its biological function. A change in just one amino acid in the sequence can sometimes lead to a misfolded protein, potentially impairing its function.
Essential and Non-Essential Monomers
Amino acids are categorized based on whether the human body can produce them. Essential amino acids are the nine amino acids that the human body cannot synthesize on its own in sufficient quantities. Therefore, these must be acquired from dietary sources. Examples include histidine, isoleucine, and leucine.
In contrast, non-essential amino acids are those that the body can synthesize internally from other compounds. The body’s ability to produce these means they do not need to be obtained through food. Examples include alanine and asparagine. To ensure the body has all the necessary building blocks to construct its diverse proteins, consuming a varied diet that includes a range of protein sources is recommended.