Ferritin is a protein that stores iron in the body and releases it in a controlled manner. A molecule of ferritin is composed of protein and iron. Which of the following types of proteins is ferritin? (A) Conjugated protein, (B) simple protein, (C) structural protein, or (D) fibrous protein.
This question asks us about a specific protein called ferritin and the type of protein it is. So let’s review some different categories of protein structure before returning to our question.
A protein is a complex biological macromolecule made of amino acid monomers that can have a wide variety of forms and functions. The roles they carry out within the body include transport, movement, structure, signaling, and protection. To be able to carry out these functions, the proteins have a range of specific structures. Let’s have a look at these structures before we go back to the question.
Firstly, proteins can be divided into fibrous and globular categories, which describe their 3D shape.
Fibrous proteins are made up of long polypeptide chains that run parallel to one another. They are insoluble in water and have a stable structure, which makes them ideal for their roles in the body: structure and movement. Examples include keratin in your hair and nails and collagen in your tendons.
Globular proteins are more spherical in shape, which means they form a rough ball shape, instead of one that is long and straight. These proteins are usually soluble in water and have a less stable structure. They are usually responsible for metabolic functions. Enzymes are globular proteins, as are antibodies, some hormones, and hemoglobin in your red blood cells, which we will come back to shortly.
Globular and fibrous proteins can also be divided into simple and conjugated proteins.
Conjugated means joined together. And conjugated proteins are proteins which are joined to another component, which is not made of protein. These nonprotein components are known as prosthetic groups. The word “prosthetic” comes from the Greek for addition. An example of a conjugated globular protein is hemoglobin, which, as we’ve already mentioned, is found in red blood cells. This is made up of four polypeptide chains, each one with a heme group containing iron attached. Its specialized structure allows it to bind to and carry oxygen all around the body.
So if conjugated proteins are joined to another component, what about simple proteins? Well, “simple” means plain or basic. So a simple protein has no additional component, in other words no prosthetic group attached. An example of a simple globular protein is albumin, found in blood plasma, and a simple fibrous protein is collagen.
Now that we have reviewed what is meant by fibrous, globular, simple, and conjugated proteins, let’s return to our question. The most important part of the information we are given says that ferritin is composed of protein and iron. Iron is not made up of amino acids so is a nonprotein component and is therefore a prosthetic group. This means that the correct answer to the question is option (A). The group of proteins that ferritin belongs to is conjugated proteins.