Video Transcript
Which of the following proteins is not an example of a structural protein? (A) Collagen: a fiber found in ligaments and skin. (B) Actin: a filament found in muscle fibers. (C) Keratin: a fiber found in hair and nails. Or (D) glucagon: a hormone secreted from the pancreas.
The question asks us about structural proteins. Let’s have a quick review of proteins in general before looking at some specific
examples of structural proteins.
Amino acids are monomers that bind together to form long chains called
polypeptides. Based on chemical interactions between the amino acids, polypeptides will fold in
specific ways to form proteins. The resulting three-dimensional shape of the protein determines its function.
We can divide proteins into two major functional categories: structural and
regulatory. As their name implies, regulatory proteins regulate a vast array of processes in the
body. Examples of regulatory proteins include enzymes, antibodies, and transcription
factors. Structural proteins are the building materials of the body. They often have repeating units and shapes that make them ideal for forming fibers
and providing mechanical stability to cells and tissues.
Now, let’s go through the answer choices to see which one is not a structural
protein.
Collagen is the most abundant protein found in mammals. There are actually 28 kinds of collagen, which play a role in mechanical support and
the organization of tissues. It is the main component in connective tissues, such as ligaments, tendons, bones,
and skin. These are definitely roles associated with structural proteins. So we can rule out collage.
Actin is a small, soluble protein that forms polymers in the body. The result is long, stiff filaments that are an important component of the
cytoskeleton and muscle tissue. Actin allows cells to maintain their shape and size, which is a function of
structural proteins. So we can rule it out as well.
Keratin refers to a group of proteins that, like actin, are critical components of
the cell’s cytoskeleton. Keratin is also found in our hair, nails, and skin and helps protect certain types of
epithelial cells from mechanical damage or stress. All of these fit our description of the functions of structural proteins. So we can go ahead and eliminate keratin.
Glucagon is a protein hormone which is secreted from the pancreas when blood glucose
levels are too low. It stimulates liver and muscle cells to release stored glucose and fatty acids,
thereby regulating the blood glucose level. This corresponds to our description of a regulatory protein. So we’ve identified the correct answer. Glucagon is not an example of a structural protein.
