Fibrous and Globular Proteins: Videos & Practice Problems

Proteins are categorized into fibrous and globular proteins. Fibrous proteins, such as alpha keratin, collagen, and silk fibroin, are insoluble and have simple linear structures, primarily serving structural roles. In contrast, globular proteins are soluble, featuring complex tertiary structures with various secondary structures like alpha helices and beta sheets. They function mainly as enzymes and regulatory proteins. Understanding these distinctions is crucial for grasping protein functionality and structure in biological systems.

Fibrous and Globular Proteins

In this video, we're going to begin our discussion on the 2 broadest and most important structural groupings of all proteins, the fibrous and globular proteins. So in this video, we're going to focus on the fibrous proteins. Fibrous proteins are relatively insoluble proteins, which means they do not dissolve well in water. Fibrous proteins are arranged in simple, long, linear strands or sheets. Usually, fibrous proteins only contain one type of secondary structure. You'll see that they usually only contain either alpha helices or beta sheets, but they don't have a mixture of the two. Because they only contain one type of secondary structure, they typically have a simple tertiary structure. Fibrous proteins function mostly as structural proteins, making up the structures of a lot of different things. Fibrous proteins provide support, shape, and external protection. In our example below, we're going to talk about 3 examples of fibrous proteins: alpha keratin, collagen, and silk fibroin.

Over here on the far left, what we have is our first example, alpha keratin. Alpha keratin is a mammalian protein that provides support and external protection, and it's found in our hair, our skin, and our fingernails. Alpha keratin is predominantly helix structure. We have our alpha helix here, and with the alpha keratin structure, what happens is two alpha helices, two separate polypeptide chains, coil up on each other to create a coiled coil, which is a type of motif. Multiple coiled coils come together to form a protofilament, and then multiple protofilaments come together to form an alpha keratin filament. An alpha keratin filament is predominantly made up of alpha helices, which is just one type of secondary structure. Alpha keratin has this long linear type of structure.

Now, our second type of fibrous protein is collagen. Like alpha keratin, collagen is used to provide support and is found in connective tissues such as cartilage, tendons, and bones. Collagen has these three separate chains intertwined on one another, so they have some turns to be able to create this turning structure. The polypeptide chains of collagen are different from alpha keratin because they are not alpha helices. These chains again all have some turns to be able to intertwine the way that they do. Collagen has a long linear type of structure, so it doesn't have so many turns that create a small spherical globular type of shape like what we'll see when we talk about globular proteins.

For our last type of fiber structure, what we have is silk fibroin. Silk fibroin is a protein produced by insects such as spiders, and it's found in spider webs. Professors like to use silk fibroin as a classic example of beta sheets. We've got these sheets here, these layers of antiparallel beta sheets that are all hydrogen-bonded together to create the silk fibroin structure.

In our next video, we're going to be able to get a little bit of practice on fibrous proteins, and then we'll talk about the globular proteins. So I'll see you guys in those videos.

Why is collagen insoluble?

• A. The polar side chains of its residues highly interact with water.
• B. Its hydrophobic residues exposed on its surface do not fold away into a core in its linear structure.
• C. Its long, linear structure increases its surface area, minimizing residue contact with water.
• D. The lack of multiple secondary structures indirectly correlates with its lack of hydrophobic residues.

Answer: (B)

Fibrous and Globular Proteins

So now that we know about fibers proteins, we can talk about our second broad group of structural proteins, the globular proteins, and we'll be able to compare and contrast fibers and globular proteins. Unlike fibrous proteins, globular proteins are actually soluble, which means that they do dissolve well in water. Unlike fibrous proteins, which often have simple tertiary structures because they normally have only one type of secondary structure, globular proteins often contain complex tertiary structures because they normally have several types of secondary structures such as alpha helices, beta sheets, beta turns, and loops. Globular proteins function mostly as enzymes as well as regulatory proteins instead of functioning as structural proteins like fibrous proteins do. Interestingly, the vast majority of proteins in nature are actually globular proteins, with only a small percentage being fibrous proteins. Most of the proteins that we will be talking about from here on out throughout the rest of our course will be globular proteins.

In our example below, we have a globular protein structure. What you can see is that it has a lot of turns and loops, alpha helices, and a spherical or compact shape, which is the most important thing about globular proteins; they do not have a very long or extended shape but rather a spherical, compact, and globular shape. In our next video, we'll be able to get some practice. I'll see you guys there.

Which of the following statements concerning protein structure is true?

• A. All globular proteins, including myoglobin and its 8 α-helices, have quaternary structure.
• B. Disulfide bond formation in fibrous proteins is incredibly rare due to the lack of a compact fold.
• C. Enzyme structures tend to have significantly more beta turns than silk fibroin or α-keratin.
• D. Hemoglobin is more soluble than collagen as a result of having a lower variety of motifs.

Answer: (C)

Here’s what students ask on this topic:

What are the main differences between fibrous and globular proteins?

Fibrous proteins are insoluble in water and have simple, linear structures, often containing only one type of secondary structure like alpha helices or beta sheets. They primarily serve structural roles, providing support, shape, and protection. Examples include alpha keratin, collagen, and silk fibroin. In contrast, globular proteins are soluble in water and have complex tertiary structures composed of multiple secondary structures such as alpha helices, beta sheets, beta turns, and loops. They function mainly as enzymes and regulatory proteins. Most proteins in nature are globular, making them more prevalent than fibrous proteins.

What are some examples of fibrous proteins and their functions?

Examples of fibrous proteins include alpha keratin, collagen, and silk fibroin. Alpha keratin is found in hair, skin, and nails, providing support and external protection. Collagen is present in connective tissues like cartilage, tendons, and bones, offering structural support. Silk fibroin, produced by insects such as spiders, is found in spider webs and is known for its strength and flexibility. These proteins are characterized by their insolubility in water and simple, linear structures, which enable them to fulfill their structural roles effectively.

Why are globular proteins more soluble in water compared to fibrous proteins?

Globular proteins are more soluble in water because of their compact, spherical shape and the presence of hydrophilic (water-attracting) amino acid residues on their surface. These hydrophilic residues interact favorably with water molecules, allowing the protein to dissolve. In contrast, fibrous proteins have a more extended, linear structure with hydrophobic (water-repelling) residues, making them insoluble in water. The solubility of globular proteins is crucial for their roles as enzymes and regulatory proteins, which often require them to be in an aqueous environment.

How do the secondary structures of fibrous and globular proteins differ?

Fibrous proteins typically contain only one type of secondary structure, either alpha helices or beta sheets, leading to a simple, linear arrangement. For example, alpha keratin is predominantly composed of alpha helices, while silk fibroin consists of beta sheets. In contrast, globular proteins have a complex tertiary structure that includes multiple types of secondary structures such as alpha helices, beta sheets, beta turns, and loops. This diversity in secondary structures contributes to the compact, spherical shape of globular proteins, enabling them to perform various functions, including enzymatic and regulatory roles.

What roles do fibrous proteins play in the human body?

Fibrous proteins play crucial structural roles in the human body. Alpha keratin provides support and protection in hair, skin, and nails. Collagen, the most abundant protein in the body, is found in connective tissues such as cartilage, tendons, and bones, where it offers structural integrity and strength. These proteins are characterized by their insolubility in water and simple, linear structures, which make them well-suited for providing support, shape, and external protection. Their structural roles are essential for maintaining the body's overall form and resilience.