Lectins - Online Tutor, Practice Problems & Exam Prep

Lectins are specific proteins that bind carbohydrates with high affinity, facilitating various biological functions such as cell adhesion and recognition. They interact non-covalently with carbohydrates, forming reversible complexes that translate the sugar code into cellular actions. Two classes of plasma membrane lectins are integrins, which mediate signals between cells and the extracellular matrix, and selectins, which enable cell-to-cell recognition. Understanding these interactions is crucial for grasping cellular communication and the role of glycoproteins in biological processes.

Lectins

In this video, we're going to begin our lesson on lectins. Now, before we actually define lectins, it's first helpful for us to understand what the sugar code is referring to. And so the sugar code is just this idea that cells can use carbohydrates or sugars for that matter in order to encode biological information that could lead to completely different cellular events. And so the letters of the sugar code are just going to be monosaccharides, and the words of the sugar code, if you will, are going to be oligosaccharides.

And so if we take a look at our image down below, we can get a better idea of what this sugar coat is. And so notice over here on the far left of our image, we have these hexagons that represent monosaccharides. And so the monosaccharides are the letters of the sugar coat. Now, notice that we have 2 blue monosaccharides and 2 red monosaccharides and these are separate, from each other here as letters. However, when we go to combine these monosaccharides to create the oligosaccharides over here, notice that we can create these words if you will. And, notice that we can actually connect these monosaccharides in different ways. We can connect them so that the 2 blue ones come first and the 2 red ones come second. Or we could connect them in a different way where the blue ones are alternating with the red ones. And so, these would create different oligosaccharides and therefore create different words. And the words can have different meanings that again lead to different biological events. And so, you can see here that through protein binding or essentially taking these oligosaccharides and linking them to proteins to create glycoproteins, over here, notice that these purple balls here represent, a protein. And so we've got these glycoproteins, and notice, the one at the top has, the blue-blue-red-red pattern, whereas the one on the bottom has the alternating blue-red-blue-red pattern. And again, these are different words so they can have different meanings and that can lead to different biological events. So perhaps, this one at the top here leads to an extended life of the protein. So a long long life for this protein. Whereas the one on the bottom perhaps means that it has a short life. And so this, of course, can have completely different can lead to completely different biological events. And so, that's important to keep in mind about the sugar coat.

And so, really the main idea here is that the sugar code is this idea that cells can use carbohydrates to encode biological information. And so now that we have a better understanding of the sugar code, in our next video, we'll be able to talk more about lectin. So I'll see you guys there.

Lectins

So now that we've introduced the sugar code in our last lesson video, in this video, we're going to talk about lectins. Lectins are proteins themselves that bind to carbohydrates with very, very high affinity and very, very high specificity, which means that lectins won't bind to any carbohydrate; they bind to very specific carbohydrates, and they do so in order to perform a wide variety of biological functions. That includes bio signaling, cell adhesion, cell recognition, intracellular localization, and much more. Really, when it comes down to it, lectin carbohydrate interactions are just like protein ligand interactions. In fact, lectin carbohydrate interactions are protein ligand interactions. The protein is, of course, the lectin, and the carbohydrate is going to be the ligand. This means that the lectin carbohydrate interactions are going to be non-covalent interactions that are reversible, just like protein ligand interactions are also non-covalent and reversible.

If we take a look at our image down below, notice that this pink ball that we see over here is representing the lectin, which is the carbohydrate-binding protein. Over here on the right, what we have is some carbohydrate that's representing the ligand. Of course, the lectin is going to bind to the ligand, the carbohydrate, and create a lectin carbohydrate complex, which is what we see right here. Notice that these black dotted lines that we see are representing non-covalent interactions. This reminds us that this is actually a reversible process, and that's why we have these equilibrium arrows here because the lectin-carbohydrate complex can also dissociate backward to reform the free carbohydrate and the free lectin. When you think about it, these lectins, these carbohydrate-binding proteins, are really the translators of the sugar code. The carbohydrates are encoding the information, and the lectins bind to these specific carbohydrates to translate that encoded information into biological action or cellular events.

Really, this is the introduction for lectins. In our next couple of videos, we'll be able to apply the concepts that we've learned. Then later, we'll talk about two very specific types of lectins that you guys should know. I'll see you guys in our next video.

The biochemical property of lectins that is the basis for most of their biological effects is their ability to bind to:

• A. Amphipathic molecules.
• B. Hydrophobic molecules.
• C. Specific lipids.
• D. Specific oligosaccharides.
• E. Specific peptides.
• F. Specific monosaccharides.

Lectins are _________________________ while glycoproteins are _________________________.

• A. Lipids ; neurotransmitters.
• B. Proteins that covalently bind to specific sugars ; proteins that are non-covalently bound to sugar residues.
• C. Storage carbohydrates ; enzyme specific hydrolysis of storage carbohydrates.
• D. Structural carbohydrates ; enzymes specific for hydrolysis of structural carbohydrates.
• E. Proteins that noncovalently bind to specific sugars ; proteins that are covalently bound to sugar residues

Lectins

In this video, we're going to introduce 2 different classes of plasma membrane embedded lectins, and these two different classes are the integrins and the selectins. Now, integrins are going to be plasma membrane embedded lectins that are going to mediate signals between the cell whose plasma membrane they're embedded in and the extracellular matrix. And so, recall from your previous biology courses that the extracellular matrix can be abbreviated as the ECM, and all it really is is just a network of molecules on the outside of cells, and they can provide structural or biochemical support. And so, really, integrins are just going to be plasma membrane embedded lectins that are mediating signals between the cell and the extracellular matrix. Now, selectins, on the other hand, are going to also be plasma membrane lectins, but they're going to mediate cell to cell recognition and adhesion, and so they are not going to interact with the extracellular matrix. And so if we take a look at our image down below over here on the left-hand side, notice that these gray circles and lines here represent the cell's plasma membrane. And notice that going down with the yellow background, we have the inside of the cell. And going up with the pink background, we have the outside of the cell. And notice on the outside of the cell, we have the ECM, the extracellular matrix, which can consist of carbohydrate molecules. So you can see these carbohydrates extending off of it. And notice embedded in our plasma membrane here, we have this structure. And so, this is representing the integrin. And so, what you'll note is that this particular integrin, notice, has 2 subunits. It's got the purple one in the back, and it's got this red one here in the front. And so it's possible for integrins to have quaternary protein structure, but notice that this integrin is embedded in the plasma membrane, making it a plasma membrane lectin, and it's also mediating a signal between the cell whose plasma membrane it's embedded in and the extracellular matrix, so it's interacting with the extracellular matrix, And really that's what makes it an integrin. Now, over here on the right-hand side, notice we have a very similar image. We have the plasma membrane here. We've got the inside of the cell with the yellow background and the outside of the cell with the green background and notice that embedded in the plasma membrane, we have this green structure right here that is representing a selectin and the reason that this is a selectin is because notice that it's mediating cell to cell recognition. And over here in orange, what we have is a neighboring cell. And notice that the neighboring cell can have structures that come off of it that contain carbohydrates on them as well. And so the selectins are going to, of course, be lectins so they are carbohydrate binding proteins that will interact with these carbohydrates and allow communication between neighboring cells. And so really this is the main difference here between these two classes of lectins, the integrins and selectins, and as we move forward in our course, we'll be able to get some practice utilizing the concepts that we've learned. So I'll see you guys in our next video.

Which of the following statements regarding integrins is FALSE?

• A. Integrins have quaternary protein structure.
• B. Integrins are transmembrane protein but most of their functional parts are extracellular.
• C. Integrins form tight junctions between cells.
• D. The same integrins can have different affinities to ligands depending upon its conformation.

Which of the following types of interactions is not used by lectins to recognize specific glycans?

• A. Hydrogen bonds.
• B. Salt bridges.
• C. Van der Waals contacts.
• D. Disulfide bonds.

Here’s what students ask on this topic:

What are lectins and what role do they play in biological processes?

Lectins are specific proteins that bind to carbohydrates with high affinity and specificity. They play crucial roles in various biological processes, including cell adhesion, cell recognition, bio-signaling, and intracellular localization. By binding to specific carbohydrates, lectins translate the encoded information in the sugar code into cellular actions. This interaction is non-covalent and reversible, similar to protein-ligand interactions. Understanding lectins is essential for grasping cellular communication and the role of glycoproteins in biological processes.

How do lectins interact with carbohydrates?

Lectins interact with carbohydrates through non-covalent, reversible interactions, similar to protein-ligand interactions. The lectin (protein) binds to a specific carbohydrate (ligand) to form a lectin-carbohydrate complex. This binding is highly specific and occurs with high affinity. The non-covalent nature of the interaction allows the complex to dissociate, making the process reversible. These interactions enable lectins to translate the sugar code into various biological actions, such as cell adhesion and recognition.

What is the difference between integrins and selectins?

Integrins and selectins are two classes of plasma membrane-embedded lectins. Integrins mediate signals between the cell and the extracellular matrix (ECM), which provides structural and biochemical support outside the cell. They are involved in cell-ECM interactions. Selectins, on the other hand, mediate cell-to-cell recognition and adhesion. They do not interact with the ECM but facilitate communication between neighboring cells by binding to carbohydrates on the cell surface. Both play essential roles in cellular communication and adhesion.

What is the sugar code and how is it related to lectins?

The sugar code refers to the concept that cells use carbohydrates to encode biological information, leading to different cellular events. Monosaccharides are the letters of the sugar code, and oligosaccharides are the words. Lectins are proteins that bind to these specific carbohydrates with high affinity and specificity, translating the encoded information into biological actions. By interacting with carbohydrates, lectins play a crucial role in interpreting the sugar code and facilitating various cellular processes.

What are some biological functions of lectins?

Lectins perform a wide variety of biological functions, including bio-signaling, cell adhesion, cell recognition, and intracellular localization. They bind to specific carbohydrates with high affinity and specificity, translating the sugar code into cellular actions. This interaction is crucial for processes such as immune response, inflammation, and tissue repair. Understanding the role of lectins in these functions is essential for comprehending cellular communication and the role of glycoproteins in biological processes.