Biochemistry Glycoconjugates

Hello and welcome back to Educator.com, and welcome back to Biochemistry.0000

In the last lesson, we talked about polysaccharides.0004

We are going to continue our discussion of polysaccharides, and talk about something called glycoconjugates.0007

These are polysaccharides that are attached to proteins and lipids.0013

Let's just go ahead and jump in, get started.0022

OK, let's make sure we have...let's start off with black here.0026

In addition to their structural and fuel storage roles - actually, I do not think I will use the word "roles", I think I will use the word "capacities" - oligo and polysacchs - excuse me - they also carry information.0031

OK, let's go over here; they also carry information.0087

In other words, in this particular case, they are involved - as we mentioned in the previous lesson - in cell to cell and cell extracellular matrix interactions.0093

In other words, they are involved in recognition.0122

OK, in these cases, the carbohydrate, oligopolysaccharide, the carbohydrate is often joined to a protein or a lipid.0125

Lipid is just a fancy word for fat, and we are going to be discussing lipids in great detail very, very soon.0160

OK, let's talk about these things called proteoglycans.0167

Let me go to blue here, and let me see.0171

Is proteoglycan like a protein?0175

Yes, that is fine; alright, proteoglycan.0177

Our first class of glycoconjugates are these things called proteoglycans.0185

OK, they are cell surface and extracellular.0192

Extracellular means that they are just proteins that are not attached to the cell; they are actually proteins that are floating around in the cytosol.0202

That is all that means- proteins with one or more glycosaminoglycans, one or more Gags attached.0207

OK, now, these things called proteoglycans, they are the carbohydrates, but they are attached to cell surface proteins or they are attached to free proteins, secreted proteins, proteins that have been created and sent outside of the cell to sort of wander around in the matrix or wherever it is that they wander around, proteins with one or more glycosaminoglycans attached.0227

What is important here is the attachment to the protein is an actual glycosaminoglycan, and you remember from the last lesson that the glycosaminoglycan is this particular polysaccharide that has alternating, it is a linear polymer.0254

It is not branched; it is linear, but it alternates A-B, A-B, A-B with a certain collection of monomers, those monomers generally being N-acetylglucosamine or N-acetylgalactosamine and the other monomer being, more often than not, a glucuronic acid or an iduronic acid.0269

Now, other things can show up; it is not a problem, but the majority of the glycosaminoglycans are going to be those.0289

So, when that particular arrangement of polysaccharide is attached to a protein, we call that glycoconjugate; the whole thing, it is called a proteoglycan- that is all this is.0296

The carbohydrate in a proteoglycan tends to dominate the structure, and that is where the biological activity is.0306

The protein just happens to be a point of attachment.0316

OK, another family of glycoconjugate is something called a glycoprotein.0320

I know, it is kind of interesting, isn't it?0328

Glycoprotein, notice, here, proteo is first, glycan is second; here, glyco is first, protein is second.0330

Now, these are proteins with one or more oligosaccharides covalently attached, and they are also covalently attached here.0337

They can be covalently or electrostatically, but here, they are covalently attached.0364

Now, the carbohydrate portion is generally more varied in the sense that there is a greater collection of monomers to choose from, that the oligosaccharide is made of, is more varied and complex than the glycosaminoglycans on the proteoglycan, the glycosaminoglycan chains on the proteoglycans.0370

If you see a protein, and it has some carbohydrate attached, if the carbohydrate attached happens to be a glycosaminoglycan, we are talking about a proteoglycan- the whole thing.0422

If it tends to have some oligosaccharide or polysaccharide attached, but it is a lot more complex and it has branching, that is a glycoprotein.0430

That is the difference between the two; I will actually write that out in just a second.0440

Let me go ahead and just run through the list here of glycoconjugates.0445

We have proteoglycans, glycoproteins, and now, we have something called a glycolipid.0450

Now, this is just lipids in the cell membrane that have oligosaccharides attached- that is it.0461

In this particular case, the carbohydrate, the oligosaccharide, the polysaccharide, is attached to a fat, not a protein - that is it - covalently linked.0486

OK, now, proteoglycan versus glycoprotein.0496

I know this is good.0505

Now, what is the difference, proteoglycan, glycoprotein?0508

Here is the difference.0510

Proteoglycan, it has a linear glycosaminoglycan attached- that is the difference.0512

The attachment to the protein is some linear glycosaminoglycan, and we know that a glycosaminoglycan is a heteropolysaccharide.0524

It is linear, has no branching; and it consists of alternating monomers, A-B, A-B, A-B- whatever those monomers happen to be.0533

OK, linear Gag attached, and, of course, the Gags are repeating disacch units - that is how you tell - whereas a glycoprotein, it has various oligos attached, different monomers, linear, branched, all kinds of crazy things, any arrangement, high binding specificity.0542

Now, we said that these oligosaccharides attached to proteins that are on the cell surface are floating around, they serve recognition purposes.0594

Well, yes, proteoglycans are involved in recognition; glycoproteins are involved in recognition.0606

Proteoglycans are involved in sort of a global recognition - attached here, attached there.0612

Because these tend to be smaller, more complex, they have a higher binding specificity.0620

They bind specific things at specific points, whereas a proteoglycan might have some glycosaminoglycan attached; and maybe, it is sort of attached at 15 or 16 different points, just sort of like, it is attached here, attached here, attached here, sort of like a claw, whereas these glycoproteins are attached in one location.0626

So, these oligosaccharides tend to be much smaller, and they have very high binding specificity- that is the thing.0648

These glycoproteins are the ones that are involved in your body's immune response.0656

The immune cells that your body sends out to attack invaders recognize these glycoproteins on the cell surface of the bacteria and viruses and things like that.0664

OK, let's take a look at a drawing here, and OK.0675

This is a cell surface; a cell is a lipid bilayer, so that is this thing right here in blue.0681

OK, we are not going to worry about that; let's just sort of take a look and see some of these things.0690

Now, notice this particular protein here; this particular protein looks like it has a couple of things attached, so it looks linear.0697

This could be a proteoglycan; I will just say PG.0708

This could be a proteoglycan.0711

This over here, this is a membrane bound protein; part of it is in the membrane.0714

Part of it is inside the cell; part of it is outside the cell.0720

This is inside the cell here; this is outside the cell, and it has this oligosaccharide attached.0723

Notice, this is not a linear oligosaccharide; this is, it branches here.0730

It branches there, and it branches there.0735

This one is a glycoprotein- that is it.0737

That is how you tell; I mean unless you do a particular analysis, there is no way to actually tell, but this pretty much gives it away.0741

If you have some extensive branching, you are pretty much looking at something which is a glycoprotein.0749

It is going to be a very specific binding site.0754

Let's see, over here, there is another in an integral protein.0757

Integral means it is part of the cell membrane.0761

There is a part of the protein that extends inside; there is a part that extends outside, and again, we have a linear.0765

Now, this could be a glycoprotein; it just depends on what the identity of this oligosaccharide is.0770

If it happens to be a glycosaminoglycan, let's say something like chondroitin-4-sulfate, we know that this whole thing, the protein and the oligosaccharide, is a proteoglycan.0775

If it happens to be just some collection of monomers - N-acetylglucosamine, N-acetylgalactosamine, maybe a mannose, maybe a galactose, something like that - then we know we are looking at a glycoprotein.0786

It serves a more specific type of recognition site- that is all it is, and this is what it is going to look like.0800

You see, all of these proteins, they have little oligosaccharides.0807

Your cells are covered in this stuff; bacteria is covered in this stuff.0811

That is how things get recognized; in this particular case, glycoprotein, proteoglycan, I just wanted you to see what this looks like.0815

Now, over here, we have this glycolipid.0825

Here, we have a lipid molecule, some fat; and again, we will be talking about lipids specifically, but here, you have an oligosaccharide that is attached to a fat.0829

It is not attached to a protein, not a proteoglycan, not a glycoprotein, but it is in the third class; it is a glycolipid- that is it.0838

It just means it has a sugar that is attached to some fat- that is it.0846

We will return to this a little bit later in the lesson when we get into a little bit more detail about proteoglycans and glycoproteins.0851

OK, let's talk about proteoglycans first, and let's talk about their common structure.0857

Proteoglycans are alright, proteoglycan common structure, and by common structure in this particular case, what I am going to discuss is the point of attachment to a protein.0869

In this case, we said that this was our proteoglycan, so we are going to be talking about - you know, I should do this in black, this right there - how it is actually attached to the protein.0894

Well, here is how it is attached to the protein; let's go ahead, for our example.0905

I wonder if I should do this on the next page.0912

Yes, that is fine; I should have enough room here.0916

Let's go ahead and actually do a chondroitin-4-sulfate.0917

OK, chondroitin-4-sulfate, it is a glycosaminoglycan; it is linear, and its particular monomers are glucuronic acid and N-acetylgalactosamine, and I am going to write that down in just a second.0930

So, I am not going to draw these structures; I am just going to write their names connected.0949

I have got a GlcA; I have got it connected to a GalNAc- that is the N-acetylgalactosamine - and I am going to write one more GlcA, right?0953

So, we have alternating A-B, A-B, A-B, and this is going to be connected to a galactose, which is going to be connected to a galactose, which is going to be connected to a xylose sugar.0967

Remember, xylose is a 5-carbon sugar; galactose is a 6-carbon sugar, and this happens to be connected to a serine residue, which is part of the polypeptide, which makes up the protein.0980

And, that is the term Gly, Hex, Gly; and, of course, the polypeptide continues on in this direction.0999

This is going to be the N-terminus.1008

Now, when it is attached to a serine residue, what tends to be attached to the serine is another glycine, some other amino acid and some other glycine.1011

In general, this is what we tend to find more often at this point of attachment, and, of course, this polymer goes on that way, the polypeptide, and this is the C-terminus.1023

OK, this tends to be the arrangement.1033

Right there, at the protein's surface, there is some serine, glycine, some other amino acid and glycine, this arrangement, and then attached to the serine, you will have a xylose sugar, a galactose, a galactose, and then, of course, you will have your molecule.1035

Here is your molecule of your chondroitin-4-sulfate.1049

This is our chondroitin-4-sulfate, and this is our trisaccharide linker that links the protein, the amino acid serine, to our glycosaminoglycan, which is the chondroitin-4-sulfate.1058

This is our trisaccharide connector.1080

OK, let's see; all these crazy words floating around.1087

OK, and let's do this one in red.1097

This right here, this glycosidic bond is going to be beta-(1,3).1102

The anomeric carbon of the glucuronic acid is connected to galactose at its no. 3 hydroxy on the no. 3 carbon.1107

This connection right here is a beta-(1,3) glycosidic bond.1118

Now, let's go ahead and draw out serine just so you see what serine looks like as a reminder.1124

We have NCC; serine has a CH₂, and it has that, and, of course, this is H.1132

This is that, and this is going to go on that way.1143

This is going to go on that way; that is serine, just as a reminder of what serine actually looks like, so that is it.1145

This is the point of attachment for a proteoglycan.1153

This is the part that is different.1160

There are going to be different glycosaminoglycans attached through a trisaccharide connector to a serine residue on a proteoglycan.1163

That is what's happening right there.1172

OK, now, let's talk about glycoproteins.1176

You know what, I think I am going to go back to blue; for some reason, I just thought I really, really like blue, and I don't know why, but there it is.1186

Now, again, the oligos are smaller and more complex, smaller and more diverse and complex than the glycosaminoglycans.1200

Now, smaller, you might think "Well, wait a minute, smaller and more complex, that doesn't make sense".1224

It does, it is referring to the branching.1229

Yes, they are smaller; you have fewer of them, but there is more complexity and there is more variation because now, you are not talking about just 2 monomers alternating A-B, A-B, A-B.1232

You are talking about maybe 5 or 6 to choose from, in general, on most of these glycoproteins; and, of course, they can have all kinds of different branching on them.1242

That is what we mean by more complex.1251

Complexity is a measure of quality; length is a measure of quantity.1254

They are smaller but they are more complex; the quality of them is different.1258

Now, OK, the first monosacch is attached to the protein by its anomeric carbon.1263

The no. 1 carbon, well, I won't say the no. 1 carbon because for ketoses, it could be the no. 2 carbon, so we will just call it the anomeric carbon, the one that originally had the carbonyl.1290

Now, the sugar is in the ring structure; now, there is a hydroxy attached to it, so that is the anomeric carbon- the one that was originally the carbonyl by its anomeric carbon.1302

OK, and now, it is attached to its anomeric carbon in 2 ways: through the hydroxy - I will draw it this way - through the hydroxy group on either serine of threonine.1314

We call this O-linked; in other words, it is an O-glycosidic bond - no worries, we will be drawing it out in just a minute - or, it could be attached through the amide nitrogen which is on asparagine, on Asn.1341

This is called N-linked because this is going to be an N-glycosidic bond.1387

So, it is either an O-linked glycoprotein or it is an N-linked glycoprotein.1393

In other words, the oligosaccharide is either attached to a serine threonine residue that is an O-linked glycoprotein, or it is attached to an asparagine residue that is going to be an N-linked.1398

So, this is an N-glycosidic bond.1409

OK, let's go ahead and draw what these things look like; let's go ahead and do an O-linked first.1417

Let me go ahead and do this in black; I think I will do it.1423

Let's use N-acetylglucosamine as our monomer that is attached to a serine residue.1429

Let's go ahead and draw our sugar unit first.1436

That is there, and let's go ahead and make it an alpha; and let's go ahead and make this, yes, NH, COO, CH₃.1439

This is OH; this is OH, and this is CH₂OH.1459

And now, I have got my CH₂; actually, you know what, I am going to do this in 2 colors here.1466

I really want you to see this in 2 colors; I am going to do this second one in, you know what, I will do it in red.1472

O, CH₂, C, this is NH; this is COO, and, of course, the polymer, the protein goes that way.1480

The protein goes that way; this right here is our serine residue.1493

OK, now, the O that is connected to the sugar that comes from the serine that is not from the hydroxy on the original sugar- that is the whole idea.1498

This is the nucleophile; it is going to get rid of that hydroxy, so it definitely comes from this serine.1509

That is important to know; that is why I did it in 2 colors.1513

I hope that is not confusing; let me see, let me write out what this is over here.1517

Let me go back to black; this is the alpha-GlcNAc.1524

Alpha, that is that; the anomeric carbon, this is alpha-1, N-acetylglucosamine, and it is attached to a serine residue.1533

This is O-linked.1542

OK, now, let's do an N-linked structure, so you see what that looks like; and this time, I am going to use the beta-N-acetylglucosamine.1546

Let me go ahead and go back to black here; let me draw my sugar that way.1556

OK, you know what, I am going to draw it a little bit lower here.1565

I need a little bit more room; excuse me.1569

I will go ahead and draw it like that, so that is that; and we said beta.1574

Let me go ahead and put in this first; this is NH.1581

I am always forgetting that H; I don't know why.1584

Well, old habits- they die hard.1587

OH, OH, and we said N-acetylglucosamine, so this is CH₂OH.1591

This is a beta-GlcNAc, right?1600

Yes, and we said, now, we will go ahead and go; we have N.1607

We have COO, CH₂, C, NH, COO.1614

The protein, the polypeptide is this thing right here.1628

This is the asparagine residue; this is the R-group.1632

Well, the whole thing is the asparagine residue; this is the R-group on the asparagine.1635

We have got CO, and there is also, let's go ahead and put an H on here too because there is an H there.1640

this right here is our Asn residue, and this happens to be our beta-1-carbon, and the nitrogen, this is an N-linked, right?1646

So, we have, this is N-linked, and the nitrogen comes from the protein, from the amino acid.1662

OK, so it is N-linked; this is a nucleophile.1674

This is what is nucleophilic; it is what is going to displace the hydroxy.1677

OK, there you go.1681

Now, let's talk about the monomers that commonly show up in these glycoproteins, in these oligosaccharides, monomers that commonly show up in the oligo portion of glycoproteins.1683

I wonder if I am going to have enough room here to write out all of them; yes, it is fine.1725

OK, let's do this in black.1729

Now, it is not exclusively these; these just tend to show up more often than any others.1736

GlcNAc- that is N-acetylglucosamine.1741

GlcNAc- this is N-acetylglucosamine, and we have Man.1750

This is mannose; it is a hexose.1762

We have Gal; that is galactose.1766

It is another hexose.1772

We have Neu5Ac; this is called N-acetylneuraminic acid, otherwise known as sialic acid.1774

It is actually A-sialic acid; it is a class of molecule, but we tend to call it sialic acid.1788

And, no worries, I will be drawing out the structure in just a minute, or you can look in your book- either way.1795

And, we have Fuc, which is fucose; it is just another sugar, and this one usually shows up as the L-isomer, L-fucose, instead of the D-isomer.1805

And, our last one - which I should have left room but that is OK, I will go ahead and write it over here - is GalNAc.1823

This is N-acetylgalactosamine; this is N-acetylgalactose.1832

Wooh, I mean this is exhausting.1842

You can see why biochemistry is drilled with acronyms - Gag, Gal, Man, PG, GP - all over the place, simply because we can't write everything out.1845

OK, let's go ahead and draw out a couple of the structures.1858

We have talked about most of these, but I am going to go ahead and draw out the sialic acid, the N-acetylneuraminic; and I am going to go ahead and draw out the L-fucose just so you see what the structures look like, just for the heck of it.1865

OK, let's go ahead and do this in red.1878

Actually, no, let's go ahead and do this in blue.1882

This is going to be, that is that, here, here, here.1885

Now, let me see, we have got COO^-, OH.1896

This one is deoxy; we have the hydroxy here, and here, we have NCOCH₃, and we have an R-group, and R happens to be equivalent to C.1904

This is OH; this is O - yes, that is right, I always forget that - CH₂OH.1923

This R-group right here, that is just this thing, just not enough room; we just do that.1929

This is our N-acetylneuraminic acid.1933

This is actually the deprotonated, so it is N-acetylneuraminate.1944

This is our Neu5Ac.1951

OK, now, let's go ahead and do our other one.1957

OK, I am going to leave the stereochemistry on this one unspecified.1964

I am going to go OH; I am going to go OH, and I am going to go OH, and there is going to be a CH₃ on this one, and this is L-fucose.1970

L that is the carbon, 1, 2, 3, 4; that carbon is what specifies the L.1983

OK, let me see what it is I have got here- example of an N-linked oligosaccharide.1988

Let me just give you a quick example of an N-linked oligosaccharide.1997

I will do this in red just to show you what it looks like, and I am not going to actually draw out the structure; I am just going to draw out little hexagons and put numbers in them.2004

We have Asn; that is going to be our Asn residue, and, of course, the peptide goes in that direction.2024

It goes in that direction, and it is going to have attached, let's say, I will just draw them out as hexagons.2033

Now, of course, these are sugar rings, so there is definitely an oxygen in the ring somewhere; but I do not know where that oxygen is going to be depending on what the connection is.2040

So, I am just going to draw them as little individual hexagons.2049

You will often see it like this.2053

Let's see, boom, boom, boom, boom; and let's go ahead and go here, boom, boom, boom, boom, boom, boom, and up here like this.2058

These are all in original units, so this is no. 1, 1.2071

Let's go ahead and do something like that and maybe something like that.2074

Maybe I will just go ahead and put one more for the hell of it.2081

1, 1, 1, 2, 3, 4, 2, 3, so again, 1, 2, 3, these 1, 2, 3, 4s, they are different monomers.2084

This can be N-acetylgalactosamine; this can be fucose.2096

This can be N-acetylglucosamine; this can be mannose.2099

It could be anything- that is it.2102

This is just sort of what it looks like, and again, these are all sugar hexoses, but we have not put the oxygen in there because we do not know exactly where the oxygen is in terms of connection.2105

Now, what is interesting about this, the degree of complexity that we talked about earlier has to also do what the glycosidic linkages.2117

In order to fully specify what this oligosaccharide arrangement is, I have to give the connection, that one, that one, that one, I have to specify each connection; and what is interesting about these glycoproteins, the oligosaccharide portion of these proteins, is you can have 1,4 glycosidic bond.2125

You can have 1,2 glycosidic bond, 1,3; you can have 1,6.2146

You can have 2,3- any combination.2152

As long as there is a hydroxy available to react with something else, you can have those connections.2155

That is why in order to fully specify, you have to specify the connection at each glycosidic bond.2160

We are just generally going to talk about it like this.2167

If we happen to need a specific glycoprotein that has this connection to this connection, we will deal with it like that, but this is what we mean by complexity- certainly a hell of lot more complex than a proteoglycan, which is just alternating monomers.2169

OK, let's take a look at the picture one more time- the cell membrane picture.2186

Here we go, so, again, let's say this one is a glycoprotein.2195

Let's say there is another glycoprotein; let's say this is a glycoprotein.2205

This one is a glycoprotein; this looks like a proteoglycan.2213

This one here, probably a proteoglycan attached.2217

Actually, you know what, that one looks like a glycolipid.2221

This one right here, a glycolipid- that is it.2224

You are just sort of identifying and taking a look at what the carbohydrate looks like and what it is attached to.2227

OK, now, let's go ahead and see; let's talk about glycolipids a little bit.2236

Glycolipids are lipid molecules, fat molecules, in the cell membrane.2247

I mean all of these are fat molecules.2267

These are fat molecules that are not necessarily, that make up, the things that make up the lipid bilayer of a cell membrane, it could be attached to that, but more often than not, it is attached to some lipid that is in there like a cholesterol or some other lipid that happens to be in that lipid bilayer, and the oligosaccharide is attached to that.2276

OK, are lipid molecules in the cell membrane with oligosacchs attached, and, of course, we said that already, but there is no harm in repeating ourselves.2302

OK, now, we are going to, of course, be discussing lipids, and glycolipids in much more detail when we specifically get to that chapter discussing the lipids and all the different types of lipids and certain oligosaccharides that are attached to those things.2318

We are definitely going to get more into detail about this, but just to get an idea because we are talking about glycoconjugates, and a glycolipid happens to be a glycoconjugate.2338

OK, now, I am going to talk about one particular type of glycolipid.2348

It is called a lipopolysaccharide.2353

And again, this is mostly just for your edification at this point; we will be discussing it in more detail later- lipopolysaccharide.2359

OK, a lipopolysaccharide, now the dominant feature - and when we say dominant, we definitely mean dominant - the dominant feature on the outer membrane of gram-negative bacteria for example E. coli is a gram-negative bacteria and certain salmonellas or salmonella.2370

OK, the lipopolysaccharide is the dominant feature on the outer membrane of gram-negative bacteria.2415

I mean, it is just covered with this stuff.2419

OK, it is a complex oligosaccharide, complex oligosacch units covalently attached to multiple lipids in the outer cell membrane.2423

In this particular case, a lipopolysaccharide is a specific example of a glycolipid, and it is peculiar to gram-negative bacteria; and it is where this particular oligosaccharide are, these things are attached to not just one lipid molecule in the membrane, but several different lipid molecules that are anchored in the membrane.2463

And now, we want to go ahead and take a look at one of these and what it looks like.2488

OK, this is what it looks like; let's take a look.2494

Here, we are talking about the cell interior; this bottom portion right here, this is inside the cell.2498

Inside the cell membrane, you have these lipids, OK, these long carbon chains, these are fats, these lipids that are inside the cell membrane.2507

Now, notice, these are attached to sugar units, and, of course, on the no. 6 carbon, it looks like one of these, that through and O-glycosidic bond, is attached to several other sugars.2520

So, this is going to be outside the cell; this part is the core oligosaccharide.2540

This part is always the same; this particular arrangement Kdd, Kdd, Kdo - I'm sorry, yes - Kdo, Hep, Hep, Glc, Gal, Gal, Glc, Ngc, this particular arrangement of oligosaccharide, this oligosaccharide right here is always going to be the same.2549

Do not worry, these are just different monomers: Kdo, Kdd, Hep.2566

These are just different types of sugars with different things attached to them.2569

What is different from bacteria to bacteria or different places along the bacteria, is this thing right here.2574

This is the thing that changes; this is the same.2582

This is the same, but this is the thing that changes; and depending on what this is, what collection of monomers and what glycosidic bonds are actually connecting them, that is going to be the point of recognition.2586

We call this the O-antigen; this is what your immune cells recognize when they run across the bacteria in your body.2599

That is what they attach to in order to do what the immune cells do, which is destroy these things or whatever else they plan on doing to it- that is it.2608

This is just an example of a lipopolysaccharide; it is a glycolipid.2618

It has a lipid core that is in the membrane.2623

In this particular case, certain portion of it contains an oligosaccharide, which is in variant; and then, of course, at the end, it is variant.2629

Different things happen up here; this is going to be the same.2642

This whole thing is the oligosaccharide portion plus these two, and this is the lipid portion- that is it.2644

We just wanted you to get an idea of what something like this can actually look like.2653

OK, thank you for joining us here at Educator.com2659

We will see you next time, bye-bye.2663