Biochemistry Membrane Lipids

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

Today, we are going to continue our discussion of lipid biochemistry by talking about membrane lipids; these are structural lipids.0004

On the last lesson, we talked about the storage lipids, the triacylglycerols.0011

Today, we are going to be talking about the structural lipids, the lipids, the fats that actually show up in biological membranes, in the cell membranes.0016

Let’s get started.0025

OK, let’s go ahead and write down a couple of things.0028

Yes, that is fine; I guess we can stick with black here.0033

Your membrane lipids are lipids that make up the biological membranes of cells, the bilayer - and we are going to be looking at some illustrations a little bit later on - in cells or reside in the membrane.0038

In the case of something like cholesterol, it is not so much that it makes up the fats in the membrane.0078

It just happens to reside there, or reside in the membrane among the other types of lipids.0085

Again, it is just sort of the other types of lipids that actually make up the membrane.0092

It is just a question of perspective, whether one considers it to be something that actually makes up the membrane or happens to just be there.0101

In another cell, it is not that important; what is important is the lipid, the structure and the function.0108

OK, they have a hydrophilic end.0114

Let’s call it polar end and a hydrophobic tail.0126

Again, it looks something like this; we generally tend to draw it like that, either a single or usually a double.0140

In the case of the lipids that we are going to be talking about today, this is the hydrophobic tail; this is the polar end.0146

And again, we will be looking at some illustrations a little bit later, after we talk about some structures.0151

OK, we are going to be talking about five major classes of membrane lipids.0157

We will discuss five major classes of the membrane lipids, and we are going to be taking about the glycerophospholipids.0166

OK, I have to warn you here.0188

As you have probably already noticed with biochemistry, the nomenclature, the number of the names, they tend to not only get very, very long, but we tend to have multiple names for the same thing.0192

For a student, it is very, very daunting because often we will be talking about, let's say we will mention 3 or 4 different names of something but we are actually talking about the same class of molecule; and I understand it is an annoyance.0203

So, sometimes we are going to be calling them glycerophospholipids; you will hear them talked about as phosphoglycerides.0216

Sometimes, the name is so completely different that you are wondering, you really think that they are different molecules and they are not.0223

I have to apologize for that; that is just the nature of biochemistry.0230

Different people from all around the world call them different things; people within a certain specific research community refer to them differently.0234

These are just names that you are going to get accustomed to hearing over and over and over again.0242

I will try my best to be consistent, but in all honesty, I think it is also a good idea to hear the multiple different names and to know that we are talking about the same class of molecule or the same molecule itself, so just a little warning; but you already figured that out.0247

OK, glycerophospholipids, and then the second major class we are going to talk about is going to be the galactolipids.0261

Let’s see, the third class, something called the tetraether lipids, and then, we have something called the sphingolipids, and then, we are going to talk about the steriles.0270

OK, let’s go ahead and start with our glycerophospholipids, and I think I will go ahead and go to blue here, so glycerophospholipids, glycerophospholipid.0293

Now, these are the ones that are also called the phosphoglycerides.0314

Now, the name pretty much says it all, glycerophospholipids; you have a glycerol backbone.0327

It contains a phosphate somewhere, and its glyceride part, it basically has some fatty acids that are attached to it.0332

Let’s go ahead and look at the general structure; I think I will go ahead and do it this way.0340

We have carbon, carbon, carbon; that is going to be our glycerol backbone.0346

We have O; we have O, and let me go ahead and put this O a little bit down here.0351

We have one fatty acid; I will go ahead and call it R1.0358

It could be any particular length that could have some double bonds.0361

It could have no double bonds, the saturated or unsaturated.0366

We have a second one attached; I will go ahead and call this one R2.0370

Rather than choosing a specific example of a fatty acid, I will just go ahead and leave them as R1 and R2.0374

And here, here is where we have our phosphate; and then, of course, we have...so, I am going to go ahead and put a little bit of an X there, this X.0380

This is the general structure of a glycerophospholipid; let me go ahead and finish with my hydrogens here, make sure I have all of those.0391

There is an H₂ here, and we have an H here.0399

We have the glycerol backbone; this is your glycerol right here.0403

And then, of course, we have a fatty acid attached to one of them, a fatty attached, another one, in an ester linkage - right - C double bond O, single bond O.0410

And then, of course, to this third oxygen, we have a phosphate group attached; and then we have this thing right here, this XO.0420

I am going to put a little square around this, and the reason I am going to do that is the following.0428

We are going to be doing some specific examples of what this X group is, but it is really, really important to know that this oxygen, over here, that is attached to the phosphate and attached to the X actually comes from the X group.0433

The X is some kind of an alcohol, something that has a hydroxy group on it.0444

This oxygen actually comes from that alcohol.0448

Later on, when you study the actual biosynthesis of lipids, then you will actually see where each individual atom comes from; but for now, it is good to know that this X group, it is actually an XOH.0452

It is actually some alcohol that is attached to this phosphate- there you go.0469

And also, this oxygen right here, it actually comes from the glycerol.0475

All right, now, and, of course, here is our phosphate group right there.0479

A couple of things, now, the X group is what changes.0486

So, this is what is variable; the rest of the molecule is pretty much fixed.0493

I mean, it is true; R1 and R2 can be different.0496

Generally, there will not be a huge variety; they can be different, but what actually characterizes that particular glycerophospholipid is this XO group, the alcohol that is attached.0500

Group is what changes and gives a particular glycerophospholipid its name.0512

OK, X is the polar group, so this is the polar group; and these R1 and R2, these long hydrocarbon chains, that is going to end up being the nonpolar tail.0531

X is the polar group and is an alcohol like we said.0549

OK, now, when X = H, when X is equivalent, when it is just a hydrogen, then the molecule is called a phosphatidic acid.0566

We often refer to these as derivatives of phosphatidic acid because the X is going to change.0594

When it is just H, when it is just POOOH, it is phosphatidic acid or A phosphatidic acid.0599

OK, R1 and R2 are variable.0608

R...well, I do not need to write that; you know that already, that is why we call them R1 and R2.0613

OK, and at physiological pH, at physio pH, the phosphate group carries a -1 charge.0616

That is very, very important.0630

Charge on these lipids is actually very, very important; it is going to affect the biochemistry.0636

OK, that is why I have this -1 here; and in general, I am going to be putting circles around my charges, so that I can actually see them clearly and add them up.0642

OK, let’s do some examples here.0651

We have examples; let’s go this way.0655

I am going to draw this a little bit different; I am going to put the tails on one end, then I am going to the polar head group on the other.0662

Let me go ahead and put C, C and C.0668

I am going to put the esters on this side.0673

COO, this is R1; this is going to be one of the tail ends in ester linkage, and this is going to be that.0676

This is going to be R2, and over here, and I hope you will forgive me if I leave off my hydrogens.0684

I tend to leave off my hydrogens; if you see a carbon that has two bonds attached to it, the other 2 are going to be hydrogens.0689

That is just how it is.0695

O phosphate, O, there is - oops, let me go ahead and make that negative sign a little bit clearer, a little circle around it - O, and then if we have C, C, NH₃⁺, this is called, so this right here - I do not know which one I should...yes, it is fine - I will go ahead and call it the whole thing.0699

This is ethanolamine; this molecule is called ethanolamine.0726

The name of this whole thing - OK - of the glycerophospholipid is, let me write it out, lamine, M-ethanolamine; the regular molecule without the attachment, is exactly what it sounds like.0734

It is ethanol, 2 carbons and a hydroxy and an amine group attached right there.0760

This is ethanolamine before it is actually attached.0765

Again, this oxygen actually comes from the alcohol.0772

Ethanolamine, this is called - depending on where you want to put the stress again with pronunciation - phosphatidylethanolamine or phosphatidylethanolamine.0776

It is up to you; again, pronunciation is unimportant.0784

You will often see this written as one word; I tend to write it as two words.0787

Again, it is going to be up to your teacher, about how strict they are with things like that.0791

What is important is the chemistry and the structure.0797

This part, the phosphatidyl, that is this basic structure; and then this other name right here, depends on what it is that is attached.0801

Let’s do another molecule; let’s do a C, a C, a C.0815

OK, let’s go O, C, R1, O, C.0821

This is R2, and then, we have O, P, our phosphate with a negative charge.0830

We have O, and this time, we have C, C; we have the nitrogen again, but this time, we have a CH₃, a CH₃ and a CH₃.0837

And again, there is a positive charge on there, negative charge here.0848

There is a positive; there is a negative charge.0853

Notice the net charge on the ethanolamine derivative is 0 because the nitrogen is carrying a positive.0856

The phosphate is carrying a -1, so this is a net charge of 0; so this is a neutral glycerophospholipid.0865

This one also, it is a neutral glycerophospholipid; these things are going to be very, very important.0871

In a minute you will see an example of one that is not neutral, so positive, negative, neutral, charges- very, very important.0876

This one is called - oops - phosphatidylcholine because we have this right here.0883

This group is called choline.0898

Ethanolamine has 3 hydrogens; choline has 3 methyl groups attached to the nitrogen.0903

And again, nitrogen has 4 things attached to it, so it is carrying a formal charge of +1.0908

OK, let’s take a look at another one.0914

We have got C, and we have got C, and C, O, C.0920

This is R1, O, C; and this is R2.0929