Solute Carriers (SLC Transporters)

00:01 Let’s now move on to the final type of carrier that we’re going to talk about in this lecture and that’s these solute carriers.

00:08 The prototype that we’re going to use is an SGLT1.

00:12 These particular transporters are very nice and in this particular example, we’re going to have a molecule that will enter into this particular cell -- into this transporter and then have it ratchet to exit.

00:28 So first, the solute binds within the carrier itself in a binding site and then finally, it changes or there’s a conformational change that happens that then allows it to enter into the cell.

00:43 Now, how the solute is released is also important, so it not only needs to bind, but the carrier needs to have that change occur and then it also needs to open up on the outside to allow it to move through the membrane.

00:58 What do you need for a solute carrier to work well? The big thing is you need a concentration gradient.

01:04 So, similar to a pore and similar to an ion channel, you need to have the concentration gradient developed already.

01:12 The other thing that you need is to make sure that the transport substance, does it transport by itself or does it need a cotransporter molecule? For our particular prototype, we are going to use one that requires a cotransporter or another transporter that’s being brought along.

01:32 If the solute is brought along in the same direction, it is called the symporter or cotransporter.

01:39 If it involves a substance in which it will go the opposite direction, it’s called an antiporter or a countertransporter.

01:47 Or finally, we have things that just move through by themselves or uniporters.

01:53 So why have a solute carrier in the first place? Why not use simple diffusion to move a molecule through the cell membrane? Well, first is it might not be able to make it through the cell membrane if it doesn’t have enough of a nonpolar or lipophilic nature.

02:10 So that’s one reason.

02:12 The second is you can move things through faster with a solute carrier than you can with simple diffusion.

02:19 So if you look at this particular graph, you have concentration of the substance along the X-axis and along the Y-axis you have the rate of diffusion.

02:28 Focus in here on the red line, the facilitated diffusion line.

02:33 You can see how it occurs.

02:35 At a lower concentration, you get more rate of diffusion with facilitated diffusion through a solute carrier than you do through simple diffusion.

02:44 So it’s faster.

02:46 The one problem that can occur is that sometimes, besides it being faster, you can do it at a lower concentration.

02:55 Meaning, that you need less of the substance available for the fast transport.

03:01 This is very important in terms of getting things through as rapidly as possible.

03:08 The one problem with a solute carrier is that you only have a certain number of solute carriers and eventually, you can eclipse the number of carriers that you have available.

03:20 And if you do that, you can then have the substance spill over.

03:25 And this sometimes occurs in places like the kidney in what we’ll get to in a few slides.

03:31 Now, the prototype that we want to focus on today is the SGLT1, which is the sodium glucose cotransporter one, and this is one of the primary ways in which you’re going to bring glucose into the body in the GI system.

03:47 So this is usually how it occurs if you take in let’s say milk or some sort of a dairy product.

03:53 You have lactose.

03:55 It gets broken down by an enzyme called lactase into glucose and galactose.

04:00 Now, both of these particular carbohydrates, simple carbohydrates, are going to need a transporter to get into the body.

04:10 SGLT1 provides that transporter.

04:13 In this case, we’re going to be moving in one glucose molecule and two sodiums.

04:20 What do you need to have this process work? The first is you need to have a concentration gradient.

04:26 So sodium needs to have a concentration gradient to move in glucose into the body.

04:34 How does that concentration gradient get developed? It’s through another transporter, the sodium-potassium ATPase, which is located on the other side of the enterocyte on the basolateral membrane is creating the gradient so you need to pull sodium through the cell and then the sodium is providing the driving force to move that particular glucose or galactose molecule through the transporter.

05:01 It’s interesting for SGLT1s you actually need two sodiums.

05:05 For SGL2s, which is another transporter in this particular family, you only need one.

05:10 So it depends on the transporter which molecules are going to be brought through it, but both of them are called symporters or cotransporters.

05:21 Now, this allows for glucose absorption and the example there we’re using with this particular SGLT1, it’s through the small intestine to allow us to reabsorb glucose or into the body and that allows us to have enough energy for ATP production.

05:39 There are other SGLT1s in the body besides those that are located in the GI tract.

05:45 One of them is located in the renal tubule and this allows us to reabsorb glucose so we don’t lose it out of the body.

05:53 And so in this case, we’re moving glucose from the tubule lumen space in through the renal cell into the blood.

06:03 And in this case, we really want to make sure we don’t lose glucose out of the body.

06:09 So it’s a very important transporter.

06:12 However, if a person is diabetic and has a high amount of blood glucose, you could eclipse the Vmax of the SGLT1 within the renal tubule cell, and therefore, glucose will be spilled over into the urine and you will urinate it out.

06:29 In this case, you’ll be losing glucose in a diabetic hyperglycemic condition.