Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
L07v02 Trp part1a final export [00:00:00.00] [00:00:01.67] SPEAKER: Hi. In this video, we'll continue our discussion on Gene Specific Transcription. In particular, we're going to focus most of this lecture on the Trp repressor. Trp is short for tryptophan, the amino acid. And we're going to talk about negative feedback, which we've already seen with enzymes. And again, allostery is going to play the key role in terms of a small molecule binding to a protein, changing its shape, which then changes the proteins function. [00:00:32.18] We'll look at more complex mechanisms of gene regulation. In particular, we'll look at the way the cells decide which sugar to use. And then we'll look at some other complex mechanisms. So here is the operon for tryptophan bio synthesis. There are five genes that make five proteins that each play a different role in the bio synthesis of this amino acid; amino end, alpha carbon, carboxylic end, and then the big side chain here. [00:01:11.29] These five genes are coordinately regulated since this is an operon, it's a polycistronic mRNA. They are all regulated by this promoter region. With the idea that if you need to make one of these enzymes, you need to make all of them. Because there's not much use in going part of the way to making the tryptophan molecule, but not completing the task. [00:01:35.25] In this example, the cells default state is that the genes are turned on if the promoter region is empty, RNA polymerase will bind to the promoter region, and start making the mRNA tryptophan bio synthesis. Since tryptophan is going into every protein that's made, it's important to have a lot of it. So the default decision for the cell is to have the gene turned on. [00:02:03.14] But what happens when you have too much tryptophan when you've already made a lot and you don't need more? In that case, tryptophan binds to the Trp repressor protein right here. And will induce a confirmation, here it's rounded in a won't bind DNA. When the repressor binds to the protein, it changes its conformation. It's now able to bind DNA in the place where RNA polymerase normally binds and so that the genes are turned off. [00:02:37.80] So this is a very simple example of negative feedback regulation. Here the default state is, let's make more tryptophan. The negative feedback is, hey, we've got too much tryptophan, stop for a little bit. And this all happens automatically since the molecule being made is also the molecule that binds to the repressor to induce the conformational change. [00:03:03.54] On this slide, we see a bit of the detail of the repressor bound to DNA. Let's look first at the case where the genes are off, where the repressor is bound to DNA. You see this is a symmetrical protein. You can see that helix is perfectly positioned to bind to the major groove here. And a second helix is binding to a second major groove here. And the spacing between these two arms is just appropriate for sitting down and having those both spaced properly. And you can see the tryptophan here is sort of in a hinge region. And you can see that the binding here can easily affect this angle here so that it's properly positioned for both sides to bind simultaneously. In general, this probably doesn't have enough binding energy to bind with just one of the two helices. [00:03:59.06] In the case where tryptophan is not bound, we have a smaller angle here. A shorter distance between these two arms, such that they will not sit down properly. Instead of sitting in over here, it sits down to maybe here and here. And so it cannot bind to both the major groups. [00:04:22.64] Again, this is just a consequence of having too much tryptophan in the cell or a sufficient amount that you don't need to make more. It automatically will regulate this protein and it will bind this, it'll prevent the genes from being made. Now I'd like to remind you about what negative feedback regulation that we saw earlier in the class, where molecules were-- in this case CTP was the molecule z, it binds to this enzyme complex here that converts b to x. And because we have enough CTP, it will bind and prevent the conversion of b to x. So all the molecules of intermediate molecules b will go on to make molecule c. [00:05:14.82] In this case, we have negative feedback regulation of enzymatic activity. Down here with the tryptophan repressor, we have negative feedback of genetic regulations. So same principle, different mechanism of controlling the regulation of the enzymes that you need. [00:05:38.81] In addition to repressors being used, soon people found activators for the protein. And it can be modulated in just the same way as repressor. First of all, you might have some repressor activators that bind without any ligands. Or you could also have ligands binding to activators to induce a fit such that they can bind a promoter region. And because they provide extra binding energy to the RNA polymerase, it can help recruit the RNA polymerase to this promoter. [00:06:11.32] Again, that would be simple thermodynamics. You have more positive interactions building up, this is going to make an event happen more frequently. And just to give a specific example, we'll talk about hormone receptors. [00:06:29.26] In this case, it's glucocorticoid hormone. It could be estrogen or any number others. A lot of them work by this mechanism. And here is the glucocorticoid receptor, when this-- which is a protein internal to the cell in the cytoplasm, whenever glucocorticoid which passes through the membrane binds to the protein, it induces a shape change. Now it interacts with a protein that's there and the promoter region. And now it will encourage the polymerase to sit down and you can have transcription of hundreds to thousands of genes. [00:07:08.50] Hormone receptors in general influence the expression levels of lots of genes. And although the book talks about genes expressed at low levels versus high levels, you can think of it as turned off or turned on. I think this is a good place to pause our discussion. And we'll resume in the next video with this slide, which is a little bit more involved than most. Thanks.