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Protein modifications

Video transcript

after a polypeptide chain is formed it's going to be folded into its secondary and tertiary structure into a very specific 3d conformation or shape and at this point we can start calling it a protein but this protein may not be ready to carry out its function just yet there might be some additional protein modifications that need to be made to this protein before it can be functional and those are called protein modifications there are two different types of protein modifications the first type is Co translational modification and that means that these are modifications or changes that happen to the protein or actually to the polypeptide while it's being translated so let's say we have the ribosome right here and we have a poly peptide that's being formed so these changes are going to happen while the polypeptide is being formed and an example of a co translational modification is acetylation and what happens during a seat elation is the first amino acid in the polypeptide which is usually Metheny n-- it's going to be removed and in its place we put in a seal group so let's just draw an acetyl group and a situation happens to 80 to 90 percent of eukaryotic proteins but the significance of this modification is not known very well we're actually trying to figure out what the purpose of this modification is the other type of protein modification that happens is post translational modification and actually most protein modifications fall into this category and the examples are going to discuss in this video are all post translational modifications and those modifications happen after translation many post translational modifications happen in the endoplasmic reticulum and the Golgi apparatus but not all of them so let's go through examples so the first post-translational modification i want to talk about is glycosylation and glycosylation well you can look at the word the prefix glyco tells us that it has something to do with a carbohydrate and so glycosylation is the adding of a carbohydrate to a protein and most of the proteins in this video are on green and glycosylation usually happens to proteins that end up being embedded in the cell membrane so you can see here we have a cell membrane and this protein embedded it and then there's there are these carbohydrate groups attached so here's a carbohydrate group attached and here's another one and glycosylation helps to identify different types of cell types of cells and one very common example of where we use glycosylated is in the [ __ ] blood groups so let's let's take four different red blood cells and let's just say that each one of these red blood cells belongs to a different person and so red blood cells have these proteins embedded in their surface and these proteins are going to have many times different carbohydrate groups attached to them so let's say that this person right here has this particular carbohydrate coop attached to it that makes him blood type a let's say that this person has a different type of carbohydrate attached to the protein let's say it looks something like that that makes them blood type B let's say that this person has actually both of those carbohydrates attached to his red blood cells that would make him blood type a B now let's say that this last person does not have any carbohydrates of this category attached to the proteins on hid on his red blood cells and so that makes them blood type O so here's a very common example of how glycosylated is used in the identification of different types of cells let's go on to a different type of post-translational modification that's pretty similar and that is lipid ation and lipid ation is when we add a lipid to a protein also a protein that's going to be attached to the cell membrane and this some lipid we're looking at is actually an example of a GPI anchor and GPI anchors our lipids that help to attach or tether proteins to the cell membrane and just to give you an idea of maybe why this would be necessary so to quickly review the structure of the cell membrane we have these hydrophilic heads that means that they are polar and then we have inside these hydrophobic tails and that means that they are nonpolar and so the protein has both polar and nonpolar parts on it and maybe it just doesn't attach well to the hydrophilic portion of the cell membrane and so this GPI anchor a lipid kind of plunges into the lipid or hydrophobic part of the cell membrane and we know that substances that are similar like substances that are both both hydrophobic attached very well to each other so this lipid which is hydrophobic attaches very well to the inner part of the cell membrane that is also hydrophobic and so that's how it helps to attach the protein to the cell membrane and both glycosylation and lapidation usually do occur in the endoplasmic reticulum or in the Golgi apparatus let's move on to some protein modifications that have more to do with the activity or the function of the enzyme and less with the structure so one very very common protein modification I want to discuss is phosphorylation and phosphorylation is basically the adding of a phosphate group to a protein or to an enzyme and then phosphorylation comes along with D phosphorylation is when you remove a phosphate group from the enzyme or the protein I'm just going to make a little bit of room over here and so what you're looking at is this schematic diagram of the sodium potassium pump that's found in basically every animal cell so the sodium potassium pump and another name for the sodium potassium pump widget is the enzyme that you're looking at is the NA K ATPase and so this enzyme or protein the sodium potassium pump is responsible for maintaining the proper osmolarity of sodium ions and potassium ions in and out of the cell and so let's see how phosphorylation regulates this protein so and again the proteins that you're looking at they really all represent one protein it's just we're going through the motions of the changes that happen to this one protein but it's not like we have six different it's not like we're looking at six different proteins in the membrane so here's our first step and when you look at this enzyme you can see that there are three receptor sites for a particular ion this is the cilium ion represented by these dark blue circles and then there are two receptor sites that look kind of more squarish and those receptor sites for the potassium ions and those are represented by these light blue squares so those are those are the potassium ions and so back to the first step and what's happening here the sodium ions will attach to the receptor sites on the enzyme and just to clarify this is the intracellular space so this is basically the cytoplasm and so these sodium ions are coming from the cytoplasm and then out here is the extracellular space the outside of the cell anyway back to our first step so the sodium ions attach to the receptor site and when the receptor sites are full it's going to cause something to happen it's going to cause an ATP molecule adenosine triphosphate to break down into adp adenosine diphosphate plus phosphate and then this phosphate will attach itself to the protein and that is phosphorylation and so when this protein gets phosphorylated when the phosphate group is attached to it it causes there to be some sort of change in the conformation of the protein and that change in conformation causes the protein to turn itself around by 180 degrees and face the outside of the cell that's what you're looking at right over here so we're still our protein is still phosphorylated I know I drew it in a different place but just you know keep in mind that there's still a phosphate group attached to that protein and then the protein releases these sodium ions into the extracellular space so the next step is number four and again we are still phosphorylated there's still phosphate group attached to our protein and so the next step the potassium ions and the outside of the cells will attach themselves to the protein on the receptor sites and take note there are three receptor sites for sodium but only two for potassium when the potassium receptor sites are full it's going to cause a different change to happen this phosphate group is going to be removed and that is called D phosphorylation so that is d phosphorylation and so the that phosphate group will is ends up in the inside of the cell and gets recycled in some other way and when this protein is d phosphorylated when that protein is removed it's going to cause a different change in the conformation of the protein and that change in conformation causes the protein now to turn around again by 180 degrees and it faces the inside of the cell so that's our fifth stuff you can see the protein is facing inside of the cell and in the last step the potassium ions are released into the inside part of the cell and then we're back to our first step and so you can see that the phosphorylation and dephosphorylation basically regulates the activity of this protein and the end result that we're trying to get to is that we want there to be on the outside of the cell a rather low concentration of sodium sorry your rather high concentration of sodium and your rather low concentration of potassium and on the inside of the cell we want there to be a rather low concentration of sodium and a relatively high concentration of potassium and that's accomplished by the fact that for every three sodium's that are pumped out to potassium are pumped in and so here's an example a very common example of how phosphorylation regulates a protein and this doesn't happen just in the sodium potassium pump it happens in many many enzymes and proteins in our bodies and in in cells let's move on to two other protein modifications that also have to do with regulating an enzyme or something similar to that so the next protein modification I want to talk about is methylation and in particular the methylation of certain proteins called histones and those are these green circles that you're looking at and histones are these proteins that around which DNA wraps itself so they're found in the chromosomes and they help to package DNA in a very tight and organized manner and sometimes histones are methylated so let's put some methyl groups on our histones and methylating and D methylating histones helps to turn certain genes on and off and so here's another example of a protein modification helps to regulate activity but in this case we're regulating the activity of genes as opposed to proteins and another protein modification I want to bring up is proteolysis and by looking at this word pro do means protein and lysis means to break something down or to cut something and proteolysis is sometimes to take a protein and activate it we need to cut it and in fact the insulin has to be cut twice before it's activated so let's cut this protein twice and you may have actually heard of the term zymogen and a zymogen is an inactive form of an enzyme and sometimes the way to activate a zymogen is by cutting it and that's pretty lysis there is one more protein modification I want to discuss and that is sometimes we add a protein ubiquitin well to another protein so that protein I just added is ubiquitin and this process is called ubiquitination and what ubiquitination does is it basically marks this green protein for segregation or for breakdown so within a short while of being ubiquity this protein is going to be destroyed and the different parts are going to be recycled let's just quickly review the post translational modifications that we discussed so we talked about glycosylation and lipid ation and those are two protein modifications that generally happen to proteins that end up being embedded in the cell membrane so glycosylation was the adding of a carbohydrate group which helps to identify certain cells and liquidation is when you add a lipid to a program that generally helps to anchor a protein to the cell membrane then we talked about phosphorylation methylation and proteolysis and these will have to do with activating or deactivating an enzyme or genes so phosphorylation was the we brought the example of the sodium potassium pump which is basically regulated with phosphorylation and dephosphorylation then we spoke about methylation which can basically helps to turn on or turn off certain genes and proteolysis is the way in which many enzymes are activated and the last post translational modification we talked about was ubiquitination and ubiquitination marks a protein for degradation and then the various parts are recycled