Non-enzymatic protein function

So we're going to be talking about non-enzymatic protein function. And before we get into what exactly that means, let's just say we are playing a game of Scattergories. And the next category that comes up is protein, and you have to list all the different types of protein that you know. And so you're like, oh, I totally got this. We've got beef and pork and chicken. Right? Those are different types of protein, and that's true. But what about the biochemistry type of protein, as in the large biomolecules that are made up of amino acids? What are the different types of proteins then? And so that's a little bit tougher. And so that's the point of what we're going to be talking about today is to talk about all the different types of proteins and the different functions that they perform. And so the key to understanding proteins as we go forward is understanding one unique characteristic about proteins. And that unique characteristic is that they can bind various biomolecules, and they bind specifically and tightly. And so just keep that concept in mind as we're talking about all the different functions of proteins, and that will help you understand how they are able to perform the vast array of functions that they do. So I like to think of proteins as being in one of two main classes. There are the enzymatic proteins, so enzymes, and then the non-enzymatic proteins, or we'll just call them the non-enzymes. And so let's back up just a minute. What exactly are enzymes? What do they do? Enzymes are, in a nutshell, little chemical reaction machines. They can catalyze all sorts of chemical reactions, so they catalyze reactions that help to sustain life. And so they are really the workhorses of the cell, helping to build up and break down things as needed. And by acting as catalyst, enzymes can help to accelerate the rate and specificity of these chemical reactions. And one good example of this that you're probably familiar with already is DNA polymerase, which catalyzes the synthesis of new strands of DNA. And probably even more familiar to you are the enzymes in your saliva. So every time you eat, there is one enzyme in particular called amylase, which is responsible for breaking down starch into sugars. So amylase is another example of a protein that has enzymatic function. And just notice for a second that both of these enzymes end in A-S-E, ase. And so in general, if you see a protein and it has this sort of ending, you should be thinking to yourself, oh, I bet this is an enzyme, an enzymatic protein. So that's a good rule of thumb. Now, non-enzymatic proteins, or non-enzymes, are all of those proteins that carry out functions that require the capacity to bind, but not necessarily to catalyze a reaction. And so what are some examples that we'll be talking about of non-enzymatic protein function? Well, there are proteins that function as receptors or ion channels in a cell membrane, and we'll talk more about that. And then there are proteins that are transport proteins. There also motor proteins. And then a special class of proteins that function as an integral part of the immune system, and those are called antibodies. And so we'll go through each of these examples of non-enzymatic protein function one by one. So let's give ourselves a little bit more room to talk about those. Now, let me quickly interject that it's important to realize that not all proteins are always either an enzyme or a non-enzyme. Oftentimes, they have characteristics of both, and so just keep that in mind as I'm going through and highlighting the non-enzymatic properties in this video. OK, so starting with receptors and ion channels. There are certain proteins that exist in the membrane of a cell and function as either receptors or ion channels. Now, receptors are proteins that receive, or bind, a signaling molecule. So let's draw a cell here. We're going to draw the membrane bilayer here. And so here would be my exterior of the cell, and here's the interior of the cell. And within this membrane bilayer, you can find a receptor protein. So we'll draw that here. And this receptor protein will bind a signaling molecule, also known as a ligand, which then induces some sort of chemical response in the cell. So one example of a receptor protein and ligand pair is an insulin receptor and insulin. So let's say this ligand is insulin, and this is the insulin receptor. Now, insulin is a hormone that's released by the pancreas in response to an increase in blood glucose levels. So let's say there's extra glucose, because you just ate a piece of pizza or something. So let's say there's an increase in glucose around, and then insulin is going to be released by the pancreas in response to this increase in blood glucose. And then once it's released, it binds to its corresponding receptor on certain cells, which leads to a cascade of signals within the cell. And this allows it to then absorb this excess glucose into the cell. And then likewise, you can also have an ion channel that also spans the membrane bilayer. So let's extend this lipid membrane bilayer, and then we'll draw an ion channel protein here. And so likewise, this protein spans the entire bilayer of the cell, and it acts as a pore or a channel through which certain ions, say calcium, can enter or exit the cell. So it can come in and come out through the same channel. So next up are the transport proteins, and now these proteins are responsible for binding small molecules and transporting them to other locations in a multicellular organism like humans. And the trick with these proteins is that they have to have a high affinity for their ligand when the ligand is present in high concentration. So at high concentration of a ligand, you have high affinity of the protein for that ligand, and at low concentration, you have low affinity. And a great example of this is hemoglobin. So hemoglobin is present in red blood cells, and it picks up oxygen in the lungs-- so here are my lungs, it's high in oxygen in your lungs-- and then delivers this oxygen to tissues. And we'll draw a tissue here, say it's muscle tissue, where its present in low concentrations. And so this is a great example of a transport protein at work. So next up are the motor proteins, which include myosin, kinesin, and dynein, all of which are capable of generating great forces. And these proteins are really crucial for cellular motility. And myosin specifically is a protein responsible for generating the forces exerted by contracting muscles. So every time you flex your bicep like so, your myosin protein in your muscles are contracting and generating that force. Now, kinesin and dynein are motor proteins that are responsible for intracellular transport. And then dynein in particular also plays a role in the motility of cilia, which are these little extensions of a cell that project out. And mutations in a particular dynein protein can lead to a rare disease called primary ciliary dyskinesia. So in primary ciliary dyskinesia, you can see there's some sort of dyskinesia or problem in movement for the cilia. And mutations in a particular dynein protein lead to this rare disease in which the action of the cilia of the cells lining the respiratory tract fail to function. And this leads to a decrease in mucus clearance from the lungs, and therefore an increase susceptibility to chronic infections like pneumonia and bronchitis. So as you can see, these motor proteins are really important. And then finally, our last class of non-enzymatic proteins that we'll be talking about are the antibodies of the immune system. Now, antibodies are protein components of the adaptive immune system whose main function is to find foreign antigens and target them for destruction. So in this case, the antigen, which comes from any foreign substance, say a virus or something like that, is the antibodies ligand. So here's an example of an antibody, and then here is the antigen. And the antigen is really just the antibodies particular ligand. So you can think of antibodies as being like little red flags for the body's immune system letting us know that hey, this thing is not supposed to be here. We need to get rid of it somehow. And it's important to know that an antibody's affinity for its target antigen is extraordinarily high. So the affinity is strong, really high. And so there you have it. You can see all the different types of non-enzymatic roles or functions that proteins can play, either as receptors or ion channels, as transport proteins, motor proteins, and then highly specific antibodies in our immune system.