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Current time:0:00Total duration:7:44

Conformational stability: Protein folding and denaturation

Video transcript

let's talk about conformational stability and how this relates to protein folding and denaturation so first let's review a couple of terms just to make sure we're all on the same page and first we'll start out with the term conformation and the term conformation just refers to a proteins folded 3d structure or in other words the active form of a protein and next we can review what the term denatured means when you're talking about proteins and denatured proteins just refer to proteins that have become unfolded or inactive so all conformational stability is really talking about are the various forces that help to keep a protein folded in the right way and these various forces are the four different levels of protein structure and we can review those briefly right here so recall that the primary structure of a protein just refers to the actual sequence of amino acids in that protein and this is determined by a proteins peptide bonds and then next you have secondary structure which just refers to the local sub structures in a protein and they are determined by backbone interactions held together by hydrogen bonds then you have tertiary structure which just talks about the overall 3d structure of a single protein molecule and this is described by distant interactions between groups within a single protein and these interactions are stabilized by van der Waals interactions hydrophobic packing and disulfide bonding in addition to the same hydrogen bonding that helps to determine secondary structure and then quaternary structure just describes the different interactions between individual protein subunits so you have the folded up proteins that then come together to assemble the completed overall protein and the interaction of these different protein subunits are stabilized by the same kinds of bonds that help to determine tertiary structure so all of these levels of protein structure help to stabilize that folded up active conformation of a protein so why is it so important to know about the different levels of protein structure and how they contribute to conformational stability well like I said a protein is only functional when they are in their proper conformation in their proper 3d form and an improperly folded or degraded denatured protein is inactive so in addition to the four levels of protein structure that I just reviewed there is also another force that helps to stabilize a proteins conformation and that force is called the solvation shell now the solvation shell is just a fancy way of describing the layers solvent that is surrounding a protein so say I have a protein who has all these exterior residues that are overall positively charged and picture this protein and the watery environment of the interior of one of our cells then the solvation shell is going to be the layer of water right next to this protein molecule and remember that water is a polar molecule so you have the electronegative oxygen atom with a predominantly negative charge leaving a positive charge over next to the hydrogen atoms the same is true for each of these water molecules so now as you can see the electronegative oxygen atoms are stabilizing all the positively charged amino acid residues on the exterior of this protein so as you can see the conformational stability of a protein depends not only on all of these interactions that contribute to primary secondary tertiary and quaternary structure but also what sort of environment that protein is in and all of these interactions are very crucial for keeping a protein folded properly so that it can do its job now what happens when things go wrong how does a protein become unfolded and thus inactive well remember that this is called denaturation and this can be done by changing a lot of different parameters within a proteins environment including changing the temperature the pH adding chemical denaturants or even adding enzymes so let's start with what happens if you alter the temperature around a protein and we can use the example of an egg when we put it into a pot of boiling water because an egg especially the white part is full of protein and this pot of boiling water is representing heat and remember that heat is really just a form of energy so when you heat an egg the proteins gain energy and literally shake apart the bonds between the parts of the amino acid chains and this causes the proteins to unfold so increased temperature destroys the secondary tertiary and quaternary structure of a protein but the primary structure is still preserved so the takeaway point is that when you change the temperature of a protein by heating it up you destroy all of the different levels of protein structure except for the primary structure so now let's say you were to take an egg and then add vinegar which is really just an acid the acid in the vinegar will break all the ionic bonds that contribute to tertiary and quaternary structure so the takeaway point when you change the pH surrounding a protein is that you have disruption of ionic bonds and if we think about this a little bit more deeply it kind of makes sense because ionic bonds are dependent upon the interaction of positive and negative charges so when you add either an acid or a base which in the case of an acid is just like adding a bunch of positive charges you kind of disrupt the balance between all these interactions between the positive and negative charges within the protein so now let's look at how chemicals denature proteins chemical denaturants often disrupt the hydrogen bonding within a protein and remember that hydrogen bonds contribute to secondary tertiary all the way up to a quaternary structure so all these levels of protein structure will be disrupted if you add a chemical teenager in so let's take our same example of a protein with an egg and say if you are 21 years or older you got your hands on some alcohol and you added this to the egg then all the hydrogen bonds will be broken up leaving you with just linear polypeptide chains and then finally let's take our hard-boiled egg from the temperature example and let's eat it so here's my beautiful drawing of a person represe you eating this hard-boiled egg once the egg enters our digestive tract we have enzymes that break down the already denatured proteins in the egg even further they take the linear polypeptide chain whose primary structure is still intact and they break the bonds between the individual amino acids the peptide bonds so that we can absorb these amino acids from our intestines into our bloodstream and then we can use them as building blocks for our own protein synthesis and that's how enzymes can alter a proteins primary structure and thus the proteins overall conformational stability so what did we learn well we learned that the conformational stability refers to all the forces that keep a protein properly folded in its active form and this includes all the different levels of protein structure as well as the solvation shell and we also learned that a protein can be denatured into its inactive form by changing a variety of factors in its environment including changing the temperature the ph adding chemicals or enzymes