Biochemistry 401 lecture 31 Today we re going to talk about protein degradation We ll begin with dietary proteins and then work on to cellular proteins We re then going to talk about amino acid catabolism This includes the urea cycle We re then going to finish with inborn errors of amino acids metabolism The digestion of dietary proteins begins in the stomach This occurs through the action of an enzyme called pepsin Pepsin is originally synthesized as an inactive zymogen called pepsinogen Pepsinogen includes a 44 amino acid portion that s stuffed into the enzyme active site and so in order for this enzyme to be activated 44 amino acids must be clipped away This happens when the pepsinogen hits the acidic environment of the stomach The low pH allows that protein to unfold just a little bit because there are amino acids that become protonated and so the salt bridges that were there the ionic bonds are no longer there And so the enzyme opens up just enough for these 44 amino acids to become vulnerable to proteolysis And so pepsinogen autoproteolyzes and the 44 amino acids are gone and the pepsinogen becomes activated to pepsin Which of these amino acids has an R group that you might expect to go from a non protonated to a protonated state following a shift from pH 7 3 what you would find inside a cell to about pH 2 This is about the pH of the interior of the stomach So which of these lysine glutamine aspartate alanine or tyrosine That s right it s aspartate There are actually aspartates in the active site that are responsible for the catalytic activity of this enzyme And so when aspartate becomes protonated it s no longer going to bind to a lysine Therefore the protein can open up just a little bit to become vulnerable to proteolysis and so this 44 amino acid segment that is present in the active site is going to be cleaved away to activate this enzyme Now the thing with pepsin is it is a random cleaver It just goes along and it chops up proteins randomly non specifically and so there s not an amino acid sequence that it favors It just goes after proteins and proteolyzes them The thing is once this pepsin reaches the duodenum the duodenum is a different place It s the first part of the small intestines you have bile coming in Now bile doesn t just have products from the liver coming in not just bile salts it also has pancreatic juices it s called These contain proteases also that are secreted from the pancreas as zymogens and then once they get to the duodenum they are activated Bile also contains bicarbonate and the bicarbonate is going to raise the pH of those stomach contents that are coming from the stomach into this first part of the small intestines and so this bile is going to increase the pH of the food as it travels into the duodenum This means that pepsin is going to be deactivated It does not do well in a high pH environment and so it s going to become deactivated This is really good for the proteases that are coming in from the pancreas Now they re not going to be randomly cleaved by this pepsin Awesome Once pancreatic zymogens get to the small intestine there is a protein on the outer surface of the enterocytes So we have the small intestine okay Let s say this is this is the surface of my small intestine and we re going to have a protein that s on the surface that is called enteropeptidase This is a transmembrane protein that has a protease activity that extends into the lumen of the small intestine So when the zymogens come in from pancreas you first start off with trypsinogen Now trypsinogen is the zymogen You know it s a zymogen because it has ogen at the end These zymogens are either going to have ogen at the end or pro at the beginning That s how you know So here comes trypsinogen It s going to encounter enteropeptidase Enteropeptidase is going to cleave off the inhibitory portions of trypsinogen to make it into trypsin Now trypsin is going to go along and activate all of those zymogens that come in from the pancreas So this is like chymotrypsinogen this is proelastase and you can see right here in the diagram that there is a cascade Trypsinogen is going to be activated by enteropeptidase It s going to become trypsin and it s going to activate the proteases in this cascade Now the thing is is that these proteases are specific cleavers They hydrolyze the peptide bond at specific amino acid sequences Why Because these particular sequences fit into the active sites of these enzymes This has to do with the architecture of the active site itself in these proteases Active site specificity Let s take a look at that For instance trypsin has an aspartate in the active site whereas elastase has a couple of valines in the active site And it is the nature of the active site that determines its specificity Each of these proteases chymotrypsin trypsin and elastase cleave the peptide bond after a specific amino acid that fits into the active site Can you pair these proteases up with their target amino acid Because trypsin has a negatively charged amino acid aspartate in its active site it s going to be specific for positively charged amino acids like lysine and arginine Chymotrypsin has a large active site without any bulky hydrophobic residues without any charged residues in it so it is going to cleave after large hydrophobic amino acids like phenylalanine or tyrosine or even tryptophan Elastase likes to cleave after an alanine or a valine Do you have to remember these specificities off the top of your head No but if I give you the active site you d have to be able to match that up with the type of amino acid that it would cleave after And so once these proteins are cleaved by these proteases both from pepsin and pancreatic proteases they re taken in to the enterocytes the epithelial cells lining the gut the small intestine and then they re broken down even more by peptidases that are in the enterocytes Finally these are shuttled in the bloodstream as amino acids What can we do with amino acids Well we can use them to make other proteins right We can also use them to make other metabolites that we need But another thing we can do is we can break those down take the amino group off and then we have a carbon skeleton right And so we can use that carbon skeleton in anabolic reactions and for energy So now we re going to turn to the breakdown of endogenous proteins There are times that we have to break these down Why Well one of the ways we control the activity of enzymes is by
View Full Document
Unlocking...