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Lecture 08

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Biological Sciences
Ian Brown

BGYC19H3F LECTURE 8 Slide 1,Zymogen Activation: During development we can get differentiation of specific cell types. They accumulate and give unique properties. Examples; • Myosin (contractile ability) • Mature Crystallin (optical properties) • Globin (binds and transports oxygen) Slide 2, Differences in the level of control of gene expression; Transcriptional control (turning on a gene to make mRNA) Translational control (mRNA is translated into protein very quickly, however in red blood cell development the globin message was made and waiting for translation until a later stage of development) Third level of control, Post translational control: Message is translated into a protein, but the protein is not active. Why? 1. protein is degraded as soon as it is made, activation would be removal of the degradation mechanism 2. protein needs a co-factor to be active, example, enzyme, activation would be to supply the co-factor 3. might be the presence of an inhibitor which might prevent it’s activity, activation would be to remove the inhibitor 4. protein may have to be modified, activation would be to add phosphate (or whatever may be needed) 5. proteins are made as a zymogen Slide 3, What is a zymogen? Proteins made are inactive because they are an inactive precursor protein. They must have a protease to remove the inactive prefix at the beginning of the protein to make it active. Most enzymes are made as inactive zymogens. Sometimes they are made in one tissue then excreted and taken up in another tissue. Activation happens in the second tissue. They need a biological change to activate the protein, which is the protease clipping off the prefix. The removal of the prefix only happens in the right time and place in development. (diagram) Slide 4/5, How zymogen protein is made. N-terminus is the first part of the protein to be made. C-terminus is the last part of the protein to be made. We need cleavage of the peptide bond at a specific place, limited proteaolysis. 1 The prefix is inactive. The protease must come along and there must be “Lock and Key” recognition. They must bind and cleave at the correct place. What is left is the active protein with the N-terminus removed. This is called limited proteolysis. Slide 6, Limited Proteolysis: Protein is prepositioned in a zymogen form and prefix is removed and it becomes active. It cleaves the peptide bond at a specific position to remove the prefix. This cleavage site is controlled by a 3D action between zymogen and the clipping protease. Zymogen activation is an irreversible reaction. It is a hydrolysis reaction and cleaves the peptide bond making it active. Slide 7, Enzyme Active Site: Enzymes need to fold up to be active. Initially they are made to be a linear peptide, then they must fold on itself, this forms the active site for binding of the substrate. Folding creates the active site. Slide 8, How does removal of the prefix activate the enzyme? Two possibilities: 1. prefix is physically obstructing the binding site of the substrate 2. prefix is preventing the folding of the protein in the first place to create the binding site Slide 9, Trypsin Studies: (Enzyme and is a zymogen) There is a limited enzymatic activity. Removal of the prefix increases binding of the substrate to the active site. This suggests zymogen can fold on itself, therefore the prefix does not prevent folding. Removal of the prefix removes the obstruction of substrate to the binding site. Therefore, possibility #1 is the binding problem. This is called Steric Hinderance, which is the presence of the prefix getting in the way of the substrate being able to bind to the active site. Slide 10, Trypsin Activation: (diagram) The zymogen is called trypsinogen and is inactive, when the peptide is removed it changes to typsin which is the activated protein product. The protease is an enzyme called enterokinase, which is the activating protease which clips off the prefix at the N-terminus. When the peptide is removed the now end terminal unit can interact and open the active site to allow for binding on the enzyme. Slide 11, Advantages of Zymogen Activation: 1. it keeps harmful enzymes inactive until they are needed. Zymogen is important in activating digestive enzymes. These enzymes are dangerous and made in the pancreas and sent to the stomach. We need these enzymes to be inactive until they are needed for digestion. It is also a useful way to activate hormones. Hormones are made in the brain and released into the blood stream. Hormones are made as pro-hormones and are converted to the hormone by the removal of the prefix at the right time and place in development. 2 2. It permits rapid responses and avoids the time lag of RNA/protein synthesis. T
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