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Chapter 9

Chapter 9.docx

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Ingo Ensminger

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Chapter 9 Page 189-200 9.3 glycolysis  Discovered by accident  1890s- Hans and Edward Buchner added sucrose( used as preservative) to yeast extract  Instead preserving yeast extract, sucrose was broken down and fermented and formed alcohol.  Fermentation and other type of cellular metabolism could be studied in vitro  Glucose and pyruvate are phosphorylated  Each step in glycolysis is catalyzed by different enzyme A closer look at glycolysis reactions  All 10 reactions of glycolysis occur in cytosol  ATP production during glycolysis occurs by substrate- level phosphorylation  For each glucose molecule, the net yield:  2 NADH  2 ATP  2 pyruvate How is glycolysis regulated?  High level of ATP inhibits a key glycolytic enzyme phosphofructokinase.  Phosphofructokinase catalyzes step 3  High level of ATP signals that cell doesn’t need to produce more ATP  Feedback inhibition occurs when an enzyme in a pathway is inhibited by the product of the reaction sequence  cells that are able to stop the reaction when ATP is abundant can store glucose  as a result natural selection favors individuals with phosphofructokinase How high levels of substrate inhibit the enzyme?  Phosphofructokinase has 2 binding sites for ATP  ATP can bind to active site or at a site that changes the enzyme’s activity – regulatory site.  When the ATP concentrations are high, the molecule binds to regulatory site and changes the conformation of enzyme in a way that decreases reaction rate (ATP acts as allosteric regulator) 9.4 pyruvate processing  In eukaryotes, Pyruvate is transported from cytosol to mitochondria  Mitochondria has 2 membranes and the interior is filled with sac like structures called cristae  The region inside the inner membrane but outside the cristae is called mitochondrial matrix  Pyruvate moves across outer membrane of mitochondria into intermembrane space through small pores  Entry into matrix occurs through active transport (though membrane protein called pyruvate carrier located in inner membrane)  What happens when pyruvate enters mitochondria?  Coenzyme A (CoA) serves as a cofactor  Cofactors are often a metal ion or a relatively small organic molecules called coenzymes.  Coenzymes bind to the active site and stabilize the reaction’s transition state  CoA act as coenzyme by accepting and then transferring an acetyl group to a substrate  In CoA, A stands for acetylation.  Acetyl CoA forms, when acetyl binds to sulfur atom on one end of the CoA.  Pyruvate reacts with CoA to form acetyl CoA.  This process occurs in an enzyme complex called pyruvate dehydrogenase.  Eukaryotes: pyruvate dehydrogenase is located in the matrix.  Prokaryotes: pyruvate dehydrogenase is located in cytosol.  The process 1. One of the C from pyruvate oxidized to CO 2 + 2. NAD reduced to NADH 3. Remaining 2 carbon acetyl molecule transferred to CoA.  Acetyl CoA is the final product of the pyruvate processing step.  When ATP is abundant, feedback inhibition takes place.  The pyruvate processing stops and the complex becomes phosphorylated  Phosphorylation changes the shape of the complex  And stops the reaction +  High concentration of NAD , CoA or AMP speeds up the reaction catalyzed by the pyruvate dehydrogenase complex. 9.5 Krebs cycle  Acetyl CoA is the key compound that feeds the Krebs cycle  Acetyl CoA reacts with oxaloacetate and forms citrate  Figure 9.19 (page 195)  Energy released by the oxidation of one molecule of acetyl CoA is used to produce 3 molecules of NADH one of FADH2 and one of the guanosine triphosphate (GTP) though substrate level phosphorylation  Prokaryote: enzymes responsible for pyruvate processing and Krebs cycle are located in cytosol  Eukaryotes: enzymes responsible for for pyruvate processing and Krebs cycle are located in mitochondrial matrix How is Krebs cycle regulated?  Krebs cycle is regulated by feedback inhibition  Feedback inhibition occurs at 2 points in the cycle. (figure 9.20 page 196)  At first point, NADH binds to the enzyme’s active site (example of competitive inhibition)  At second point, ATP binds to allosteric regulatory site.  Krebs cycle slows down when ATP and NADH are plentiful and ATP acts as an allosteric and NADH acts as competitive inhibitor. What happens to NADH and FADH ? 2  Each molecule of glucose that’s fully oxidized to 6
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