Fermentation Pyruvate Decarboxylase Lecture

OUTLINE: Glycogenolysis Glycolysis Introduction & overview; 2 phases Phase I Phase II Summary: labeling studies, logic, energetics Other sugars Pasteur: Anaerobic vs Aerobic Fermentations Lactate pyruvate dehydrogenase Acetoacetate Acetoacetate decarboxylase Ethanol pyruvate decarboxylase alcohol dehydrogenase Fates of Pyruvate Pasteur effect No Oxygen (anaerobic) +O 2à growth ↑ & fermentation ↓ sometimes anaerobic conditions In 1857, Louis Pasteur was studying fermentation by yeast, which are facultative anaerobes. They can produce energy with or without oxygen. When oxygen is low, the product of glycolysis, pyruvate, is turned into ethanol and carbon dioxide. When oxygen is high, pyruvate is converted to acetyl CoA and completely oxidized. More ATP is made aerobically than anaerobically. Therefore, about 15 times more glucose is consumed anaerobically as aerobically. Oxygen available (aerobic) 1 Fates of Pyruvate Pasteur effect +O 2à growth ↑ & fermentation ↓ No Oxygen (anaerobic) In 1857, Louis Pasteur was studying fermentation by yeast, which are facultative anaerobes. They can produce energy with or without oxygen. When oxygen is low, the product of glycolysis, pyruvate, is turned into ethanol and carbon dioxide. When oxygen is high, pyruvate is converted to acetyl CoA and completely oxidized. More ATP is made aerobically than anaerobically. Therefore, about 15 times more glucose is consumed anaerobically as aerobically. The beginnings of biochemistry…… … Dr. Kornberg: Lecture 01.27.17 (2:52-9:22/11:27-13:30) Fates of Pyruvate No Oxygen (anaerobic) sometimes anaerobic conditions Fermentation to acetate and other hydrocarbons in some strict anaerobes Acetoacetate 2 Fermentation: Why? • Regenerates NAD+ for further glycolysis under anaerobic conditions • Generation of energy (ATP) without consuming oxygen • Reduction of pyruvate to another product, there is no net change in oxidation state of the sugars • The process is used in the production of food from beer to yogurt to soy sauce. R NAD+ (oxidized) R=Rib-PP-Rib-Ade Glycolysis can continue 2 Ethanol 2 Acetaldehyde 2 acetate Acetoacetate Fates of Pyruvate No Oxygen (anaerobic) sometimes anaerobic conditions Fermentation to acetate and other hydrocarbons in some strict anaerobes Acetoacetate 3 Fermentation: Lactic acid • Pathway in animals • Reduction of pyruvate to lactate, reversible • Highly thermodynamically favorable/reversible (DG°’ = –6 kcal/mol) • During strenuous exercise, lactate builds up in the muscle. – generally less than 1 minute • The lactate can be transported to the liver and converted to glucose there. Called the Cori cycle. – Requires a recovery time – high amount of oxygen consumption to fuel gluconeogenesis – restores muscle glycogen stores Fermentation: Lactic acid Mechanism Lactate dehydrogenase ① ② Stereo-specific ③ ④ ⑤ Acid catalysis 4 Fates of Pyruvate No Oxygen (anaerobic) sometimes anaerobic conditions Fermentation to acetate and other hydrocarbons in some strict anaerobes Acetoacetate Fermentation: Acetoacetate decarboxylase Cleavage of C-C bonds beta to carbonyl P P Strict anaerobe: Clostridium acetobutylicum In 1916, the enzyme, acetoacetate decarboxylase, was discovered by Chaim Weizmann….. on to serendipity! 5 Fates of Pyruvate No Oxygen (anaerobic) sometimes anaerobic conditions Fermentation to acetate and other hydrocarbons in some strict anaerobes Strict anaerobe: Clostridium acetobutylicum Chaim Weizmann Fermentation: Acetoacetate decarboxylase Dr. Kornberg: Lecture 02.01.17 (33:24-48:02)-Weizmann 6 Fates of Pyruvate No Oxygen (anaerobic) sometimes anaerobic conditions Fermentation to acetate and other hydrocarbons in some strict anaerobes Acetoacetate Fermentation: Ethanol • Two-step reduction of pyruvate to ethanol • Humans do not have pyruvate decarboxylase. • Humans express alcohol dehydrogenase for ethanol metabolism, but is largely used in the reverse reaction, then aldehyde dehydrogenase takes it to acetate (recall the different forms with different Km values for why some people get flush). • Both steps require cofactors. – pyruvate decarboxylase: Mg++ and thiamine pyrophosphate (TPP) – alcohol dehydrogenase: Zn++ and NAD+ • CO2 produced in the first step is responsible for: – carbonation in beer – dough rising when baking bread Cleavage of C-C bonds alpha to carbonyl 7 Fermentation: Pyruvate decarboxylase Vitamin B1 TPP is a Common Acetaldehyde Carrier Fermentation: Pyruvate decarboxylase TPP Is a Common Acetaldehyde Carrier :B-E • TPP forms a covalent bond with carbonyl carbon, forming an alcohol, and resulting in release of CO2. • TPP allows for resonance stabilization, and subsequent protonation of carbonyl carbon, then release from complex. 8 Fermentation: Pyruvate decarboxylase Cleavage of C-C bonds alpha to carbonyl Like LDH, highly thermodynamically favorable (DG°’ = –6 kcal/mol) This reaction pulls the entire fermentation pathway. Fermentation: Ethanol 34:08) Humans are missing the gene for this enzyme. 34:08) 9