Fermentation Catabolism of Other Sugars

OUTLINE: Glycogenolysis Glycolysis Introduction & overview; 2 phases Phase I Phase II Summary: logic, energetics, labeling studies Other sugars Pasteur: Anaerobic vs Aerobic Fermentations Lactate pyruvate dehydrogenase Ethanol pyruvate decarboxylase alcohol dehydrogenase Acetoacetate decarboxylase Catabolism of Other Sugars •Ingestion yields free glucose from glycogen and starch by a-amylase, maltase, and isomaltase •In the cell, glucose molecules are cleaved from glycogen and starch by glycogen phosphorylase. – yielding glucose 1-phosphate – uses inorganic phosphate for lysis (phosphoro-lysis) •Disaccharides are hydrolyzed. – – – – lactose: glucose and galactose sucrose: glucose and fructose trehalose: glucose Monosaccharides fructose, galactose, and mannose enter glycolysis at different points. 1 Catabolism of Other Sugars Glc(a1àa1)Glc hexokinase hexokinase PGI hexokinase PFK-1 triose phosphate isom erase aldolase Pyruvate Catabolism of Other Sugars Glc(a1àa1)Glc Oligosaccharide Biosynthesis hexokinase hexokinase UDP-mannose PGI hexokinase a -Mannose 1-phosphate p ho sp ho m a n n o se m u ta se PFK-1 triose phosphate isom erase aldolase Pyruvate 2 Catabolism of Other Sugars ATP ADP Galactokinase • Use of nucleotide sugars discovered by Argentinian biochemist Luis Leloir in 1940 for which he received the Nobel Prize. • Defects in the human genes for these enzymes causes what are called inborn errors in metabolism. • The cause of one of these due to galactose ingestion was one of the first discovered in 1956 by Herman Kalckar. • Galacotsemia causes enlarged liver, cirrhosis, kidney failure, cataracts, hypoglycemia, lethargy, and brain damage. Without treatment, mortality is about 75%. • Defects in transferase are the underlying cause, with incidence of about 1:60,000 births. Catabolism of Other Sugars ATP ADP Galactokinase • Use of nucleotide sugars discovered by Argentinian biochemist Luis Leloir in 1940 for which he received the Nobel Prize. • Defects in the human genes for these enzymes causes what are called inborn errors in metabolism. • The cause of one of these due to galactose ingestion was one of the first discovered in 1956 by Herman Kalckar. • Galacotsemia causes enlarged liver, cirrhosis, kidney failure, cataracts, hypoglycemia, lethargy, and brain damage. Without treatment, mortality is about 75%. • Defects in transferase are the underlying cause, with incidence of about 1:60,000 births. 3 Fates of Pyruvate Pasteur effect No Oxygen (anaerobic) +O 2à growth ↑ & fermentation ↓ 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) Fates of Pyruvate No Oxygen (anaerobic) Oxygen available (aerobic) 4 Fermentation • 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. 2 Acetaldehyde 2 Ethanol 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 5 Fermentation: Lactic acid Mechanism Lactate dehydrogenase ① ② Stereo-specific ③ ④ ⑤ Acid catalysis Fermentation: Lactic acid Mechanism Lactate dehydrogenase ① ② Stereo-specific ③ ④ ⑤ Acid catalysis 6 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 (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 Fermentation: Pyruvate decarboxylase Vitamin B1 TPP Is a Common Acetaldehyde Carrier 7 Fermentation: Pyruvate decarboxylase :B-E • TPP forms a covalent bond with carbonyl carbon, forming an alcohol, and resulting in release of CO2. • TPP then allows rearrangement of and protonation of carbonyl carbon to release from complex. 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 then allows rearrangement of and protonation of carbonyl carbon to release from complex. 8 Fermentation: Pyruvate decarboxylase Like LDH, highly thermodynamically favorable (DG°’ = –6 kcal/mol) This reaction pulls the entire fermentation pathway. Fermentation: Alcohol Dehydrogenase (ADH) Base catalysis Acid catalysis 9 Fermentation: Alcohol Dehydrogenase (ADH) Base catalysis Acid catalysis 10