FERMENTATION: Anaerobic Glycolysis

CATABOLIC FATES OF PYRUVATE

HOMOLACTIC FERMENTATION Utilized by muscles when the demand for ATP is high and oxygen availability is low. ATP is rapidly regenerated compared to oxidative phosphorylation. The reaction is freely reversible

HOMOLACTIC FERMENTATION Net reaction: Glucose + 2ADP+ 2Pi 2 lactate + 2ATP+ 2H 2 O+ 2H + Lactate formed can either exported from the cell or converted back to pyruvate The lactate formed in muscles is carried by the blood to the liver, where it is converted to glucose

CORI CYCLE

ALCOHOLIC FERMENTATION The NAD+ regenerated in this reaction will be utilized by GAPDH TPP is an important cofactor of Pyruvate decarboxylase

ALCOHOLIC FERMENTATION

GLYCOLYSIS AND CANCER Utilization of glucose and glycolysis proceed faster in cancer cells Because of hypoxia, cancer cells depend on anaerobic glycolysis for ATP production Tumor cells also have smaller amount of mitochondria Some tumor cells overproduce several glycolytic enzymes due to the presence of HIF-1 HIF-1 acts at the level of mRNA synthesis to stimulate the production of at least 8 glycolytic enzymes

Entry of other sugars: Lactose

Entry of other sugars: Fructose

SYNTHESIS OF ACETYL Co-A Pyruvate dehydrogenase complex is composed of 3 enzymes and requires 5 coenzymes

PYRUVATE DEHYDROGENASE COMPLEX E1: pyruvate dehydrogenase (30 heterodimers) E2: dihydrolipoamide transacetylase (20 trimers) E3: dihydrolipoamide dehydrogenase (12 dimers) ~10,000 kD

PYRUVATE DEHYDROGENASE COMPLEX

Thiamine = Vitamin B1 (ribo)Flavin = Vitamin B2 Niacin = Vitamin B3 Pantothenic Acid = Vitamin B5

REGULATION OF THE COMPLEX The eukaryotic complex contains two regulatory enzymes: a kinase that phosphorylates three serine residues in E1 and the phosphatase that removes those phosphates The activity of the complex is controlled by allosteric inhibition and covalent modification that is in turn controlled by the energy state of the cell. ATP is an allosteric inhibitor of the complex; AMP is an activator E2 is inhibited by acetyl-CoA and activated by CoA-SH E3 is inhibited by NADH and activated by NAD +

REGULATION OF THE COMPLEX Regulation also occurs by covalent modification of E1 (de/phosphorylation) NADH and acetyl-CoA activate the pyruvate dehydrogenase kinase which phosphorylates the 3 specific serine residues in E1 rendering it inactive Pyruvate dehydrogenase phosphatase removes the phosphate groups. This enzyme is activated by Ca 2+ and Mg 2+

KREBS CYCLE Aka tricarboxylic acid cycle and citric acid cycle central oxidative pathway Composed of 8 reactions that oxidizes acetyl CoA to 2 molecules of CO2 Occurs in the mitochondrial matrix

Citrate Synthase Reaction (First) Claisen condensation OAA must bind first before Acetyl-CoA (sequential mechanism) -32.2kJ

Aconitase Reaction Forms isocitrate Goes through alkene intermediate (cis-aconitate) – elimination then addition 13.3kJ

Isocitrate Dehydrogenase All dehydrogenase reactions make NADH or FADH 2 Oxidative decarboxylation -20.9kJ Energy from increased entropy in gas formation

α-ketoglutarate dehydrogenase Same as pyruvate dehydrogenase reaction Formation of thioester – endergonic – driven by loss of CO 2 increases entropy exergonic -33.5kJ

Succinyl CoA synthetase Hydrolysis of thioester – Releases CoASH – Exergonic Coupled to synthesis of GTP – Endergonic – GTP very similar to ATP and interconverted later -2.9kJ

Succinate dehydrogenase Dehydrogenation Uses FAD – NAD used to oxidize oxygen-containing groups Aldehydes alcohols – FAD used to oxidize C-C bonds – 0kJ

Fumarase Addition of water to a double bond -3.8kJ

Malate Dehydrogenase Oxidation of secondary alcohol to ketone Makes NADH Regenerates oxaloacetate for another round 29.7 kJ

REGULATION OF KREBS CYCLE 3 rate determining enzymes: citrate synthase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase 3 mechanisms used by the enzymes: – Substrate availability (acetyl CoA and oxaloacetate) – Product inhibition (NADH) – Competitive feedback inhibition by intermediates (citrate and succinyl CoA ADP is an effector of isocitrate dehydrogenase Ca2+ activates pyruvate dehydrogenase phosphatase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase

Counting ATP’s: one molecule of glucose ATPNADHFADH2 Glycolysis220 PDC (X2)020 TCA (X2)262 TOTAL4102 After OxPhos 430 ATPs4 ATPs