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+ 2H2O+ 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 B2Niacin = Vitamin B3Pantothenic 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 Ca2+ and Mg2+

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)

O

SCoA

acetyl CoA

OO

O

O

Ooxaloacetate

CoASH

citrate synthase

OO

OH

O

O

O

O

citrate

OH2

+

• Claisen condensation• OAA must bind first before Acetyl-CoA (sequential

mechanism)• -32.2kJ

Aconitase Reaction

OO

OHO

O

OO

citrate

aconitase

OO

O

O

OO

OH

isocitrate

• Forms isocitrate• Goes through alkene intermediate (cis-aconitate)

– elimination then addition

• 13.3kJ

Isocitrate Dehydrogenase

OO

O

O

OO

OH

isocitrate

NAD NADH CO2

OO

OO

O

isocitrate dehydrogenase

alpha ketoglutarate

• All dehydrogenase reactions make NADH or FADH2 • Oxidative decarboxylation• -20.9kJ• Energy from increased entropy in gas formation

α-ketoglutarate dehydrogenaseOO

OO

O

alpha ketoglutarate

NAD NADHCoASH

CO2

OO

O

SCoA

succinyl CoA

alpha ketoglutaratedehydrogenase

• Same as pyruvate dehydrogenase reaction• Formation of thioester

– endergonic – driven by loss of CO2

• increases entropy• exergonic

• -33.5kJ

Succinyl CoA synthetase

OO

O

SCoA

succinyl CoA

GDP GTP CoASH

succinate

succinyl CoAsynthetase

OO

OO

• Hydrolysis of thioester – Releases CoASH– Exergonic

• Coupled to synthesis of GTP– Endergonic– GTP very similar to ATP and interconverted later

• -2.9kJ

Succinate dehydrogenase

OO

OO

HH

succinate

FAD FADH2

succinyl CoAdehydrogenase

OO

OO

fumarate

• Dehydrogenation• Uses FAD

– NAD used to oxidize oxygen-containing groups• Aldehydes• alcohols

– FAD used to oxidize C-C bonds– 0kJ

Fumarase

OO

OO

HHfumarate

OH2

OO

OO

OH

malate

fumarase

• Addition of water to a double bond• -3.8kJ

Malate Dehydrogenase

O

O

O

O

O

oxaloacetate

OO

OO

OH

malate

NAD NADH

malatedehydrogenase

• 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 glucoseATP NADH FADH2

Glycolysis 2 2 0PDC (X2) 0 2 0TCA (X2) 2 6 2

TOTAL 4 10 2After OxPhos

4 30 ATPs 4 ATPs