Glycolysis and Gluconeogenesis Glycolysis n What is glycolysis

Glycolysis and Gluconeogenesis

Glycolysis n What is glycolysis? n sequence of reactions that converts one molecule of glucose to two molecules of pyruvate with the formation of two ATP molecules n anaerobic

Glycolysis n Why is glucose such a commonly used fuel? n tends to exist in ring form, very stable, doesn’t generally glycosylate proteins n formed from formaldehyde under prebiotic conditions

Glycolysis n What are the possible fates of glucose?

Glycolysis n What’s the difference between a facultative anaerobe and an obligate anaerobe? n Can you give an example of habitatdependent anaerobiosis? n What about activity-dependent anaerobiosis?

Glycolysis n All the intermediates in glycolysis have either 3 or 6 carbon atoms n All of the reactions fall into one of 5 categories n n n phosphoryl transfer phosphoryl shift isomerization dehydration aldol cleavage

Glycolysis n Entire reaction sequence may be divided into three stages n glucose is trapped and destabilized n six carbon molecule is split into two three carbon molecules n ATP is generated

Glycolysis – Stage 1 n glucose converted to glucose-6 -PO 4 n ATP is needed n catalyzed by hexokinase or glucokinase n induced fit n G 01= -4. 0 kcal/mole

Glycolysis – Stage 1 n phosphoglucoisomerase n aldose is converted to ketose n G 01=+0. 4 kcal/mole

Glycolysis – Stage 1 n rate limiting enzyme – allosteric n inhibited by high ATP, citric acid, long-chain fatty acids n stimulated by ADP or AMP n G 01= - 3. 4 kcal/mole

Glycolysis

Glycolysis – Stage 2 n six carbon molecule split into 2 - 3 carbon molecules n aldose and ketose n G 01=+ 5. 73 kcal/mole

Glycolysis – Stage 3 n At equilibrium most mixture exists as dihydroxyacetone phosphate n G 01=+ 1. 83 kcal/mole

Triose Phosphate Isomerase

Glycolysis – Stage 3 n redox reaction n energy from redox used to form acyl phosphate n G 01= +1. 5 kcal/mole

Glycolysis – Stage 3 n Consists of two coupled processes

Glycolysis – Stage 3 n formation of ATP – substrate level phosphorylation

Glycolysis – Stage 3 n phosphoryl shift – uses 2, 3 bisphoglycerate G 01= +1. 1 kcal/mole n dehydration G 01 = +. 44 kcal/mole n phosphoryl transfer G 01 = -7. 5 kcal/mole

Glycolysis

Fate of Pyruvate

Alcoholic Fermentation n Which organisms carry out this process? n yeast n other microorganisms n PDC requires thiamine pyrophosphate as coenzyme n NAD+ is regenerated

Lactic Acid Fermentation n Occurs in muscle cells, microorganisms n Regenerates NAD+

NAD+ and Dehydrogenases n Various dehydrogenases have a similar binding domain for NAD+ showing their common origin n Rossman fold

Glycolysis n How can fructose be used for energy?

Glycolysis n To use galactose it must be converted to glucose-6 -PO 4

Glycolysis

Glycolysis n What causes lactose intolerance?

Glycolysis n What is galactosemia? n inability to metabolize galactose n missing galactose 1 -phosphate uridyl transferase n liver disease n development of cataracts n CNS malfunction

Control of Glycolysis n Of what value is glycolysis for cells? n provides energy in form of ATP n provides building blocks for synthetic reactions n Where are most control points found? n enzymes that catalyze irreversible reactions n n n hexokinase phosphofructokinase pyruvate kinase

Phosphofructokinase n Most important control point in mammalian glycolytic pathway n allosteric enzyme n activated by AMP and fructose 2, 6 bisphosphate n inhibited by high levels of ATP, citrate, fatty acids

Phosphofructokinase

Hexokinase n Hexokinase is inhibited by its product glucose-6 -PO 4 n glucose remains in blood n Glucokinase, an isozyme of hexokinase is not inhibited by glucose-6 -PO 4 n found in liver n has lower affinity for glucose

Pyruvate Kinase n Pyruvate kinase exists as isozymes n L form – predominates in liver n M form – mostly in muscle and brain n PK is an allosteric enzyme n activated by fructose 1, 6 bisphosphate n inhibited by ATP, alanine n L form of PK influenced by covalent modification n inhibited by phosphorylation

Pyruvate Kinase

Glucose Transport n What is the role of glucose transporters in animal cells? n facilitate movement of glucose across cell membrane n What kind of molecule is a transporter and where is it located? n small protein embedded in plasma membrane

Glucose Transport n mammalian glucose transporter

Glucose Transport

Glycolysis and Cancer n Why are rapidly growing tumor cells dependent upon glycolysis? n insufficient oxygen supply n What is the function of HIF-1? n hypoxia-inducible transcription factor stimulates synthesis of many glycolytic enzymes and GLUT-1 and 3 n also stimulates vascular endothelial growth factor

Gluconeogenesis n What is gluconeogenesis? n synthesis of glucose from non-carbohydrate precursors n Why is this an important pathway? n What are some of the major precursors? n lactate, amino acids, glycerol n Where does this process occur? n liver, kidney

Gluconeogenesis n If gluconeogenesis involves the conversion of pyruvate to glucose why is it not simply the reverse of glycolysis? n glycolysis contains several irreversible reactions n Which reactions in glycolysis are irreversible? n phosphoenolpyruvate to pyruvate n fructose 6 -phosphate to fructose 1, 6 bisphosphate n glucose to glucose 6 -phosphate

Gluconeogenesis n What is the first reaction?

Gluconeogenesis n Why is pyruvate carboxylase of special interest? n structural properties n contains ATP-grasp domain at N-terminal end n contains biotin-binding domain at C-terminal end

Gluconeogenesis n What is the role of biotin in this reaction? n prosthetic group lined to -amino group of lysine residue n carrier of activated carbon dioxide

Gluconeogenesis n Pyruvate carboxylase is an allosteric enzyme activated by acetyl Co. A n needed to form carboxybiotin n

Gluconeogenesis n Carboxylation of pyruvate occurs in the mitocondria but next step in reaction sequence occurs in cytosol

Gluconeogenesis Decarboxylation of oxaloacetate is coupled with phosphorylation by GTP enzyme is phosphoenolpyruvate carboxykinase

Gluconeogenesis n Which other steps in glycolysis are irreversible? n conversion of fructose 1, 6 -bisphosphate to fructose 6 -phosphate n conversion of glucose 6 -phosphate to glucose

Gluconeogenesis n G° = -16. 7 k. J mol-1 n fructose-1, 6 -bisphosphatase is an allosteric enzyme, inhibited by AMP and activated by ATP

Gluconeogenesis n Enzyme that catalyzes last reaction not found in all tissues n liver and kidney cortex

Gluconeogenesis n Is gluconeogenesis an energetically favorable reaction in the cell? n What drives this reaction? n Are glycolysis and gluconeogenesis active at the same time?

Regulation of Glycolysis and Gluconeogenesis n What are some of the factors that ensure the reciprocal regulation of these processes? n allosteric regulators of key enzymes n energy charge n fructose 2, 6 -bisphosphate n hormones

Regulation of Glycolysis and Gluconeogenesis

Regulation of Glycolysis and Gluconeogenesis n fructose 2, 6 -bisphosphate stimulates PFK and inhibits fructose 1, 6 bisphosphase n controlled by insulin and glucagon and reflects the nutritional status of the cell

Regulation of Glycolysis and Gluconeogenesis n How do hormones influence the enzymes associated with these processes? n influence gene expression n change transcription rate n influence degradation of m-RNA – insulin PFK, PK – glucagon PEPCK, fructose 1, 6 -bisphosphatase

Regulation of Glycolysis and Gluconeogenesis n What are substrate cycles and why are they important? can amplify metabolic signals n can generate heat n

Regulation of Glycolysis and Gluconeogenesis n What is the Cori cycle and why is it important?

Regulation of Glycolysis and Gluconeogenesis