Lecture 5 - Carbohydrate Production

Front 1

Glycogen

Back 1

Carbohydrate store in animals (glucose) - muscle and liver. Remobilised to provide energy production or allow lover to regulate blood glucose.

Highly branched polymer - alpha 1,4 links (chains) and alpha 1,6 links (branches). One unit of glucose can be released at a time due to branches). Stored as granules, with metabolic enzymes

Front 2

The breakdown of glycogen

Back 2

Glycogen phosphorylase (G1P) - remove glucose from the branches - up to 4 units from end of a branch. Each unit released as glucose 1 phosphate units.

Transferase -- moves the three glucose units to another branch to be broken down by G1P.

One residue left - broken down b debranching enzyme in form of glucose

Front 3

Products of glycogen breakdown

Back 3

Glucose (debranching enzyme) - minor product

Glucose-1-phosphate - phosphorylase and transferase enzyme. Converted to glucose-6-phosphate by phosphogucomytase to enter glycolysis pathways. Can be converted to glucose by glucose-6-phosphate.

Front 4

Uses of glycogen breakdown products in liver and muscle

Back 4

Liver - glucose 6 phosphate converted into glucose and released into blood. Presence of glucose-6-phosphatase

Muscle - glucose 6 phosphate powers glycolysis directly, glucose-6-phosphatase not present

Front 5

Glycogen synthesis

Back 5

Glucose-6-phosphate converted to Glucose-1-phosphate by phosphoglucamatase

Converted to UDP-glucose by UDP-glucose pyrophosphorylase. Pyrophosphate released.

Glycogen unit added to produce glycogen using enzyme glycogen synthase. UDP released.

Front 6

The enzymes controlling breakdown and synthesis of glycogen

Back 6

Breakdown - glycogen phosphorylase

Synthesis - glycogen synthase

Control mediated by metabolites and hormones

Front 7

Glycogen breakdown in the liver

Back 7

Glucagon released by pancreas in times of fasting, detected by specific membrane receptors.

Binds to receptors which activates adenylyl cyclase converting ATP to cAMP.

cAMP activates protein kinase A by binding to it. Active kinase A can add phosphate to inactive phosphorylase kinase-b forming active phosphorylase kinase - a (P). This adds phosphate to phosphorylase kinase - b forming phoshorylase - a (P). Catalyses conversion of glycogen to glucose -1-phosphate

Front 8

Glycogen breakdown in muscle

Back 8

Hormone used: adrenaline - adrenaline receptors on muscle. Same phosphorylation cascade occurs.

In both tissues - reversible covalent modification of enzyme occurs, covalent attachment of phosphate group. Enzymes switched on by phosphorylation - amplification occurs.

Front 9

How is the pathway switched off?

Back 9

Protein phosphatase 1 - inactivates the newly activated phosphorylase-a (P) enzyme. Reforms phosphorylase-b.

Occurs in both muscle and liver

Front 10

Metabolite and hormonal control of glycogen breakdown

Back 10

Phosphorylase kinase-a (P) activated by calcium ion, which signal for muscle contraction

AMP - activates phosphorylase-b. AMP rises as muscle contract as ATP used.

Front 11

Metabolite and hormonal control of glycogen synthesis

Back 11

Protein kinase a directly phosphorylates glycogen synthase (active enzyme) to form inactive glycogen synthase-P enzyme.

Same hormone can switch on glycogen breakdown and switch off glycogen synthesis - allows for perfect regulation.

Front 12

Control of metabolism in muscle contraction

Back 12

Exercise releases adrenaline, glycogen breakdown needed for glycolysis

Calcium ions (stimulate phosphorylase kinase) and AMP (stimulase phosphorylase -b) released from muscle.

Synchronises muscle contraction with glycogen breakdown

Front 13

Control of metabolism - blood glucose

Back 13

Low blood glucose - glucagon secreted, glycogen broken down in liver to release glucose

In times of plenty - phosphorylase a-(p) binds glucose in liver, inhibiting breakdown. Insulin levels high - promote uptake of glucose by GLUT4 and activated glycogen synthase. Glycogen phosphorylase activity low

Front 14

Glycolysis

Back 14

Degradation of 6C glucose to 3C pyruvate to produce ATP

Anaerobic, catabolic (6C to 3C) but also anabolic (biosynthetic intermediates to produce larger molecules).

Control - glucose transporters and control of enzymes

Front 15

The stages of glycolysis and enzymes used

Back 15

Glucose -> Glucose-6-phosphate by hexokinase -> fructose-6-phosphate -> fructose-1,6-bisphosphate by phosphofructokinase. Split into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

Glyceraldehyde-3-phosphate converted to 1,3diphosphoglycerate, then to phosphoenolpyruvate. This converted to pyruvate by pyruvate kinase

Front 16

Inhibitors of Phosphofructokinase

Back 16

Fructose-6-phosphate to fructose-1,6-bisphosphate

Inhibited by ATP, allosteric binding and lowering affinity for F6P. High energy - so switch off glycolysis. Opposed by AMP (ATP decreases so glycolysis needs to increase).

Inhibited by citrate, enhances ATP effects. In TCA cycle, signals abundance of biosynthetic precursors.

Front 17

Stimulators of phosphofructokinase

Back 17

Fructose 2,6-bisphosphate - allosteric. In liver glucagon acts to reduce levels of F26BP and switch off glycolysis (blood glucose released into blood).

Made by enzyme phosphofructokinase 2: F6P + ATP -> F26P2 + ADP

Front 18

Fructose-2,6,-bisphosphate

Back 18

Liver enzyme - inhibited by cAMP, dependent on phosphorylation.

Heart muscle - adrenaline stimulates production of fructose-2,6-bisphosphate and stimulates glycolysis to power muscle contraction. Stimulated by cAMP

Front 19

Hexokinase

Back 19

Glucose to glucose-6-phosphate

Inhibited: glucose 6 phosphate (own product). If phosphofructokinase inhibited then G6P rises, prevents unnecessary conversion of glucose

Isoenzymes - different affinities for substrates (km values). 3 have low Km values but isoenzyme IV (glucokinase) has high Km value (found in liver). Not inhibited by G6P

Front 20

Pyruvate kinase

Back 20

Converts phosphoenolpyruvate to pyruvate

Allosteric - ATP inhibits (abundance of energy). F1,6BP stimulates, keeping glycolysis occurring. If F1,6BP increases it inhibits phosphofructokinase, feeding forward to pyruvate kinase so glycolysis happens quicker.

Hormonal - glucagon stimulates cAMP production, phosphorylates pyruvate kinase and inhibit it.

Front 21

Chronic (longer term control of glycolysis)

Back 21

Change in permanent diet - changes in pathways or production of certain enzymes

Insulin promotes synthesis of some glycolytic enzymes

Glucagon promotes synthesis of some gluconeogenic enzymes (create glucose).

Front 22

Pyruvate dehydrogenase

Back 22

Pyruvate + NAD + CoA -> acetylCoA + NADH + CO2 (acetyl CoA for TCA cycle).

Decarboxylation, not reversible, multiple controls.

Reversible covalent modification - cAMP independent

Exists in an active and inactive form

Front 23

Regulation of pyruvate dehydrogenase

Back 23

Inactivation - phosphorylated by kinase, forming PDH-P. Kinase enzyme regulated by acetyl coA, NADH and CO2

Activated - dephosphorylation by phosphatase. Calcium ions released in muscle contraction activates phosphatase - more pyruvate converted in the TCA cycle.