Glycolysis

Question 1

Tissues that can only use glucose as a metabolic fuel:

Answer 1

brain, RBCs, renal medulla, cornea, testes, exercising muscle

Question 2

What is the end product of glycolysis?

Answer 2

2 pyruvates

Question 3

What occurs to the 2 pyruvates after glycolysis?

Answer 3

converted into 2 acetyl coAs, which enters the TCA cycle

Question 4

How does glucose enter cells?

Answer 4

• can’t simply diffuse into cells

- carried by either facilitated diffusion (GLUT transport proteins in membrane) or Na+-dependent co-transport (SGLT)

Question 5

What are the GLUT transporter isoforms and their locations?

Answer 5

• GLUT1: brain and RBCs
• GLUT2: hepatocytes
• GLUT3: neurons
• GLUT4: adipose tissue and skeletal muscle

Question 6

What is the phosphorylation of glucose to trap it in cells catalyzed by?

Answer 6

• Hexokinase: most cell types

- Glucokinase: liver and pancreatic islet cells

Question 7

Describe the enzymatic activity of glucokinase vs. hexokinase:

Answer 7

Hexokinase has a lower Km than glucokinase but also has a lower Vmax

Question 8

Functions of glucokinase:

Answer 8

• helps beta-cells in pancrease sense rising glucose concentrations in order to trigger insulin release
• allows liver to mop up high conc. of glucose in the portal circulation after a meal

Question 9

How are glucokinase and hexokinase regulated?

Answer 9

• glucokinase regulated by fructose-6-phosphate

- hexokinase regulated allosterically by glucose-6-phosphate

Question 10

What is the rate limiting step of glycolysis?

Answer 10

phosphofructokinase-1 (PFK1)

Question 11

What is the PFK1 in glycolysis regulated by?

Answer 11

• inhibited by ATP and citrate

- stimulated by AMP and fructose-2,6-bisphosphate

Question 12

What is the function of PFK2?

Answer 12

converts fructose-6-phosphate into fructose-2,6-bisphosphate

Question 13

How does PFK-2 exist in the body?

Answer 13

exists in a bifunctional enzyme complex with FBP-2–activity controlled by phosphorylation

Question 14

Describe the hormonal regulation of glycolysis:

Answer 14

1. high insulin/glucagon ratio–> decreased cAMP and reduced levels of active PKA
2. decreased PKA favors dephosphorylation of PFK2/FBP2
3. dephosphorylated PFK2 active whereas FBP2 is inactive–> favors formation of F2,6BP
4. elevated conc. of F2,6BP activates PFK1, leading to increased rate of glycolysis

Question 15

What happens to F2,6BP in response to insulin after a high carbohydate meal?

Answer 15

increases and therefore acts as a signal of high glucose levels, promoting glycolysis

Question 16

What happens after the formation of F1,6BP

Answer 16

• Aldolase A produces glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP)
• triose phosphate isomerase can interconvert G3P and DHAP

Question 17

How is ATP produced?

Answer 17

• Directly: substrate level phosphorylation

- Indirectly: oxidative phosphorylation (NADH+H and FADH2 generation)

Question 18

What is the approximate conversion of electron intermediates to ATP in the ETC?

Answer 18

• NADH+H–> 3 ATP

- FADH2–> 2 ATP

Question 19

Why is aerobic metabolism much more efficient than anaerobic metabolism?

Answer 19

allows the complete oxidation of fuel molecules to CO2 and H2O

Question 20

What is the net energy production from aerobic glycolysis?

Answer 20

• glucose–> pyruvate
• 2 ADP–> 2 ATP
• 2 NAD+–> 2 NADH

Question 21

What is the net energy production from anaerobic glycolysis?

Answer 21

• glucose–> 2 lactate

- 2 ADP–> 2 ATP

Question 22

What is the total max ATP that can be produced from one molecule of glucose?

Answer 22

38 ATP

Question 23

Lactic acidosis:

Answer 23

• elevated plasma lactic acid secondary to: circulatory collapse (MI, PE, hemorrhage) or shock
• potentially fatal

Question 24

Where does the TCA cycle occur?

Answer 24

in the mitochondrial matrix

Question 25

Pyruvate dehydrogenase

Answer 25

converts pyruvate to acetyl coA in the matrix (irreversible reaction, glucose cannot be made from acetyl coA)

Question 26

What is pyruvate dehydrogenase regulated by?

Answer 26

- regulated by phosphorylation (PDH kinase/phosphatase activity) - ATP, acetyl coA and NADH promote phosphorylation--> inactive - pyruvate levels inhibit PDH kinase activity--> promoting active form - Ca2+ promotes dephosphorylation (activation)

Question 27

Pyruvate dehydrogenase complex

Answer 27

- a multi-enzyme complex - three distinct enzyme activities: pyruvate decarboxylase, dihydrolipoyl transacetylase, dihydrolipoyl dehydrogenase - needs 5 coenzymes - intermediates passed from one active site to the next without being released

Question 28

Describe the PDH complex reactions

Answer 28

1. pyruvate decarboxylated into a hydroxyethyl derivative bound to the reactive carbon of thiamine pyrophosphate, the coenzyme of pyruvate dehydrogenase 2. the hydroxyethyl intermediate is oxidized by transfer to the disulfide form of lipoic acid covalently bound to dihydrolipoyl transacetylase 3. the acetyl group, bound as a thioester to the side chain of lipoic acid, is transferred to CoA 4. the sulfhydryl form of lipoic acid is oxidized by FAD-dependent dihydrolipoyl dehydrogenase, leading to the regeneration of oxidized lipoic acid 5. the reduced flavoprotein is reoxidized to FAD by dihydrolipoyl dehydrogenase as NAD+ is reduced

Question 29

Pyruvate dehydrogenase deficiency: what is it, symptoms, treatment

Answer 29

- x-linked disease resulting in congenital lactic acidosis - pyruvate converted to lactic acid instead of acetyl coA - Symptoms: developmental defects (esp. of CNS), muscular spacticity, early death - no effective therapy exists

Question 30

Arsenic poisoning

Answer 30

- arsenic is an inhibitor of enzymes that use lipoic-acid as a cofactor - inhibits pyruvate dehydrogenase - symptoms: neurological disturbances and death

Question 31

What vitamins do the following coenzymes require for their synthesis?

Answer 31

- CoA--> panthotenic acid (B5) - NAD--> niacin (B3) - FAD--> riboflavin (B2) - TPP--> thiamine (B1)

Question 32

Symptoms of vitamin deficiencies affecting pyruvate dehydrogenase and its coenzymes:

Answer 32

- increased pyruvate, lactate and alanine levels (pyruvate reduced to lactate or transaminated to alanine) - severe lethargy and fatigue - complications affecting the cardiovascular, nervous, muscular and GI systems - examples: Wernicke-Korsakoff and Beri Beri

Question 33

Hereditary myopathy with lactic acidosis: presentation, symptoms, biochemical features

Answer 33

- presentation in childhood - symptoms: muscle becomes hard and tender with exercise, cramping, dyspnea, heart palpitations - Biochemical features: conc. of lactate and pyruvate in the blood increased disproportionately for the workload (increased lactate and pyruvate w/ exercise, increased arterial alanine conc. during exercise in patients)

Question 34

What are the biochemical defects in hereditary myopathy and lactic acidosis?

Answer 34

- succinate dehydrogenase and iron-sulphur proteins in muscle mitochondria - markedly reduced capacity of TCA cycle and Ox-phos - increased production of lactate by muscle glycolysis - high levels of pyruvate are converted to alanine