Glycolysis and Glucose Oxidation

Question 1

How is glucose taken up by cells?

Answer 1

via GLUT receptors

Question 2

What is a ‘Km value’ and what does it mean?

Answer 2

indicative of the affinity of a glucose transporter to glucose, a high Km indicates a low affinity and a low Km indicates a high affinity

Question 3

Describe the GLUT1 receptor

Answer 3

low Km value (high affinity) and is found on erythrocytes and most cell membranes and provides basal glucose transport to cells at constant rate

Question 4

Describe the GLUT2 receptor

Answer 4

found in liver and beta cells of pancreas and have a higher Km than GLUT1 (lower affinity) and these receptors are only active when there is a high blood glucose level (fed-state)

Question 5

Describe the GLUT4 receptor

Answer 5

found in muscle and fat cells which ‘store’ these transporters in vesicles and release these in response to insulin to promote glucose uptake and have a low affinity for glucose ordinarily

Question 6

Where is the hexokinase enzyme found?

Answer 6

in most tissues

Question 7

Where is the glucokinase enzyme found?

Answer 7

liver (and the pancreas)

Question 8

What is the role of hexokinase?

Answer 8

phosphorylates glucose to glucose-6-phosphate to prevent it from leaving the cell

Question 9

What is the role of glucokinase?

Answer 9

converts glucose to glucose-6-phosphate

Question 10

How is the action of hexokinase controlled?

Answer 10

inhibited by it’s product; glucose-6-phosphate

Question 11

Describe the function of phosphofructokinase-1 (PFK1)

Answer 11

phosphorylates fructose-6-phosphate to fructose-1,6-phosphate using ATP which is irreversible and is the rate determining step of glycolysis

Question 12

Describe how ATP is produced by substrate level phosphorylation in glycolysis

Answer 12

conversion of 1,3-BPG to 3-PG produces ATP and the conversion of PEP to pyruvate produces ATP

Question 13

What is the net production of ATP per glycolysis cycle?

Answer 13

4 ATP produced, 2 ATP used –> 2 ATP

Question 14

How does glycerol enter glycolysis?

Answer 14

IN THE LIVER: glycerol is converted to glyceraldehyde-3-phosphate by glycerol kinase (using ATP) and this GAP is then converted into DHAP (via reduction of NAD+ –> NADH) and enters into the glycolysis pathway directly

Question 15

How does fructose enter glycolysis?

Answer 15

Can be metabolised by hexokinase to form fructose-6-phosphate to enter glycolysis direcly.
More often fructose –> fructose-1-phosphate (by fructokinase in the liver) –> converted to DHAP and glyceraldehyde –> both form GAP which enters the glycolysis pathway (converted by aldolase B)

Question 16

Why can fructose be metabolised by hexokinase?

Answer 16

Due to low specificity of enzyme, mistakes it for glucose

Question 17

Outline the potential fates of the pyruvate produced by glycolysis

Answer 17

aerobic respiration - converted to acetyl CoA by PDH –> TCA cycle –> ETC
anaerobic respiration - pyruvate converted back to lactate through lactate dehydrogenase to oxidise NADH back to NAD+

Question 18

How may the deficiency of vitamin B1 affect metabolism?

Answer 18

vitamin B1 is the vitamin from which the coenzyme TPP is derived which is involved in stimulating the pyruvate dehydrogenase complex, so it could prevent aerobic respiration from taking place (as pyruvate wouldn’t be converted to acetyl CoA, so couldn’t enter Krebs cycle)

Question 19

How is PDC regulated by allosteric control?

Answer 19

NADH (E3 enzyme competition) and acetyl CoA (E2 enzyme competition) compete for binding sites on the PDC enzymes to inhibit its action

Question 20

How is PDC regulated by reversible phosphorylation?

Answer 20

Active when non-phosphorylated and inactive when phosphorylated. PDH kinase is involved in phosphorylating and deactivating the complex and is stimulated by high levels of ATP as this signifies there is enough energy so TCA cycle doesn’t need to be entered. Conversely, PDH phosphatase dephosphorylates to activate the enzyme and allow aerobic respiration and this is stimulated by insulin and Ca2+

Question 21

Outline the role of NAD in glycolysis

Answer 21

NAD+ required to produce first ATP and this needs to be regenerated from the NADH product to feed the respiratory chain

Question 22

How does glucokinase differ from hexokinase?

Answer 22

It has a higher Km than hexokinase –> lower glucose affinity, but has a greater specificity for glucose. Hexokinase is sensitive to feedback inhibition (causes reduced glucose uptake) whereas glucokinase is not

Question 23

What is the location of the glucokinase enzyme controlled by?

Answer 23

Glucokinase regulator protein (GKRP)

Question 24

What stimulates the glucokinase regulator protein?

Answer 24

Promoted by the presence of glucose and fructose-1-phosphate –> stimulates glucokinase to convert glucose into glucose-6-phosphate but only at high blood glucose levels

Question 25

What is the rate-determining step of glycolysis?

Answer 25

The conversion of fructose-6-phosphate –> fructose-1,6-bisphosphate by PFK-1 as this is an irreversible step

Question 26

Which substances inhibit the activity of PFK-1?

Answer 26

High ATP levels inhibit PFK1 (no more energy needed)

Citrate inhibits PFK1 (signifies enough metabolic intermediates are present)

Question 27

Which substances promote the activity of PFK-1?

Answer 27

Increased levels of AMP and fructose-2,6-bisphosphate is strongest allosteric activator of PFK1

Question 28

What is the most potent allosteric activator of PFK-1 (and subsequently, glycolysis)?

Answer 28

Fructose-2,6-bisphosphate; this is formed by phosphorylation of fructose-6-phosphate but using PFK-2 instead of PFK-1

Question 29

Which enzymes require Mg2+ as cofactors in order to function?

Answer 29

All kinases

Question 30

Where does most glycerol come from?

Answer 30

Breakdown of fat stores in the body, however, some also comes from dietary sources

Question 31

What is the function of aldolase B?

Answer 31

Conversion of fructose-1-phosphate to DHAP and GAP which can enter glycolysis directly

Question 32

What is the function of the pyruvate dehydrogenase complex?

Answer 32

Oxidative carboxylation of pyruvate to acetyl CoA in the mitochondrial matrix

Question 33

What is the role of PDH phosphatase and what activates this enzyme?

Answer 33

Dephosphorylates, and thus activates the pyruvate dehydrogenase complex, and this enzyme is activated by Ca2+ and insulin (as these stimulate too much blood glucose –> need to use more up OR muscle respiration –> need to produce more energy)

Question 34

What is the role of PDH kinase, and what activates this enzyme?

Answer 34

Catalyses the phosphorylation, and thus, inactivation of the pyruvate dehydrogenase complex, and this enzyme is activated by an increase in the ATP:AMP (signifies enough energy), NADH and acetyl CoA (in addition to their allosteric effect)