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BIOL201: Biochemistry > Glucose metabolism > Flashcards

Glucose metabolism Flashcards

1
Q

How is glucose obtained from diet, digested, absorbed, and then stored in the liver?

A

Digestion: salivary and pancreatic amylases hydrolyses the a1,4 glycosidic bonds in amylose and amylopectin of starch, intestinal epithelium secretes maltase, sucrase and lactase to breakdown disaccharides into monosaccharides, such as glucose.

Absorption: enterocytes: SGLT1 (cotransport of glucose and Na+ - Na+ kept low in cell by basolateral Na+/K+ ATPase), GLUT2 (facilitated diffusion across basolateral membrane into interstitial space the blood), GLUT5 (facilitated diffusion of fructose into enterocyte)

Liver: glucose transport to liver by hepatic portal vein and taken up into hepatocyte by GLUT2 facilitator, where it is phosphorylated to glucose-6-phosphate

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2
Q

List the steps of glycogenesis

A

1) Glucose 6 phosphate –> glucose 1 phosphate
2) glucose 1 phosphate –> UDP-glucose
3) high energy bond of UDP-glucose is broken and glucose moiety is added to existing glycogen by 1,4 glycosidic bond
4) 1, 6 glycosidic branches are formed by cleaving off linked glucose moieties and transferring them to a C6 further up the chain

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3
Q

name the enzymes involved in the 4 steps of glycogenesis

A

1) phosphoglucomutase
2) UDP-glucose pyrophosphorylase
3) glycogen synthase
4) amylo(1,4–>1,6)transglycosylase

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4
Q

List the steps of glycogenolysis

A

1) glycogen phosphorylase breaks 1,4 bond between terminal glucoses to loberate glucose-1-phosphate until within 4 glucose residues from a branch point.
2) three end residues transferred to another branch by oligo(a1,4–>a1,4)glucantransferase.
3) remaining glucose removed by a(1-6)glucosidase.
4) glucose 1 phosphate –> glucose 6 phosphate by phosphoglucomutase

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5
Q

What happens to glucose 6 phosphate in the muscle cells?

A

can be channelled into glycolysis to produce ATP

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6
Q

What happens to glucose 6 phosphate in the liver and kidney cells?

A
  • can be channelled into glycolysis to produce ATP

- can be converted into glucose by glucose-6-phosphatase and released into the blood when low

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7
Q

What are the two ways in which glycogenesis and glycogenolysis reciprocally regulated? give details about the molecules involved and how this increases or decreases enzyme activity.

A

Allosterically by glucose-6-phosphate (high G6P reflects high blood sugar):

  • binding of G6P to glycogen synthase increases activity
  • binding of G6P to glycogen phosphorylase decreases activity

Reversible phosphorylation under hormonal regulation (peptide hormones insulin, glucagon and adrenaline)

  • Insulin signalling activates protein phosphatases:
    - hypophosphorylation of glycogen synthase = active (stores glucose when levels are high)
    - hypophosphorylation of glycogen phosphorylase = inactive
  • Glucagon and adrenaline signalling activates cAMP dependent protein kinases:
    - hyperphosphorylation of glycogen synthase = inactive
    - hyperphosphorylation of glycogen phosphatase = active (liberation of glucose when levels are low or increase free energy for fight or flight response).
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8
Q

What are the 5 steps of the glycolysis investment phase? name the enzyme at each step (specifically 1 and 3)

A

1) glucose to glucose-6-phosphate
- enzyme = hexokinase (muscle), glucokinase (liver)

2) glucose-6-phosphate to fructose-6-phosphate
- enzyme = glucose-6-phosphate isomerase

3) fructose-6-phosphate to fructose-1,6-bisphosphate
- enzyme = phosphofructokinase/PFK

4) fructose-1,6-bisphosphate to glycerate-3-phosphate and DHAP
- enzyme = fructose-1,6-bisphosphate aldolase

5) DHAP to glycerate-3-phosphate
- enzyme = triosephosphate isomerase

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9
Q

How many glycerate-3-phosphate molecules are produced at the end of the investment phase of glycolysis?

A

2

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10
Q

What are the 5 steps of the glycolysis pay-off phase? name the enzyme at each step (specifically 6, 7, and 10)

A

6) Glycerate-3-phosphate to 1,3-bisphosphoglycerate
- enzyme = glyceraldehyde-3-phosphate dehydrogenase

7) 1,3-bisphosphoglycerate to 3-phosphoglycerate
- enzyme = phosphoglycerate kinase

8) 3-phosphoglycerate to 2-phosphoglycerate
- enzyme = phosphoglyceromutase

9) 2-phosphoglycerate to phosphoenolpyruvate (PEP)
- enzyme = enolase

10) phosphoenolpyruvate (PEP) to pyruvate
- enzyme = pyruvate kinase

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11
Q

Which steps are the major regulatory control points of glycolysis?

A

steps:

1) glucose to glucose-6-phosphate (by hexokinase/glucokinase)
3) fructose-6-phosphate to fructose-1,6-bisphosphate (by PFK)
10) PEP/phosphoenolpyruvate to pyruvate (by pyruvate kinase)

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12
Q

Which steps of the glycolysis investment phase hydrolyse ATP?

A

steps 1 and 3

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13
Q

how many times does the pay-off phase occur per glucose molecule?

A

twice (as 2 G3P’s are produced)

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14
Q

Which steps of the glycolysis pay-off phase produce ATP?

A

steps 7 and 10

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15
Q

What is the net production of ATP by glycolysis?

A

two ATP consumed in investment phase (steps 1 and 3)
two ATP produced in pay-off phase (steps 7 and 10) but since pay-off phase occurs twice per glucose as two G3P are produced, the net ATP production is:

2 ATP.

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16
Q

What is the net production of NADH? which step of glycolysis produces this?

A 
2 NADH (one for every pay-off phase, but it occurs twice as 2 G3P's are produced)
- step 6
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17
Q

What are the 4 metabolic fates of pyruvate produced in glycolysis?

A

1) amino acid synthesis - converted to alanine by transamination
2) gluconeogenesis - converted to oxaloacetate by carboxylation
3) reduced to lactate in oxygen deprived conditions by lactate dehydrogenase
4) conversion to acetyl-CoA through oxidative decarboxylation by pyruvate dehydrogenase - enters the citric acid cycle to produce ATP

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18
Q

What percentage of available energy in glucose is released by glycolysis?

A

about 3% = inefficient

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19
Q

Which steps of glycolysis are exergonic?

A

1, 3, 7, and 10.

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20
Q

glycolysis is energetically ___________, where as gluconeogenesis is energetically _______________

A

favourable

unfavourable

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21
Q

Which step of glycolysis is most tightly regulated? why?

A

3 because it is the first committed step to glycolysis

22
Q

What are the two regulatory methods of PFK regulation at step 3?

A
  • allosteric regulation

- indirect hormonal regulation

23
Q

How is PFK allosterically regulated?

A

Regulation relating to cells energy status
- indicators of high energy state or alternative metabolic/energy sources = inhibition of PFK to conserve glucose

Inhibited:

  • high ATP/AMP ratio
  • high NADH
  • citrate (amino acid metabolism)
  • long chain fatty acids
  • H+ in muscles (decreased pH)

Stimulated:

  • low high AMP/ATP ratio
  • fructose-2,6-bisphosphate
24
Q

Which enzymes are involved in the production of fructose-2,6-bisphosphate in the glycolysis regulatory off-shoot pathway?

A

PFK2/fructose bisphosphatase 2 bifunctional enzyme produces fructose-2,6-bisphosphate using PFK2, which can be reversed by fructose bisphosphatase 2

25
Q

How is PFK indirectly regulated by hormonal control?

A

the activity of PFK2/fructose bisphosphatase 2 bifunctional enzyme in the regulatory off-shoot are regulated by reversible phosphorylation under hormonal control = allow reciprocal control of PFK of glycolysis and fructose-1,6-bisphosphatase of gluconeogenesis

26
Q

What is the effect of insulin on enzymes at step 3?

A

insulin signalling when blood glucose levels are high = activates protein phosphatase 2A = dephosphorylate the PFK2/fructose bisphosphatase 2 bifunctional enzyme = increases PFK2 activity = more fructose-2,6-bisphospate effector = PFK stimulated (glycolysis occurs), fructose-1,6-bisphosphatase inhibited (prevents gluconeogenesis)

27
Q

What is the effect of glucagon on enzymes at step 3?

A

glucagon signalling when blood glucose levels are low = increases cAMP = activates cAMP dependent kinases = phosphorylation of PFK2/fructose bisphosphatase 2 bifunctional enzyme = increases fructose bisphosphatase 2 activity = reduction in fructose-2,6-bisphosphate effector = removes inhibition of fructise-1,6-bisphosphatase (stimulates gluconeogenesis), reduced stimulation of PFK (prevents glycolysis)

28
Q

What are the methods of hexokinase/glucokinase regulation at step 1?

A
  • enzyme kinetics
  • compartmentalisation
  • allosteric regulation
29
Q

How is step 1 regulated by enzyme kinetics?

A

Hexokinase has a low Km and Vmax - high affinity means that it binds to glucose before the glucokinase

Glucokinase has a high Km and Vmax - low affinity and saturates a higher concentrations to ensure that glucose is only phosphorylated in the liver when levels are high

30
Q

How is step 1 regulated by allostery?

A

hexokinase is subject to product inhibition by glucose-6-phosphate, where as glucokinase is not. This ensures that phosphorylation of glucose decreases when there is sufficient G6P and the excess can be stored as glycogen by liver glucokinase.

31
Q

How is step 1 regulated by compartmentalisation in high and low glucose situations?

A

low levels of glucose = gluconeogenesis occurs = increases levels of fructose-6-phosphate = increases affinity of interaction between glucokinase and regulator (with a nuclear localisation signal/NLS) to keep glucokinase in nucleus and prevent it accessing scarce glucose and breaking it down via glycolysis.

high levels of glucose = diffuses into hepatocyte through GLUT2 and into nucleus = disrupts binding between glucokinase and regulator = glucokinase moves into cytoplasm = phosphorylates glucose to G6P trapping it.

32
Q

What is the benefit of glucokinase regulation by compartmentalisation?

A
  • rapid mobilisation as it is not under transcriptional control (which would take too long)
33
Q

What are the methods of pyruvate kinase regulation at step 10?

A
  • allosteric regulation

- phosphorylation isomeric forms (M and L) under hormonal control

34
Q

How is step 10 regulated by allostery?

A

Downregulation of PK:

  • ATP (high energy state)
  • citrate (amino acid metabolism)
  • alanine (amino acid metabolism)
  • Long chain fatty acids (alternative energy)

Upregulation of PK = feed-forward control

  • Phosphoenolpyruvate (PEP) (glycolytic intermediate - product of step 9)
  • Fructose-1,6-bisphosphate (glycolytic intermediate - product of step 3)
35
Q

How is step 10 regulated by reversible phosphorylation under hormonal control?

A

the L-form pyruvate kinase (liver) is regulated by phosphorylation (the M-form isn’t)

glucagon signalling when glucose is low = activates cAMP dependent kinases = phosphorylation of PK = L-form pyruvate kinase inactivated = inhibits glycolysis

insulin signalling when glucose is high = activates phosphatases = dephosphorylation of PK = L-form pyruvate kinase activated = stimulates glycolysis

36
Q

what is gluconeogenesis?

A

The formation of glucose from non-carbohydrate sources

37
Q

Which steps of the glycolytic pathway require additional enzymes to reverse in gluconeogenesis?

A

1, 3, and 10

38
Q

Describe the reversal of glycolysis step 10 during gluconeogenesis? name the enzyme at each step and any energy expense
(5 steps)

A

1) pyruvate to oxaloacetate in the mitochondria
- enzyme = pyruvate carboxylase
- energy = uses 2 ATP

2) oxaloacetate to malate
- enzyme = mitochondrial malate dehydrogenase

3) malate moves into cytoplasm through malate transporter

4) malate to oxaloacetate in the cytoplasm
- enzyme = cytoplasmic malate dehydrogenase

5) oxaloacetate to PEP
- enzyme = phosphoenolpyruvate carboxykinase
- energy = uses 2 GTP

39
Q

What molecules are utilised in the reversal of glycolysis steps 9 to 4 in gluconeogenesis?

A 
2 ATP (reversal of step 7) - occurs twice
2 NADH (reversal of step 6) - occurs twice
40
Q

Describe the reversal of glycolysis step 3 during gluconeogenesis? name the enzyme

A

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

- enzyme = fructose-1,6-bisphosphatase

41
Q

Which step is the main regulatory control point of gluconeogenesis?

A

Step 3 : fructose-1,6-bisphosphatase

42
Q

What is the product of the reversal of glycolysis step 2 in gluconeogenesis? what are its two metabolic fates?

A

Glucose-6-phosphate

metabolic fates:

1) store are glycogen
2) converted into glucose (liver and kidney only as possess enzyme glucose-6-phosphatase)

43
Q

Describe the reversal of glycolysis step 1 during gluconeogenesis? name the enzyme and the location of this reaction

A

Glucose-6-phosphate to glucose

enzyme = glucose-6-phosphatase

location = liver and kidney only

44
Q

What four molecules are utilised during gluconeogenesis? how many of each and at which steps?

A

2 pyruvate at step 10
4 ATP at steps 10 and 7
2 GTP at step 10
2 NADH at step 6

45
Q

Why must gluconeogenesis and glycolysis be reciprocally controlled?

A

To prevent a costly futile cycle

46
Q

How is step 3 of glycolysis/gluconeogenesis reciprocally regulated?

A

Allosterically:

  • ATP and citrate inhibit PFK (inhibit glycolysis) and stimulate fructose-1,6-bisphosphatase (stimulate gluconeogenesis)
  • fructose-2,6,-bisphosphate stimulates PFK and inhibits fructose-1,6-bisphosphatase

Reversible phosphorylation under hormonal control:

  • insulin activates phosphatases = dephosphorylation of bifunctional enzyme = PFK2 more active = production of more fructose-2,6-bisphosphate = stimulates PFK to carry out glycolysis
  • glucagon activates cAMP dependent kinases = phosphorylation of bifunctional enzyme = fructose bisphosphatase 2 more active = less fructose-2,6-bisphosphate produced = reduced stimulation of PFK, removal of fructose-1,6-bisphosphatase inhibition to allow gluconeogenesis.
47
Q

Which enzyme catalyses the conversion of pyruvate to oxaloacetate in the reversal of glycolysis step 10 in gluconeogenesis?

A

Pyruvate carboxylase

48
Q

How is step 10 of glycolysis/gluconeogenesis reciprocally regulated?

A

Allosterically:

  • pyruvate carboxylase is stimulated by acetyl-CoA and ATP (high energy state so able to carry out gluconeogenesis)
  • pyruvate kinase is inhibited by ATP, alanine and citrate (high energy state and other energy sources so don’t need to break down glucose)
49
Q

Name 3 gluconeogenesis precursors

A

pyruvate
lactate
amino acids
glycerol

50
Q

What is the Cori cycle?

A

glycolysis is occurring in the muscle to produce pyruvate, which is converted into lactate and transported in the blood to the liver. The liver takes up the lactate and converts it to pyruvate by lactate dehydrogenase and the pyruvate is used in gluconeogenesis to produce glucose. Glucose is then transported back to the muscle.

BIOL201: Biochemistry (13 decks)

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