L6: Energy production- Carbohydrates II Flashcards

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1
Q

What are the important intermediate in glycolysis?

A

Glycerol phosphate
- Triglyceride and phospholipid biosynthesis
- Produced from DHAP in adipose and liver
- Lipid synthesis in liver requires glycolysis
2,3-bisphosphate
- Produced from 1,3-bisphophoglycerate in RBC
- Important regulator of O2 affinity in haemoglobin
- Present in RBCs at the same molar concentration as haemoglobin

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2
Q

Why is lactate dehydrogenase useful?

A

Allows glycolysis to continue when there is no O2
- RBC or anaerobic respiration
Produced from pyruvate
NADH + H+ + pyruvate → NAD+ + lactate
Then reconverted back to pyruvate in heart, liver and kidney

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3
Q

How do the lactate levels vary?

A
Without major exercise 40-50g/24hrs
→ RBC, skin, brain, skeletal msucle, GI tract
Strenous exercise 30g/5 min
→ plasma levels x10 in 2-5 mins 
→ back to normal after 90mins
Pathological situations
→ shock, congestive heart disease
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4
Q

What determine the plasma lactate levels?

A
  1. Production
  2. Utilisation (liver, heart, muscle)
  3. Disposal (kidney)
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5
Q

What happens if lactate levels rise?

A

Hyperlactarmia

  • 2-5mM
  • Below renal threshold
  • No change in blood pH due to buffering capacity

Lactic acidosis

  • Above 5mM
  • Above renal threshold
  • Blood pH lowered, more H+ ions in the blood
  • Critical marker in acutely unwell patient
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6
Q

What is fructose?

A

Monosaccharide found in fruit
Sucrose= fructose + glucose
Broken down by fructase

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7
Q

How is fructose metabolised?

A

In the liver
Catalysed by enzymes frucokinase and aldolase to glyceraldehyde-3-phosphate
Intermediate of glycolysis

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8
Q

What are some clinical complications associated with fructose metabolism?

A
Essential fructosuria
→ fructose in urine 
→ No clinical signs 
→ fructokinase enzyme missing
Fructose intolerance
→ Fructose-1-phosphate accumulates in the liver 
→ Aldolase enzyme missing 
→ treatment- remove fructose from diet
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9
Q

What is galactose?

A

Monosaccharide
Lactose- disaccharide of galactose and glucose
Metabolised by lactase

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10
Q

How is galactose metabolised?

A
In liver
Galactokinase 
Uridyl transferase 
UDP-galactose epimerase 
Galactose → Galactose-1-P → glucose-1P
using enzymes galactokinase and uridyl transferase
Glucose-1P can enter glycolysis
OR
Galactose-1-P → UDP-galactose 
using enzymes UDP-galactose epimerase
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11
Q

What is galactosemia?

A

Deficiency in transferase enzyme or kinase enzyme
Unable to utilise galactose
Galactokinase deficiency (rare)→ galactose accumulates
Transferase deficiency (common)→ galactose and galactose-1-P accumulates

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12
Q

What is the problem with galactosemia?

A

Galactose enters other pathways
Galactose → Galactitol
using enzyme aldose reductase and NADPH (coverted to NAD+)
Uses up NADPH stores→ prevents maintenance of free sulphydryl groups on proteins
Innappropriate disulphide bond formation
Eyes→ cataracts
In addition non-enzymatic glycosylation of less proteins due to high galactitol
Galactose-1P affect liver, kidney and brain
Increased galactose + galactitol→ raised intraoccular pressure→ glucoma → blindness
Treatment→ remove galactose from the diet

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13
Q

What is the pentose phosphate pathway for?

A
  • Hexosemonophosphate shunt
  • In high energy state (↑[ATP]) glucose-6-phosphate enters a different pathway
  • Major function
    → NADPH production
    → production of C5 ribose sugars
    No ATP or CO2 produced but it is oxidative
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14
Q

What are the two phases of the pentose phosphate pathway?

A

Phase 1- Oxidative decarboxylation

Phase 2- Rearrangement of glycolytic intermediates

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15
Q

What happens during phase 1?

A

6C is converted to 5C sugar with production of 2NADPH and 2H+ and CO2

Glucose-6P + 2NADP+ → C5 sugar phosphate + 2NADPH + 2H+ + CO2
Using enzyme glucose 6-phosphate dehydrogenase

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16
Q

What happens during phase 2?

A

Complex reactions convert any unused C5 sugar phosphates into intermediates of glycolysis
- 2 fructose-6-phosphates
- 1 Glyceraldehyde-3-phosphate
No ATP produced
Loss of CO2 therefore irreversible
Controlled by NADP+/NADPH ratio at G6P dehydrogenase

17
Q

What is NADPH used for?

A

Biosynthetic reducing power
- e.g. lipid synthesis (high activity in liver and adipose tissue)
Maintain free -SH (cysteine) groups on certain proteins
- Prevent oxidation to S-S bonds (disulphide bonds)
Detoxification process

18
Q

What are C5 sugars required for?

A

Needed for nucleic acid synthesis for DNA and RNA

High activity in dividing tissues

19
Q

What is the rate limiting enzyme in the pentose phosphate pathways?

A

Glucose-6 phosphate dehydrogenase

20
Q

What is glucose-6-phosphate dehydrogenase deficiency?

A

Common inherited disorder
Deficiency in enzyme G6PDH
Important for production of NADPH
Maintain -SH and prevent disulphide bond formation
Deficiency means function and structure of proteins is altered
↓G6PDH in RBC → ↓NADPH→ disulphide bonds formation→ aggregated proteins → Heinz bodies → haemolysis (destruction of RBC)→ lens of eye→ cataracts