Carbohydrate Metabolism

Question 1

Define catabolism.

Answer 1

reactions involving the breaking down of the organic substrates, typically by oxidation to provide chemically available energy and/or to generate metabolic intermediates used in subsequence anabolic reactions

Question 2

Define anabolism.

Answer 2

process of metabolism that results in the synthesis of cellular components from precursors of low molecular weight

Question 3

Why is the TCA cycle important in humans?

Answer 3

for ATP synthesis

Question 4

What is the glyoxylate cycle used for?

Answer 4

• happens in plants, bacteria and fungi

- provides glucose synthesis from acetyl CoA (therefore fatty acids)

Question 5

What are the main differences between the TCA cycle and the Glyoxylate cycle?

Answer 5

• glyoxylate cycle conserves CO2
• no decarboxylation steps that are seen in the TCA cycle
• allows CO2 to be conserved for carbohydrate synthesis via succinate
• oxaloacetate regeneration
• 2 molecules of acetyl CoA are used

Question 6

What is the first step of the glyoxylate cycle?

Answer 6

1. cleavage of isocitrate catalysed by isocitrate lyase

- goes from isocitrate to succinate (goes to TCA and gluconeogensis) and glyoxylate

Question 7

What is the second step of the glyoxylate cycle?

Answer 7

1. condensation of glyoxylate and acetyl coA catalysed by malate synthase
   - glyoxylate + acetyl CoA goes to Malate + CoA (goes back into the cycle)
   - malate then forms into oxaloacetate
   - will then synthesise citrate
   - bypasses the two decarboxylated steps

Question 8

Draw the structures of isocitrate, succinate, glyoxylate, acetyl coA and malate.

Answer 8

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Question 9

Why is the glyoxylate cycle important in plants?

Answer 9

• allows the conversion of fatty acids through acetate to carbohydrates
• carbohydrates are needed in plant seeds as they can be carbohydrate poor and cannot synthesise them through photosynthesis
• need carbohydrates for germination and growth

Question 10

Is the glyoxylate cycle in humans?

Answer 10

ICL and MS enzymes were detected in the liver

• many mammals may be able to convert fatty acids to carbohydrates
• a lot of evidence disputing so is controversial

Question 11

Why is the Pentose Phosphate Pathway (PPP) important?

Answer 11

• production of NADPH for use in reduction biosynthesis
• to synthesise ribose-5-phosphate for purine metabolism (leading to nucelotides, RNA & DNA)
• to synthesise erythose-4-phosphate for use in aromatic a.a production

Question 12

What are the two stages of the Pentose Phosphate Pathway?

Answer 12

Stage 1 is the irreversible oxidative phase
- reduction of NADP+ to NADPH
- by oxidation of G6P to ribulose-5-phosphate
Stage 2 is the reversible, non-oxidative phase
- conversion of ribulose-5-phosphate to fructose-6-phosphate and G3P

Question 13

What is the first step of Stage one of the PPP?

Answer 13

conversion of glucose-6-phosphate to 6-phosphoglucono-lactone via G6P-DH enzyme

• the NADP+ is reduced to NADPH + H+
• is a reversible reaction

Question 14

What is the second step of Stage one of the PPP?

Answer 14

hydrolysis of 6-phosphoglucono-lactone to 6-phosphogluconate via Lactonase

• H2O to H+
• is a reversible reaction

Question 15

What is the third step of Stage one of the PPP?

Answer 15

decarboxylation of 6-phosphogluconate to ribulose-5-phosphate

• the NADP+ is reduced to NADPH + H+
• release of CO2
• is a reversible reaction

Question 16

Draw the structures of glucose-6-phosphate, 6-phosphoglucono-lactone, 6-phosphogluconate and ribulose-5-phosphate.

Answer 16

-

Question 17

What is the net reaction of stage 2 of the PPP?

Answer 17

3 ribulose-5-phosphate to 2 fructose-6-phosphate and 1 G3P

Question 18

What are the overall reactions of stage 2?

Answer 18

1. ribulose-6-phosphate (C5) forms Xylulose-5-phosphate (C5) and ribose-5-phosphate (C5)
2. X5P and R5P make sedoheptulose-7-phosphate (C7) and G3P (C3)
3. S7P and G3P make Erythrose-4-phosphate (C4) and Fructose-6-phosphate (C6)
4. E4P and X5P form to make fructose-6-phosphate (C6) and G3P (C3)

Question 19

What is step 1 of Stage 2 of the PPP?

Answer 19

1. isomerisation of ribulose-5-phosphate
   - ribose-5-phosphate isomerase converts ribulose-5-phosphate to ribose-5-phosphate
   - ribose-5-phosphate 3- Epimerase converts ribulose-5-phosphate into Xylulose-5-phosphate

Question 20

What is step 2 of Stage 2 of the PPP?

Answer 20

2. transketolase catalyses the transfer of glycoaldehyde or ketol unit
   - ribose-5-phosphate + xylulose-5-phosphate make glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate

Question 21

What is step 3 of Stage 2 of the PPP?

Answer 21

3. transaldolare transfers dihydroxy acetone moiety of sedoheptulose-7-phosphate
   - G3P + sedoheptulose-7-phosphate turn fructose-6-phosphate + erythrose-4-phosphate

Question 22

What is step 4 of Stage 2 of the PPP?

Answer 22

4. transketolase catalyses the transfer of glycoaldehyde or ketol unit
   - xylulose-5-phosphate + erythrose-4-phosphate into fructose-6-phosphate + G3P

Question 23

What is NADPH important for?

Answer 23

• NADPH is important in protecting against reactive oxygen species
• reducing equivalent in fatty acid synthesis and cholesterol synthesis

Question 24

What does the deficiency of glucose-6-phosphate dehydrogenase lead to?

Answer 24

• lower production of NADPH
• lower capacity to reduce oxidised glutathione
• protection against malaria as malaria parasites require reduced glutathione

Question 25

How is glucose-6-phosphate co-ordinated?

Answer 25

levels of NADP+

• low levels of NADP+ reduces glucose-6-phosphate conversion in PPP
• high levels of NADP+ and low levels of NADPH increases glucose-6-phosphate conversion in PPP