Carbohydrate Metabolism Flashcards
Define catabolism.
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
Define anabolism.
process of metabolism that results in the synthesis of cellular components from precursors of low molecular weight
Why is the TCA cycle important in humans?
for ATP synthesis
What is the glyoxylate cycle used for?
- happens in plants, bacteria and fungi
- provides glucose synthesis from acetyl CoA (therefore fatty acids)
What are the main differences between the TCA cycle and the Glyoxylate cycle?
- 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
What is the first step of the glyoxylate cycle?
- cleavage of isocitrate catalysed by isocitrate lyase
- goes from isocitrate to succinate (goes to TCA and gluconeogensis) and glyoxylate
What is the second step of the glyoxylate cycle?
- 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
Draw the structures of isocitrate, succinate, glyoxylate, acetyl coA and malate.
-
Why is the glyoxylate cycle important in plants?
- 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
Is the glyoxylate cycle in humans?
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
Why is the Pentose Phosphate Pathway (PPP) important?
- 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
What are the two stages of the Pentose Phosphate Pathway?
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
What is the first step of Stage one of the PPP?
conversion of glucose-6-phosphate to 6-phosphoglucono-lactone via G6P-DH enzyme
- the NADP+ is reduced to NADPH + H+
- is a reversible reaction
What is the second step of Stage one of the PPP?
hydrolysis of 6-phosphoglucono-lactone to 6-phosphogluconate via Lactonase
- H2O to H+
- is a reversible reaction
What is the third step of Stage one of the PPP?
decarboxylation of 6-phosphogluconate to ribulose-5-phosphate
- the NADP+ is reduced to NADPH + H+
- release of CO2
- is a reversible reaction
Draw the structures of glucose-6-phosphate, 6-phosphoglucono-lactone, 6-phosphogluconate and ribulose-5-phosphate.
-
What is the net reaction of stage 2 of the PPP?
3 ribulose-5-phosphate to 2 fructose-6-phosphate and 1 G3P
What are the overall reactions of stage 2?
- ribulose-6-phosphate (C5) forms Xylulose-5-phosphate (C5) and ribose-5-phosphate (C5)
- X5P and R5P make sedoheptulose-7-phosphate (C7) and G3P (C3)
- S7P and G3P make Erythrose-4-phosphate (C4) and Fructose-6-phosphate (C6)
- E4P and X5P form to make fructose-6-phosphate (C6) and G3P (C3)
What is step 1 of Stage 2 of the PPP?
- 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
What is step 2 of Stage 2 of the PPP?
- transketolase catalyses the transfer of glycoaldehyde or ketol unit
- ribose-5-phosphate + xylulose-5-phosphate make glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate
What is step 3 of Stage 2 of the PPP?
- transaldolare transfers dihydroxy acetone moiety of sedoheptulose-7-phosphate
- G3P + sedoheptulose-7-phosphate turn fructose-6-phosphate + erythrose-4-phosphate
What is step 4 of Stage 2 of the PPP?
- transketolase catalyses the transfer of glycoaldehyde or ketol unit
- xylulose-5-phosphate + erythrose-4-phosphate into fructose-6-phosphate + G3P
What is NADPH important for?
- NADPH is important in protecting against reactive oxygen species
- reducing equivalent in fatty acid synthesis and cholesterol synthesis
What does the deficiency of glucose-6-phosphate dehydrogenase lead to?
- lower production of NADPH
- lower capacity to reduce oxidised glutathione
- protection against malaria as malaria parasites require reduced glutathione
How is glucose-6-phosphate co-ordinated?
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
