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    Hi,

    In the CGP Textbook (OCR A2 Biology), it describes each stage in the Krebs cycle, it states:

    1. Formation of citrate:
    Acetyl CoA (coenzyme A) from the link reaction combines with oxaloacetate to form citrate. Coenzyme A goes back to the link reaction to be used again.

    However, I have some questions:

    - How does Acetyl CoA from the link reaction combine with oxaloacetate?
    - How does Coenzyme A go back to the link reaction to be used again?
    - Why does Acetyl CoA come from the link reaction?
    - Why does Coenzyme A go back to link reaction to be used again? What is the purpose of this?
    - What happens if Coenzyme A does not go back to the link reaction to be used again? What are the consequences?

    Thanks,
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    Moreover, it then describes the second stage of the Krebs cycle:

    2. Formation of a 5-carbon compound:
    The 6-carbon citrate molecule is converted to a 5-carbon citrate molecule. Decarboxylation occurs, where carbon dioxide is removed. Dehydrogenation also occurs. The hydrogen is used to produced reduced NAD from NAD.

    However, I have a few questions in mind as the text does not seem to explain:
    (a) How is the 6-carbon citrate molecule converted to a 5-carbon citrate molecule?
    (b) Why does decarboxylation occur? Is this because of the conversion of a 6-carbon citrate molecule to a 5-carbon citrate molecule?
    (c) Why does dehydrogenation occur? What is the purpose of this?
    (d) How is the hydrogen used to produce reduced NAD from NAD? What happens if hydrogen is not used in this case? Why is hydrogen used?
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    (Original post by LionKing101)
    Hi,

    In the CGP Textbook (OCR A2 Biology), it describes each stage in the Krebs cycle, it states:

    1. Formation of citrate:
    Acetyl CoA (coenzyme A) from the link reaction combines with oxaloacetate to form citrate. Coenzyme A goes back to the link reaction to be used again.

    However, I have some questions:

    - How does Acetyl CoA from the link reaction combine with oxaloacetate?
    - How does Coenzyme A go back to the link reaction to be used again?
    - Why does Acetyl CoA come from the link reaction?
    - Why does Coenzyme A go back to link reaction to be used again? What is the purpose of this?
    - What happens if Coenzyme A does not go back to the link reaction to be used again? What are the consequences?

    Thanks,
    A lot of questions! I'll try to answer them the best I can, although I'm answering them off the top my head (I finished A levels months ago and I'm now at Uni).

    (a) An enzyme will catalyse this reaction so Acetyl CoA can combine with oxaloacetate.
    (b) Acetate is offloaded by CoA, and it may be returned via transport proteins.
    (c) Acetate is produced when pyruvate is decarboxylated and dehydrogenated, which then combines with CoA.
    (d) It goes back so it can combine with other acetate molecules, which can then be transported to the Krebs cycle to produce more reduced NAD and FAD for oxidative phosphorylation.
    (e) If CoA did not get back to the link reaction for some reason, then less reduced NAD and FAD would be produced, thus less would be available for oxidative phosphorylation. This means less ATP would be produced.
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    (Original post by LionKing101)
    Moreover, it then describes the second stage of the Krebs cycle:

    2. Formation of a 5-carbon compound:
    The 6-carbon citrate molecule is converted to a 5-carbon citrate molecule. Decarboxylation occurs, where carbon dioxide is removed. Dehydrogenation also occurs. The hydrogen is used to produced reduced NAD from NAD.

    However, I have a few questions in mind as the text does not seem to explain:
    (a) How is the 6-carbon citrate molecule converted to a 5-carbon citrate molecule?
    (b) Why does decarboxylation occur? Is this because of the conversion of a 6-carbon citrate molecule to a 5-carbon citrate molecule?
    (c) Why does dehydrogenation occur? What is the purpose of this?
    (d) How is the hydrogen used to produce reduced NAD from NAD? What happens if hydrogen is not used in this case? Why is hydrogen used?
    (a) Decarboxylation removes a carbon atom from the compound.
    (b) Exactly.
    (c) So the hydrogen atoms can bind to NAD and FAD molecules in order to reduce them, so the electrons can later be donated in the process of oxidative phosphorylation.
    (d) See above. If hydrogen is not used, there will be no reduced NAD/FAD, and so there will be no oxidative phosphorylation.
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    (Original post by Lighfy)
    A lot of questions! I'll try to answer them the best I can, although I'm answering them off the top my head (I finished A levels months ago and I'm now at Uni).

    (a) An enzyme will catalyse this reaction so Acetyl CoA can combine with oxaloacetate.
    (b) Acetate is offloaded by CoA, and it may be returned via transport proteins.
    (c) Acetate is produced when pyruvate is decarboxylated and dehydrogenated, which then combines with CoA.
    (d) It goes back so it can combine with other acetate molecules, which can then be transported to the Krebs cycle to produce more reduced NAD and FAD for oxidative phosphorylation.
    (e) If CoA did not get back to the link reaction for some reason, then less reduced NAD and FAD would be produced, thus less would be available for oxidative phosphorylation. This means less ATP would be produced.

    Thank you so much!
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    Hi,

    In the next part of the Krebs Cycle (Stage 3), the textbook describes the 'Regeneration of oxaloacetate':

    3. Regeneration of oxaloacetate:
    The 5-carbon citrate molecule is converted into a 4-carbon citrate molecule. There are some intermediate compounds formed during the conversion. Decarboxylation and dehydrogenation occur, producing one molecule of reduced FAD and two of reduced NAD.
    ATP is produced by the direct transfer of a phosphate group from an intermediate compound to ADP.
    When a phosphate group is directly transferred from one molecule to another it is called substrate-level phosphorylation. Citrate has now been converted into oxaloacetate.

    I have a few questions which the text does not explain:

    (a) How is the 5-carbon citrate molecule converted into a 4-carbon citrate molecule? (Is this applying the same principle as the above explanation from Lighfy). In the sense that decarboxylation removes another carbon atom from the compound?
    How does decarboxylation remove another carbon atom from the compound?
    Why does decarboxylation remove another carbon atom from the compound?
    Where does the process of decarboxylation take place?
    Where does the process of dehydrogenation take place?

    (b) Which intermediate compounds are formed during the conversion? (The textbook states you do not need to know them, however, I do not trust the examiners).

    (c) "Decarboxylation and dehydrogenation occur - producing one molecule of reduced FAD and two of reduced NAD".
    Why is one molecule of reduced FAD and two molecules of reduced NAD produced? Why is this not vice versa? What is the significance of this?
    What happens if one molecule of reduced NAD was produced instead of two molecules? What are the consequences of this on the Krebs cycle, and or other processes?

    (d) "ATP is produced by the direct transfer of a phosphate group from an intermediate compound to ADP".
    What does this mean by 'direct transfer'? Why is this a direct transfer and not a slow transfer? What is special about the process of ATP being produced by a phosphate group from an intermediate compound to ADP that it is stated as a 'direct transfer'?

    (e) How is citrate converted into oxaloacetate? (The sentence does not explain in depth detail).

    Thank you.
 
 
 
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