Chapter 10- Carbohydrate Metabolism II Flashcards Preview

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Flashcards in Chapter 10- Carbohydrate Metabolism II Deck (60)
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2 other names for the citric acid cycle

krebs cycle and tricarboxylic acid (TCA) cycle


main function of citric acid cycle?

oxidation of acetyl-CoA to CO2 and H2O

-this cycle also produces NADH and FADH2


what happens to pyruvate after its formed

1. enters mitochondrion via active transport
2. pyruvate is oxidized and decarboxylated by pyruvate dehydrogenase complex (multienzyme complex)-- becomes acetyl-CoA


enzymes in pyruvate dehydrogenase complex

1. pyruvate dehydrogenase (PDH): oxidizes pyruvate into a 2 carbon molecule, yielding CO2 with TPP (coenzyme) and Mg2+
2. dihydrolipoyl transacetylase: 2 carbon molecule gets bonded to TPP and transferred to lipoic acid (coenzyme)... end result is lipoic acid in the reduced form
3. dihydrolipoyl dehydrogenase: FAD used as coenzyme to oxidize lipoic acid to help acetyl-CoA formation... FAD reduces to FADH2

*first three convert pyruvate to acetyl-CoA

4. pyruvate dehydrogenase kinase
5. pyruvate dehydrogenase phosphatase

*4 and 5 regulate actions of PDH


coenzyme A (CoA)

CoA-SH is a thiol (has an SH group)--- this is a thioester which has very high-energy properties


what are different ways of forming acetyl-CoA (other than glycolysis forming pyruvate to turn into acetyl-CoA)

1. fatty acid oxidation (B-oxidation)
2. amino acid catabolism
3. ketones
4. alcohol


B-oxidation (pre-steps)

aka. fatty acid oxidation. before B-oxidation can occur the fatty acid must go through activation which causes a thioester bond to form between carboxyl groups of fatty acids and CoA. once carnitine brings the complex into the inner membrane of the mitochondria then acyl-CoA is formed which then undergoes B-oxidation.



brings CoA-SH from the cytosol (intermembrane space) to the inner membrane of the mitochondria

cytosolic CoA-SH ---> mitochondrial CoA-SH


amino acid catabolism

only with certain amino acids (ketogenic aa). lose their amino group via transamination. the carbon skeleton then forms ketone bodies.


ketones forming acetyl-CoA

ketone bodies are essentially transportable molecules of acetyl-CoA

acetyl-CoA is typically used to produce ketones when pyruvate dehydrogenase complex is inhibited the reverse reaction can occur as well to produce acetyl-CoA. (DONT NEED TO KNOW ENZYMES)



when alcohol is consumed in moderate amounts the enzymes alcohol dehydrogenase and acetaldehyde dehydrogenase convert it to acetyl-CoA (primarily used for fatty acid synthesis b/c the krebs cycle is inhibited due to NADH buildup from alcohol consumption)


overall reaction of pyruvate dehydrogenase complex?

pyruvate + CoA-SH + NAD+ ---> acetyl-CoA + CO2 + NADH + H+


general krebs cycle

acetyl-CoA (2C) goes into the cycle and reacts with oxaloacetate (OAA, 4C) using citrate synthase (enzyme that aids in condensation reaction). this forms citrate. through a variety of reactions citrate

makes 3NADH, 1GTP, 1FADH2 per pyruvate

each NADH = 2.5ATP
each FADH2 = 1.5ATP



enzymes that form new covalent bonds without needing significant energy


citrate formation

KREBS STEP 1: acetyl-CoA + Oxaloacetate + H2O ---> Citrate + CoA-SH + H+


citrate isomerized to isocitrate

KREBS STEP 2: citrate (aconitase) and releases H2O cis-aconitate D-Isocitrate (aconitase) and adds water in a different place as before


a-ketoglutarate and CO2 formation

KREBS STEP 3: isocitrate (isocitrate dehydrogenase) ---> produces NADH + oxalosuccinate ---> releases CO2 and adds H+ + a-Ketoglutarate

*Note: isocitrate dehydrogenase is the rate-limiting enzyme for the citric acid cycle.
The first NADH is produced and CO2 is released here as well.


Succinyl-CoA and CO2 formation

KREBS STEP 4: a-ketoglutarate (a-ketoglutarate dehydrogenase complex) + CoA-SH + NAD+ ---> Succinyl-CoA + CO2

*Note: NADH produced.



subtype of oxidoreductases (enzymes that catalyze redox reactions). transfer hydride ion (H-) to an electron acceptor, ususally NADH or FADH2.

*when you see a dehydrogenase look for a high-energy electron carrier being formed.


Succinate Formation

KREBS STEP 5: Succinyl-CoA + CoA-SH (succinyl-CoA synthetase) ---> Succinate

*Note: GDP becomes GTP in this step as well, this GTP undergoes nucleosidediphosphate kinase (also does GDP to GTP) which catalyzes the phosphate transfer from GTP to ATP (ONLY time in krebs cycle where ATP is produced directly).


whats the difference between a synthetase and a synthase?

sythetases create new covalent bonds with energy input.

synthases don't use energy.


Fumarate Formation

KREBS STEP 6: only step that takes place in the inner membrane of the mitochondria instead of the matrix.
succinate is oxidized to yield fumarate by succinate dehydrogenase-- considered a flavoprotein b/c its covalently bonded to FAD (which is reduced to FADH2 during this process).


how many ATP are produced for each molecule of FADH2 and each molecule of NADH?

FADH2 ---> 1.5 ATP
NADH ---> 2.5 ATP


Malate Formation

KREBS STEP 7: fumarase catalyzes the hydrolysis of the alkene bond in fumarate (giving rise to malate, only the L conformation)


Oxaloacetate Formed Anew

KREBS STEP 8: oxidation of malate to oxaloacetate by malate dehydrogenase. NAD+ is reduced to NADH as well.


Substrates for each step of the Citric acid cycle

Please, Can I Keep Selling Sex For Money, Officer?

1. Pyruvate
2. Citrate
3. Isocitrate
4. a-Ketoglutarate
5. Succinyl-CoA
6. Succinate
7. Fumarate
8. Malate
9. Oxaloacetate


what are the products (results) of the pyruvate dehydrogenase complex?



what the producs (results) of the citric acid cycle?

Important products
- 3 NADH

other products
- 2CO2
- CoA-SH
- 3 H+


How many total ATP are made after the citric acid cycle (including pyruvate dehydrogenase)

12.5 ATP per pyruvate and a total of 25 ATP per glucose


how does the pyruvate dehydrogenase complex regulate the citric acid cycle?

when levels of ATP rise, phosphorylating the pyruvate dehydrogenase kinase inhibits acetyl-CoA production.

This enzyme is then reactivated by pyruvate dehydrogenase phosphatase in response to high levels of ADP.

*Note: overall the citric acid cycle regulation is determined by ATP/ADP and NADH/NAD+ ratios.