Chapter 17: The Citric Acid Cycle

Question 1

What does pyruvate dehydrogenase do? Draw the mechanism.

Answer 1

Converts pyruvate, CoASH and NAD+ —–> Acetyl CoA, CO2 and NADH

Question 2

Net Krebs Reaction

Answer 2

3NAD+ + FAD+ + GDP + Pi + AcetylCoa —-> 3NADH + 1FADH2 + 1GTP+ Coa + 2CO2

Question 3

What’re the 3 Irreversible steps of the TCA cycle?

Answer 3

1) Citrate synthase, that converts oxa + acetyl coA to citrate
2) Isocitrate dehydrogenase, which converts isocitrate into alphaketoglutarate (creates NADH and CO2)
3) alphaketoglutarate dehydrogenase complex, which converts alphaketoglu into succinyl coA, with the help of CoASH and NAD+ (CO2 is released)

Question 4

What step of the Krebs cycle creates GTP/

Answer 4

when succinylcoA gets converted into succinate (coASH is also released) via succinylcoAsyntheTASE

Question 5

What stage of the TCA cycle is a hydration stage?

Answer 5

when fumarate is converted into malate via fumarase

Question 6

in the Krebs cycle, ___NADH, ___ FADH2 and ____ GTP are produced, which all add up to roughly 10 ATP.

Answer 6

3 NADH,
1 FADH2
1 GTP

Question 7

How many electrons is NADH worth

Answer 7

2 electrons

Question 8

at what point in the Krebs cycle does substrate level phosphorylation occur?

Answer 8

when succinyl coA gets converted into succinate and creates GTP

Question 9

Synthase vs synthetase

Answer 9

synthetase creates a compound but ALSO CREATES ATP

Question 10

In terms of Krebs and all that mitochondria stuff, how does NADH and FADH2 get rid of their electrons?

Answer 10

they donate their electrons into the ETC, (FADH2 donates it into complex 2), where the electrons are used to turn O2 into H2O, and to pump protons out to create a concentration gradient, which in turn houses energy for ATP synthesis.

Question 11

Generally, if you were to label acetylcoA with C14, which COO group is tagged when citrate, isocitrate, or alphaketoglu etc. are formed?

Answer 11

labelled carbon always appears on the COO- group that is not near the OH or ketone group.

Question 12

If you were to labelled a carboxyllic group oxaloacetate with C14, which carbon would you expect to be labelled in fumarate?

Answer 12

NONE. if you were to label the oxaloacetate carboxyllic carbon, it would be lose as CO2

Question 13

if you labelled the CH- group in Oxaloacetate, which carbon would be labelled in malate, after one cycle? what about in citrate?

Answer 13

either carbon 1 or 4 (a COO- group) in malate, a CH2 group in citrate (this will later be labelled as a the Carbon with the hydroxyl group in isocitrate)

Question 14

If you labelled ketone of acetyl-coA, what carbon would be labelled in malate, after one cycle?

Answer 14

either carbon 1 or 4 (a COO- group) recall.

Question 15

What compounds allosterically inhibit pyruvate dehydrogenase?

Answer 15

ATP, Acetyl coA, NADH, Fatty acids (basically things that Krebs produces)

Question 16

What compounds allosterically activate Pyruvate dehydrogenase?

Answer 16

AMP, coA, NAD+, Ca2+

Question 17

Describe the mechanism as to how Pyruvate dehydrogenase is covalently inhibited or activated.

Answer 17

pyruvate dehyd has an associated kinase and phosphatse. when ATP IS HIGH, pyruvate dh. gets PHOSPHORYLATED (kinase adds a phosphate), which INHIBITS IT, and therefor, pyruvate will no longer get converted to acetylCoA

INSULIN activates pyr-dh via DEPHOSPHORYLATION (phosphatase dephosphorylates pry-dh), which is what happens when a person is well fed. This causes pyr-dh to turn pyruvate into acetylcoA, to make ATP in Krebs.

Question 18

Pyruvate deh. phosphatase _____ pyruvate dh. via _____

Answer 18

pyr.dh Phosphatase ACTIVATES pyr-dH via DEPHOSPHORYLATION. This process is driven by INSULIN

Question 19

Pyruvate dH. Kinase ____ Pyruvate deh. via ____

Answer 19

Pyr-dH. Kinase DEACTIVATES Pyr-DH by phosphorylating Pyruvate dH.

Question 20

What (covalently) inhibits Pyruvate dH Kinase?

Answer 20

ADP or Ca2+. When there is ADP, the body needs to make more ATP and thus needs to create more acetyl coA. Pyruvate dH needs to be turned ON, and therefore Kinase (which turns it off via phosphorylation) gets inhibited.

Question 21

What’re the 3 regulation sites of the Krebs cycle?

Answer 21

1) Citrate synthase, that converts oxa + acetyl coA to citrate
2) Isocitrate dehydrogenase, which converts isocitrate into alphaketoglutarate (creates NADH and CO2)
3) alphaketoglutarate dehydrogenase complex, which converts alphaketoglu into succinyl coA, with the help of CoASH and NAD+ (CO2 is released)

Question 22

T/F Krebs cycle is inhibited by increased NADH. Why?

Answer 22

True. Krebs is inhibited by NADH concentration because it produces even more NADH. If the body does not need NADH, Krebs is slowed down.

Question 23

T/F the 3 irreversible steps of Krebs are dependent on substrates, such as acetyl coA and oxa, as well as NADH

Answer 23

true.

Question 24

ADP is an allosteric ____ of isocitrate dehydrogenase, which makes alphaketoglu.

Answer 24

ADP is an allosteric activator. The presence of ADP drives the Krebs cycle reactions forward

Question 25

ATP is an allosteric ____ of isocitrate dehydrogenase, which makes alphaketoglu

Answer 25

ATP is an alloteric inhibitor. The presence of ATP will slow down the Kreb cycle.

Question 26

T/F: Pyruvate dehydrogenase is unaffected by cAMP

Answer 26

True.

Question 27

T/F Acetyl CoA is a competitive inhibitor of Pyruvate dehydrogenase

Answer 27

true

Question 28

What is a catapleurotic reaction?

Answer 28

reactions that utilize and therefore drain citric acid cycle intermediates. Ex/ gluconeogenesis. utilizes oxaloacetate and pyruvate in order to make glucose. this slows down krebs because the substrates are being used up.

Question 29

What is an anapleurotic reaction? give a balanced example

Answer 29

reactions that replace Krebs intermediates that were siphoned off for a catapleurotic reaction. Creation of a Krebs intermediate from a non-krebs.

pyruvate + CO2 + ATP —> Malate + ADP

Question 30

How many oxaloacetates are burned during 1 cycle of Krebs?

Answer 30

none, oxaloacetate is regenerated.

Question 31

T/F Humans can undergo the glycoxylate cycle

Answer 31

false. Humans cannot make glucose from fats, (exception is odd chain fatty acids)

Question 32

Draw glycoxylate

Answer 32

its like acetyl coA but with no SCoA, just a hydrogen instead

Question 33

what’re the 2 new enzymes in the glycoxylate cycle?

Answer 33

isocitrate lyase, which converts isocitrate into succinate and glycoxylate.

malate synthase, which converts glycoxylate with acetyl coA to make malate.

Question 34

In the glycoxylate cycle, fatty acids get converted into _____ (in a previous step), which enter the cycle to produce a NEW succinate and a ____, which can converted into oxaloacetate in the cytosol, which can be used to make _____ .

Answer 34

In the glycoxylate cycle, fatty acids get converted into ACETYL-COA (in a previous step), which enter the cycle to produce a NEW succinate and a GLYCOXYLATE, the succinate can move to mitochondria and get converted into oxaloacetate in the cytosol, which can be used to make GLUCOSE. the glycoxylate stays in the glycoxysome to maintain the substrates of the glycoxylate cycle).

Question 35

Why is the glycoxylate cycle anapleurotic?

Answer 35

because isocitrate lyase creates succinate and glycoxylate. succinate moves into the mitochondria to form malate, which can either move to the cytosol to undergo gluconeogenesis (by being first converted into oxaloacetate), or it can stay in the mitochondria to ACT AS A KREBS INTERMEDIATE, making it anapleurotic.

Question 36

Draw the glycoxylate cycle

Answer 36

refer to notes to see if correct

Question 37

Where does pyruvate to acetyl coA occur?

Answer 37

in the mitochondrial matrix