10.3 Citric Acid Cycle reactions

Question 1

what are the 3 substrates for step 0 of the citrate cycle?

Answer 1

1. pyruvate
2. NAD+
3. CoA

Question 2

what are the 3 products of step 0 of the citrate cycle?

Answer 2

1. acetyl CoA
2. NADH
3. CO2

Question 3

what enzyme catalyzes step 0 of the citrate cycle?

Answer 3

the pyruvate dehydrogenase complex (E1, E2, E3)

Question 4

when does step 0 of the citrate cycle occur?

Answer 4

before entering the citrate cycle

Question 5

describe the free energy of step 0 of the citrate cycle

Answer 5

highly exergonic (very negative delta G)

Question 6

give the net reaction for the citrate cycle for one molecule of acetyl CoA entering

Answer 6

Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O —> CoA + 2CO2 + 3NADH + 3H+ + FADH2 + GTP

Question 7

what does the substrate acetyl CoA produce in the citrate cycle

Answer 7

CoA and CO2

Question 8

what do the 3 NAD+ substrates of the citrate cycle produce

Answer 8

3NADH + 3H+

Question 9

what does the FAD substrate of the citrate cycle produce?

Answer 9

FADH2

Question 10

what do the GDP and Pi substrates of the citrate cycle produce?

Answer 10

GTP

Question 11

what are the 3 substrates, 2 products, and enzyme for step 1 of the citrate cycle?

Answer 11

substrates: acetyl CoA, oxaloacetate, and H2O
products: citrate, CoA
enzyme: citrate synthase

Question 12

what are 2 important facts about step 1 of the citrate cycle?

Answer 12

1. highly exergonic (very negative delta G)
2. IRREVERSIBLE

Question 13

describe the ordered mechanism of citrate synthase

Answer 13

oxaloacetate must bind to citrate synthase first to induce a conformational change in citrate synthase that allows acetyl CoA to then bind; the apo version cannot bind acetyl-CoA!!

Question 14

give the substrate, product, and enzyme of step 2 of the citrate cycle

Answer 14

substrate: citrate
product: isocitrate
enzyme: aconitase

Question 15

describe the relationship between citrate and isocitrate, including the intermediate

Answer 15

citrate and isocitrate are isomers, but the reaction to make isocitrate from citrate is not an isomerization because of the intermediate cis-aconitate, where the H2O is removed and then added back to get isocitrate

Question 16

how is aconitase inhibited?

Answer 16

by fluorocitrate, which originates from fluoroacetate, which is found in african and autstralian trees; fluorocitrate inhibits aconitase activity and blocks citrate export

Question 17

how is fluorocitrte used?

Answer 17

as rat poison

Question 18

give the substrates, products, and enzyme for reaction 3 of the citrate cycle

Answer 18

substrates: isocitrate, NAD+
products: alpha-ketogluterate, NADH, CO2
enzyme: isocitrate dehydrogenase

Question 19

list 3 key features of reaction 3 of the citrate cycle

Answer 19

1. highly exergonic
2. IRREVERSIBLE
3. oxidative carboxylation reaction (carbonyl lost as carbon dioxide)

Question 20

give the substrates, products, and enzyme of reaction 4 of the citrate cycle

Answer 20

substrates: alpha-ketogluterate, NAD+, CoASH
product: succinyl-CoA, NADH + H+, CO2
enzyme: alpha-ketogluterate dehydrogenase

Question 21

list 3 key factors of reaction 4 of the citrate cycle

Answer 21

1. highly exergonic
2. IRREVERSIBLE
3. oxidative decarboxylation reaction (carbonyl lost as CO2 in same mechanism as pyruvate dehydrogenase)

Question 22

list the substrates, products, and enzyme for step 5 of the citrate cycle

Answer 22

substrates: succinyl-CoA, Pi, GDP
products: succinate, CoA, GTP
enzyme: succinyl-CoA synthetase

Question 23

what type of reaction is step 5 of the citrate cycle where succinyl CoA is converted to succinate?

Answer 23

substrate level phosphorylation

Question 24

list the substrates, products, and enzyme of step 6 of the citrate cycle

Answer 24

substrates: succinate, FAD
products: fumarate, FADH2
enzyme: succinate dehydrogenase

Question 25

list 2 key factors about step 6 of the citrate cycle

Answer 25

1. FAD prosthetic group serves as electron transport intermediate
2. linked to the electron transport system

Question 26

list the substrates, product, and enzyme of step 7 of the citrate cycle

Answer 26

substrates: fumarate, H2O
product: malate
enzyme: fumarase

Question 27

list 2 key factors about reaction 7 of the citrate cycle

Answer 27

1. hydrates the C-C double bond
2. fumarase is highlystereospecific and generates L-malate only

Question 28

list the substrates, products, and enzyme of reaction 8 of the citrate cycle

Answer 28

substrates: malate, NAD+
products: oxaloacetate, NADH, H+
enzyme: malate dehydrogenase

Question 29

describe the free energy of reaction 8 of the citrate cycle

Answer 29

highly endergonic

Question 30

relate the free energy of step 1 and step 8 of the citrate cycle

Answer 30

step 1 is hella exergonic, enough to immediately use the products of step 8, so since step 1 and step 8 are coupled, step 1 drives step 8 forward too

Question 31

what enters the citrate cycle?

Answer 31

1 acetyl CoA (or 2 if coming from glycolysis)

Question 32

what leaves the citrate cycle (3)

Answer 32

1. 3 NADH
2. 1 FADH2
3. 1 GTP
   (double if starting from glycolysis)

Question 33

what are the 3 irreversible steps in the citrate cycle? describe their free energy

Answer 33

steps 1, 3, and 4; are highly exergonic

Question 34

what do dehydrogenase enzymes do in the citrate cycle?

Answer 34

make the reductants NADH and FADH2

Question 35

in what 3 steps is NADH produced in the citrate cycle?

Answer 35

steps 3, 4, and 8

Question 36

in what step in the citrate cycle is FADH2 produced?

Answer 36

step 6

Question 37

in what step is GTP produced in the citrate cycle?

Answer 37

step 5