citric acid cycle

Question 1

TCA cycle - a biochemical hub of the intermediary metabolism:

1. oxdizing carbons fuels for harvesting:
2. it is ____ (e.g. catabolism and anabolism)
3. source of precursors for
4. takes place inside ____

Answer 1

1. high energy electrons
2. amphibolic
3. biosynthesis
4. mitochondria

Question 2

other names of TCA cycle

Answer 2

Tricarboxylic acid cycle (TCA)

citric acid cycle

krebs cycle

Question 3

precursor stage 1 of TCA

Answer 3

fats, polysaccharides, and proteins are reduced to fatty acids and glycerol, glucose and other sugars, and amino acids

Question 4

precursor stage 2 of TCA

Answer 4

fatty acids and glycerols, glucose and other sugars, and amino acids are all converted to Acetyl CoA (common currency)

Question 5

1 mol of NADH =

Answer 5

2.5 moles ATP

Question 6

1 mol of FADH2 =

Answer 6

1.5 moles ATP

Question 7

1 mol of GTP =

Answer 7

1 mol ATP

Question 8

TCA cycle overview: oxidation of 2-carbon units to produce:

Answer 8

2 CO₂ molecules

1 GTP

High energy electrons (3 NADH and 1 FADH₂)

Question 9

how is glucose converted into acetyl CoA?

Answer 9

glucose (6C) –oxidation–> 2 pryuvate (3C) –decarboxylation–> 2 Acetyl CoA (2C)

Question 10

how are lipids converted into acetyl CoA?

Answer 10

TAG –> fatty acids –betaoxidation–> acetyl CoA (2C)

Question 11

how are proteins converted to Acetyl CoA?

Answer 11

breakdown into various amino acids –> acetyl CoA

Question 12

how is pyruvate converted to acetyl CoA?

Answer 12

1. decarboxylation
2. oxidation
3. transfer acetyl group to CoA (by coupling with NAD+ –> NADH rxn)

Question 13

aerobic glycolysis coupled with oxidative phosphorylation:

1. pyruvate must enter:
2. pyruvate utilizes:
3. ______ catalyzes the decarboxylation of pyruvate

Answer 13

1. the mitochondira
2. mitochondrial pyruvate carrier (MPC)
3. pyruvate dehydrogenase complex (PDC) or (PDH)

Question 14

pyruvate dehydrogenase reaction:

1. requires 3 enzymes:
2. requires 5 coenzymes: catalytic cofactors and stoichiometric cofactors

Answer 14

1. E₁ (TPP), E₂(Lipoic acid), E₃(FAD)

2.

catalytic cofactors:

• thiamine pyrophosphate (TPP/Vitamin B₁)
• lipoic acid
• FAD

stoichiometric cofactor:

• CoA
• NAD+

Question 15

phosphorylated version of PDC/PDH is

Answer 15

inactive

Question 16

dephosphorylated version of PDC/PDH is

Answer 16

active

Question 17

overall reaction of pyruvate –> Acetyl CoA

Answer 17

pyruvate + NAD⁺ + CoA → Acetyl CoA + CO₂ + NADH + H⁺

Question 18

high [aceytl CoA] directly inhibits

Answer 18

E₂

Question 19

accumulation of ADP and pyruvate activates phosphatases which dephosphorylates ____

Answer 19

PDH/PDC (making it active)

Question 20

increased levels of acetyl CoA will inhibit

Answer 20

PDH/PDC

Question 21

the TCA cycle occurs under

Answer 21

aerobic conditions and procuces more energy from glucose than glycolysis

Question 22

the TCA cycle takes place in the

Answer 22

mitochondria

Question 23

what enzyme links glycolyis to the TCA cycle

Answer 23

pryuvate dehydrogenase (PDH/PDC)

Question 24

first step of TCA cycle

Answer 24

4-carbon oxaloacetate + 2-carbon acetyl CoA +H₂O → citrate

catalyzed by citrate synthase

Question 25

step 2 of TCA cycle

Answer 25

**citrate** → **isocitrate** catalyzed by aconitase (because hydroxl group is not in proper location of oxidative decarboxylation)

Question 26

step 3 of TCA cycle

Answer 26

**isocitrate** + NAD⁺ → **a-ketoglutarate** + **NADH** + H⁺ + **CO₂** catalyzed by isocitrate dehydrogenase (rate-limiting step) (first of 4 redox rxns) (NAD⁺ → NADH)

Question 27

step 4 of TCA cycle

Answer 27

**a-ketoglutarate** + NAD⁺ + CoA → **Succinyl CoA** + **NADH** + H⁺ + **CO₂** via a-ketoglutarate dehydrogenase complex

Question 28

step 5 of TCA cycle

Answer 28

**succinyl CoA** + GDP ⇔ **Succinate** + **GTP** + CoA catalyzed by succinyl CoA synthetase

Question 29

succinyl CoA contains a

Answer 29

high energy thioester bond

Question 30

succinyl CoA synthetase is the only step that yields

Answer 30

a high energy phopho-transfer compount (ATP or **GTP**) (substrate level phosphorylation)

Question 31

succinyl CoA synthetase isozymic forms: 1. form that produces GTP is used in: 2. form that produces ATP is used in:

Answer 31

1. tissues that perform many anabolic reactions (liver) 2. tissues that perofrm large amount of cellular respiration (skeletal and heart muscle)

Question 32

step 6 of TCA cycle

Answer 32

**succinate** + FAD → **Fumarate** + **FADH₂** catalyzed by succinate dehydrogenase (FADH₂ is not released from the enzyme, but electrons are passed directly to Co-Q in the ETC

Question 33

succinate dehydrogenase is located

Answer 33

in the inner mitochondrial membrane directly associated with ETC

Question 34

step 7 of TCA cycle

Answer 34

**fumarate** → **L-malate** catalyzed by fumarase

Question 35

step 8 of TCA cycle

Answer 35

**L-malate** + NAD⁺ → **Oxaloacetate** + **NADH** + H⁺ catalyzed by malate dehydrogenase

Question 36

high [acetyl CoA] directly inhibits

Answer 36

PDH/PDC complex subunit E₂ (**pyruvate** + NAD⁺ → **acetyl CoA** + NADH → **TCA cycle**)

Question 37

NADH inhibits

Answer 37

PDH/PDC complex subunit E₃ (**pyruvate** + NAD⁺ → **acetyl CoA** + **NADH** → TCA cycle) (ATP levels also inhibit PDH/PDC)

Question 38

high [pyruvate] and high [ADP] activate

Answer 38

PDH/PDC activity

Question 39

phosphatases are stimulated by

Answer 39

Ca²⁺ which increases to initiate muscle contraction

Question 40

insulin can stimulate

Answer 40

fatty acid synthesis by activating phosphatases and increasing the conversion of pyruvate to acetyl CoA (a precursor for fatty acids)

Question 41

activators of PDH/PDC

Answer 41

Ca²⁺, Mg²⁺ (via allosteric activation) ADP, CoA, NAD⁺, pyruvate insulin in adipose tissue, catecholamiens in cardiac muscle

Question 42

inhibitors of PDH/PDC

Answer 42

acetyl CoA, NADH acetyl CoA, ATP arsenite

Question 43

citrate synthase prevents

Answer 43

the wasteful hydrolysis of acetyl CoA (first control site of TCA cycle)

Question 44

oxaloacetate binds to citrate synthase first, then the enzyme

Answer 44

undergoes configurational changes to accept acetyl CoA

Question 45

second control site of TCA cycle

Answer 45

isocitrate dehydrogenase (allosterically stimulated by ADP) (NADH inhibits by directly displacing NAD⁺)

Question 46

third control site of TCA cycle

Answer 46

a-ketoglutarate dehydrogenase (this complex is similar to PDH/PDC) (allosterically inhibited by its products- succinyl CoA and NADH)

Question 47

buildup of citrate (due to regulation of isocitrate dehydrogenase) can be transported

Answer 47

to the cytosol and signal PFK to halt glycolysis

Question 48

a-ketoglutarate that builds up from enzyme inhibition can be used for

Answer 48

synthesis of amino acids and purine bases

Question 49

why is the TCA cycle anaplerotic

Answer 49

"fill up" rxns provide intermediates to replenish TCA cycle

Question 50

2 major anaplerotic reactions 1. degradation of amino acids 2. carboxylation of pyruvate

Answer 50

1. degradation of amino acids: * Gln, Pro, His, Arg → replenishes a-ketoglutarate * Thr, Met, Ile, Val → replenishes succinyl CoA * Phe, Tyr, Asp → Fumurate 2. carboxylation of pyruvate * pyruvate (ATP + PC) → replenishes oxaloacetate

Question 51

TCA cycle under fasting conditions

Answer 51

oxaloacetate → malate → gluconeogenesis → glucose

Question 52

when energy needs are met, TCA cycle intermediates are

Answer 52

drawn for biosynthesis of other molecules (citrate leaves mitochondria and forms fatty acids and sterols) (a-ketoglutarate → glutamate → other amino acids → purines) (succinyl CoA → porphyrins, heme, chlorophyll) (oxaloacetate → asparate → other amino acids, purines/pyrimidines)

Question 53

anaplerotic rxns are required during

Answer 53

states of low energy