TCA cycle

Question 1

Where does the Krebs cycle not take place?

Answer 1

mature red blood cells, as they lack mitochondria

Question 2

Citric Acid Cycle definition

Answer 2

A series of chemical reactions used by all aerobic organisms to release stored energy through oxidation of acetyl-CoA derived from carbohydrates, fats and proteins into ATP and carbon dioxide

Question 3

What else does the TCA cycle produce?

Answer 3

precursors of certain amino acids

NADH and FADH2

Question 4

What is FAD+ an example of?

Answer 4

cofactor

Question 5

Cofactor definition

Answer 5

a substance whose presence is essential for the activity of an enzyme

Question 6

Features of FAD+

Answer 6

Bound solidly onto the enzymes they’re involved with

less negative redox potential than NAD

can’t diffuse through membrane, thus reactions must be attached to membrane

lower reducing potential, so feeds electrons into ETC at ubiquinone at complex II

Question 7

Features of NAD+

Answer 7

higher redox potential

not bound onto enzyme

type of coenzyme

NADH product diffuse to complex I in ETC

Question 8

What process occurs before TCA ?

Answer 8

pyruvate decarboxylation

Question 9

Stages of pyruvate decarboxylation

Answer 9

1. Pyruvate transported into mitochondrion via specific pyruvate-H+ symport
2. Cofactor TPP decarboxylares pyruvate, forming CO2 and a 2C compound, catalysed by pyruvate decarboxylase
3. CoA added which is catalysed by dihydrolipoyl transactylase, forming Acetyl CoA
4. Formation of NADH, catalysed by dihydrolipoyl dehydrogenase. Two electrons are transferred first to a FAD then to NAD

Question 10

Brief structure of pyruvate dehydrogenase complex

Answer 10

three subunits: E1, E2 and E3

Lipoamide arm bound to E2 which guides the substrate from one subunit to the next, which ensures that the pyruvate decarboxylation occurs in the right direction

Question 11

Why pyruvate dehydrogenase is so important?

Answer 11

irreversible reactions, thus become committed to Krebs

beyond PDH glucose cannot be resynthesised

it is inhibited by ATP and stimulated by ADP, thus acts as a key regulator of TCA

Question 12

What inhibits pyruvate dehydrogenase + importance?

Answer 12

ATP- prevents excess breakdown of unnecessary glucose- inactivated by phosphorylation of a kinase stimulated by high ATP levels

NADH-

Question 13

How are levels of pyruvate dehydrogenase upregulated?

Answer 13

magnesium, calcium and insulin stimulate PDH phosphatase, dephosphorylating the enzyme, thus activating it

all markers or energetic contractions

Question 14

What causes poisoning when arsenic is ingested?

Answer 14

arsenite forms stable complex with thiol group of lipoic acid, which is found attached to the E2 group in the pyruvate dehydrogenase complex

leads to the inhibition of pyruvate dehydrogenase, which causes an increase in pyruvate and lactate

Question 15

1st reaction in TCA + Enzyme + type of reaction

Answer 15

oxaloacetate (4) + acetylcoA (2) –> citrate

water –> CoA + H+

citrate synthase

condensation

Question 16

2nd reaction in TCA + Enzyme

Answer 16

citrate —> isocitrate

Question 17

3rd reaction in TCA + Enzyme + type of reaction

Answer 17

isocitrate –> alpha- ketoglutarate

NAD+ –> NADH + H+ + CO2

isocitrate dehydrogenase

process of oxidative decarboxylation

Question 18

4th reaction in TCA + enzyme + type of reaction

Answer 18

alpha-ketoglutarate –> succinyl CoA

NAD+ + CoA –> NADH + CO2 + H+

alpha ketoglutarate dehydrogenase

oxidative decarboxylation

Question 19

What type of enzyme is alpha ketoglutarate dehydrogenase?

Answer 19

lipoic acid coenzyme and thiamine pyrophosphate TPP coenzyme

Question 20

5th reaction in TCA + enzyme + type of reaction

Answer 20

Succinyl CoA–> succinate + CoA

Pi + GDP –> GTP

succinyl-CoA synthase
thioesterase deacylation

Question 21

GTP function

Answer 21

phosphoryl donor in protein synthesis, gluconeogenesis and signal transductions

translocation of proteins into the mitochondrial matrix

conversion into ATP

Question 22

how is GTP converted into ATP?

Answer 22

GTP + ADP —> ATP + GDP

nucleoside diphosphokinase

Question 23

6th reaction in TCA + Enzyme + type of reaction

Answer 23

succinate —> fumarate

FAD —> FADH2

succinate dehydrogenase

oxidation

Question 24

Key features of succinate dehydrogenase

Answer 24

embedded in mitochondrial membrane

directly linked to ETC

only enzyme common to both TCA and ETC

Question 25

Why is reduced FAD produced, not NADH?

Answer 25

FAD is the hydrogen acceptor because the free energy is insufficient to reduce NAD+

Question 26

7th reaction in TCA + Enzyme

Answer 26

fumarate to malate

Question 27

8th reaction in TCA + Enzyme + type of reaction

Answer 27

malate --> oxaloacetate NAD+ --> NADH + H+ malate dehydrogenase

Question 28

advantages of a cycle

Answer 28

1. catalytically small amounts of cycle intermediates required to oxidise large amounts of acetyl CoA 2. only small amount of oxaloacetate needed, as it is regenerated 3. block the cycle at succinate dehydrogenase and it becomes an open pathway, less efficient- now for each acetyl CoA oxidised, 1 molecule of oxaloacetate is needed

Question 29

disadvantage of a cycle

Answer 29

the removal of cycle intermediates for biosynthesis can deplete levels of cycle intermediates need anaplerotic reactions to maintain concentrations

Question 30

Examples where anapleurotic reactions replenish TCA cycle intermediates

Answer 30

aspartate --> oxaloacetate amino acids/purine nucleotides--> fumarate amino acids --> glutamate --> a ketoglutarate odd numbered FAs/valine isoleucine --> propionyl CoA --> succinyl CoA

Question 31

Control of TCA cycle

Answer 31

most control of energy metabolism is not related to substrate availability instead a response for the demand for ATP

Question 32

Other functions of TCA cycle

Answer 32

routes of disposal for amino acids and odd chain fatty acids- 3 carbon units that occur during beta oxidation intermediates can be used as starting points for biosynthesis

Question 33

intermediates and their potential products

Answer 33

citrate - fatty acids alpha ketoglutarate- glutamate succinyl CoA - porphyrins, haeme oxaloacetate- aspartate, pyrimidines, glucose

Question 34

How much ATP is produced per molecule of TCA product?

Answer 34

NADH- 2.5 FADH2- 1.5 GTP - 1

Question 35

different sources of acetyl CoA

Answer 35

fatty acid beta oxidation ketone body oxidation amino acid degradation sugars glycolysed into pyruvate

Question 36

What can mutations in isocitrate dehydrogenase cause?

Answer 36

found in 60-90% of secondary gliomas 12-18% leukemias lead to the production of an oncometabolite

Question 37

What is beriberi?

Answer 37

thiamine deficiency, which forms TPP which is the prosthetic group of pyruvate dehydrogenase, alpha ketoglutarate dehydrogenase and transketolase symptoms: sheep-like gait, weakness mostly in asian populations, as rice has a low thiamine content and alcoholics

Question 38

Albert Szent- Gyorgyi's experiment

Answer 38

minced tissue and showed that it took up oxygen rapidly oxygen uptake was greatly increased when carbohydrates or other C3 and C4 products were added- more TCA

Question 39

What did Krebs and Johnson show?

Answer 39

Citrate was not only oxidised rapidly but it was also reformed, thus suggesting a cycle