Metabolism part 1

Question 1

What is metabolism?

Answer 1

The complete set of catabolic (exergonic) and anabolic (endergonic) reactions that occur within living cells

Question 2

Where is energy in ATP stored?

Answer 2

In the phosphoanhydride bonds

Question 3

How do we obtain energy?

Answer 3

Through the oxidation of organic molecules (food)

Question 4

What is the difference between NAD and NADP?

Answer 4

NADP has a phosphate
The molecules pick up energy in the form of 2 high energy electrons and a proton - hydride ion

Question 5

What is glycolysis and when does it occur?

Answer 5

Obtaining energy from glucose
Consists of 10 steps
Occurs in both aerobic and anaerobic conditions

Question 6

What is step 1 of glycolysis?

Answer 6

Glucose is trapped in the cell and it is phosphorylated by hexokinase which adds a phosphate from ATP hydrolysis. This forms glucose 6 phosphate which cant leave the cell

Question 7

What is step 2 of glycolysis?

Answer 7

Glucose 6 phosphate undergoes isomerisation to form fructose 6 phosphate

Question 8

What is step 3 of glycolysis?

Answer 8

The fructose 6 phosphate is phosphorylated to form Fructose 1,6 bisphosphate catalysed by PFK

Question 9

What is step 4 and 5 of glycolysis?

Answer 9

Aldolase splits the F 1,6 BP into G-3-P and DHAP but DHAP is not on the glycolysis pathway but is an isomer of G-3-P so is converted to it

Question 10

What is the importance of converting from glucose to fructose?

Answer 10

It would have split into 2 x isomers (2 and 4C molecules instead)

Question 11

What is step 6 of glycolysis?

Answer 11

Enzyme 1 binds to the G-3-P and catalyses its oxidation
NAD+ accepts the electrons and is reduced
The enzyme is displaced by Pi to create a high energy sugar phosphate (1,3- bisphosphoglycerate)

Question 12

What is step 7 of glycolysis?

Answer 12

The intermediate binds to enzyme 2 which catalyses the transfer of Pi to ADP
thus creating ATP and generating a carboxylic acid 3-phosphoglycerate

Question 13

What is step 8 of glycolysis?

Answer 13

3- phosphoglycerate is converted to its isomer 2 - phosphoglycerate

Question 14

What is step 9 and 10 of glycolysis?

Answer 14

2- phosphoglycerate loses a molecule of water becoming phosphoenolpyruvate (PEP) which is unstable so donates a phosphate group to ADP forming ATP and PEP is converted to pyruvate.

Question 15

Where does lactate come from?

Answer 15

In anaerobic cells glycolysis is the principle source of ATP
NAD must be regenerated to enable glycolysis to continue
When no O2 can act as the final electron acceptor NADH passes electrons to pyruvate to form lactate

Question 16

How is PFK regulated?

Answer 16

Regulatory molecules bind to the allosteric site to either increase or decrease the affinity of PFK for F-6-P
Activated by - AMP, F 2,6 BP
Inhibited by - ATP, decrease in pH and citrate

Question 17

What is gluconeogenesis?

Answer 17

Occurs in the liver and is the formation of glucose from non carbohydrate precursors e.g. lactate, glucogenic amino acids

Question 18

Why is gluconeogenesis not the reverse of glycolysis?

Answer 18

It would be energetically unfavourable

Question 19

What is step 1 of gluconeogenesis?

Answer 19

The mitochondria converts pyruvate to oxaloacetate by pyruvate carboxylase which adds CO2 as it has a biotin prosthetic group

Question 20

What is step 2 of gluconeogenesis?

Answer 20

Oxaloacetate is converted to malate which can leave the mitochondria (very impermeable) it is then converted back to oxaloacetate in the cytosol by NADH. This is then converted into PEP

Question 21

What is step 3 of gluconeogenesis?

Answer 21

The reverse of glycolysis occurs and fructose 1-6 bisphosphate forms it is then converted to fructose 6 phosphate which is isomerised to form glucose 6 phosphate which is then converted to glucose

Question 22

How is glycolysis and gluconeogenesis regulated?

Answer 22

Glycolysis generates ATP so AMP activates glycolysis and inhibits gluconeogenesis

Question 23

What is the Cori cycle?

Answer 23

During heavy exercise - high glycolytic flux in the muscles- lactate can enter the blood and then enter the liver where it undergoes gluconeogenesis converting it back to glucose

Question 24

Outline the entry into the TCA cycle

Answer 24

Pyruvate moves into the mitochondria where pyruvate dehydrogenase complex converts it to acetyl co A
It does this by decarboxylating it, then it is oxidised by NADH, co enzyme A is added forming acetyl co A

Question 25

Outline the TCA (Krebs) cycle

Answer 25

Citrate synthase sequentially binds a 4C oxaloacetate then the 2C acetyl co A which forms an enol bond and forms citryl CoA which citrate synthase can break down to form a 6C citric acid and prevents acetate and co A forming
Energy is released and captured by - 3x NADH, 1x FADH2, 1x GTP

Question 26

How is the TCA cycle regulated?

Answer 26

Matched to the cell’s energy need
2 enzymes can be allosterically regulated
Isocitrate dehydrogenase
stimulated by ADP NAD
Inhibited by ATP NADH
Alpha ketoglutarate dehydrogenase
Stimulated by ADP
Inhibited by ATP NADH

Question 27

How does the TCA cycle aid biosynthesis?

Answer 27

Citrate can been taken out to form fatty acids
Oxaloacetate can be taken out to form amino acids

Question 28

What is anapleurosis?

Answer 28

Reactions that replenish the TCA cycle intermediates

Question 29

What is an example of anapleurosis?

Answer 29

Oxaloacetate drawn off the TCA cycle for amino acid synthesis, increase acetyl coA which increases pyruvate carboxylase activity which increases oxaloacetate thus increasing TCA flux.