3 glucose metabolism 1

Question 1

where does digestion of starch start?

Answer 1

• mouth cavity - broken down to amylose
• gastrointestinal tract - until all carbs broken down into monosaccharides
• monosaccharides are absorbed through walls of small intestine and travel to the liver via portal blood circulation
• cellulose and other fibers remain undigested and will be expelled

Question 2

What is glucose rich in?

Answer 2

potential energy

Question 3

what is glucose a precursor of?

Answer 3

metabolic intermediates for biosynthetic reactions to make AA, nucleotides, coenzymes, and fatty acids etc

Question 4

what are the 4 fates of glucose?

Answer 4

1. stored as polysaccharide or as sucrose
2. oxidised to pyruvate (3C compound) vis glycolysis to provide ATP and metabolic intermediates
3. oxidised via pentose phosphate pathway to give ribose 5-phosphate for nucleic acid synthesis and NADPH
4. used for synthesis of structural polymers (e.g. chitin)

Question 5

What are the 3 stages of glucose breakdown?

Answer 5

1. glycolysis
2. citric acid cycle
3. electron transport chain

Question 6

what phases are glycolysis seperated into?

Answer 6

Phase 1: preparatory phase
Phase 2: payoff phase

Question 7

what is used in glycolysis?

Answer 7

• 1 glucose
• 2 ATP
• 2 NAD+

Question 8

what is made in glycolysis?

Answer 8

• 2 pyruvate (various fates)
• 4 ATP
• 2 NADH - must be reoxidized to NAD+ in order for glycolysis to continue

Question 9

What is the net gain of ATP in glycolysis?

Answer 9

2
generated 4 but used 2 in phase 1

Question 10

Summary of glycolysis

Answer 10

Glucose + 2NAD+ + 2ATP + 2Pi -> 2 pyruvate + 2NADH + 2H+ + 2ATP

Question 11

What are the fates of 2 pyruvate in hypoxic or anaerobic conditions?

Answer 11

• 2 ethanol + 2 CO2 (fermentation to ethanol in yeast)
• 2 lactate (muscles)

Question 12

What is the fate of 2 pyruvate in aerobic conditions?

Answer 12

• 2 acetyl-CoA (release 2 CO2)
• go into citric acid cycle (kreb cycle)
• can be fully oxidised into 4CO2 and 4 H20

Question 13

What are carbohydrates other than glucose transformed into when broken down? how

Answer 13

one of the glycolytic intermediates to enter glycolysis
via the feeder pathways of glycolysis

Question 14

How can sucrose enter glycolysis?

Answer 14

sucrose can be broken into glucose and fructose by sucrase enzyme

Fructose can be converted to fructose 6 phosphate and it goes into the glycolytic pathway

Question 15

How can dietary glycogen and starch enter glycolysis?

dk if need to know

Answer 15

can be broken down into simple glucose before being fed to the glycolytic pathway

glycogen into glucose 1 phosphate then glucose 6 phosphate

Question 16

What is the major catabolic fate of G6P (glucose-6-phospahte) in most animal tissue?

Answer 16

is glycolysis to pyruvate

Question 17

What are the fates of G6P (glucose-6-phosphate)

Answer 17

besides glycolysis and converted into pyruvate, it can go to the pentose phosphate pathway (PPP)

Question 18

why is the pentose phosphate pathway (PPP) important?

Answer 18

1. ribose-5-phosphate are made for the synthesis of nucleotides
2. NADPH is produced for providing reducing power for biosynthetic reactions (e.g. making lipids and AA etc), important to when making biomolecules

Question 19

how is NADPH produced in the pentose phosphate pathway?

Answer 19

conversion of G6P into ribose generates one NADPH

Question 20

What does G6P depend on to decide whether to enter glycolysis or PPP?

Answer 20

• current need of the cell or
• the concentration of NADPH

if low NADPH conc G6P will go to PPP
if sufficient, NADPH will act as inhibitor and block G6P from the PPP and will go to glycolysis to provide ATP

Question 21

What goes in glycolysis and PPP and what comes out?

Answer 21

glucose -> G6P (glucose w phosphate from ATP->ADP)

into glycolysis = 2 pyruvate + 2NADH + 2H+ + 2ATP
into PPP = ribose 5 phosphate and NADPH

Question 22

What is gluconeogenesis?

Answer 22

synthesising glucose

Question 23

what do some tissues (e.g. brain, nervous system, RBC etc) solely depend on for fuel?

Answer 23

glucose from blood

Question 24

is supply of glucose from stores and diet enough?

Answer 24

may not be sufficient

Question 25

What does the gluconeogenic pathway convert?

Answer 25

converts pyruvate to glucose

Question 26

How do non carbohydrates precursors enter the gluconeogenesis pathway ?

Answer 26

they are converted to pyruvate or enter the pathway at later stages

Question 27

What are the 3 major non-carbohydrate precursors?

Answer 27

1. lactate
2. AA
3. glycerol - from fats

Question 28

How is lactate readily converted to pyruvate?

Answer 28

• lactate dehydrogenase
• under hypoxic or anaerobic conditions

Question 29

Where does gluconeogenesis take place in mammals?

Answer 29

mainly in the liver

Question 30

What happens once glucose is made in the liver via glyconeogenesis?

Answer 30

glucose is supplied to other tissues via blood circulation

Question 31

Are glycolysis and gluconeogenesis identical, just running in opposite directions?

Answer 31

Not identical!
pyruvate can go back to glucose but not straigh reversal of the 10 steps

some reactions are reversable not some not so direct

Question 32

Why is glycolysis and gluconeogenesis not identical and just running in opposite directions?

Answer 32

• if all enzymes are the same, the 2 pathways will be impossible to regulate
• the equilibrium of glycolysis favours pyruvate formation
• the reverse reaction is energetically unfeasible

glycolysis = glucose into pyruvate, releasing energy (ATP and NADH).
Gluconeogenesis = synthesizes glucose from non-carbohydrate precursors (like pyruvate), which requires energy input (ATP and GTP).

Question 33

How many irreversible steps in glycolysis?

Answer 33

3
either on top or bottom
cant use the same enzyme to go back once happened

top 2: ATP -> ADP (harder to add phosphate back sometimes)
bottom: 2ADP -> 2 ATP

Question 34

How can be bypass the irreversible glycolytic reactions in glyconeogenesis?

Answer 34

find new enzymes

Question 35

What are the 3 bypass reactions from pyruvate up

Answer 35

1. conversion of pyruvate to phosphoenolpyruvate

Question 36

What are the 3 irreversible glycolytic reactions?

Answer 36

from pyruvate up
1. rxn of phosophoenolpyruvate to pyruvate
2. rxn of fructose-6-phosphate to fructose 1,6-bisphosphate
3. rnx of glucose to glucose-6-phosphate

Question 37

Bypass 1: How to bypass the rxn of phosphoenolpyruvate to pyruvate

Answer 37

by converting pyruvate to phosphoenolpyruvate
* go thru a sequence of rxn using enzymes in both the cytosol and mitochondria

via 2 routes
* route 1 is the predominant pathway in liver
* route 2 mainly in anaerobic muscles with lactate produced after vigorous exercise

Question 38

Bypass 2: How to bypass the rxn of fructose 6 phosphate into fructose 1,6 bisphosphate

Answer 38

by converting fructose 1,6 bisphosphate into fructose 6 phosphate
glycolysis: fructose 6 phosphate is phosphorylated to fructose 1,6 bisphosphate by phosphofructokinase-1
use another enzyme to dephosphorylate: fructose 1,6, biphosphatase, no ATP needed

reversal of phosphorylation needed ATP, so instead uses another enzyme that just cleaves off the phosphate, more energetically favourable

Question 39

Bypass 3: How to bypass the rxn of glucose to glucose 6 phosphate

Answer 39

by converting glucose 6 phosphate to glucose

glycolysis: glucose -> glucose 6 phosphate by enzyme hexokinase
bypass: use another enzyme glucose 6 phosphatase

cleave off the phosphate group into inorganic phosphate, dephosphorylation, similar to bypass 2