3 glucose metabolism 1 Flashcards
where does digestion of starch start?
- 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
What is glucose rich in?
potential energy
what is glucose a precursor of?
metabolic intermediates for biosynthetic reactions to make AA, nucleotides, coenzymes, and fatty acids etc
what are the 4 fates of glucose?
- stored as polysaccharide or as sucrose
- oxidised to pyruvate (3C compound) vis glycolysis to provide ATP and metabolic intermediates
- oxidised via pentose phosphate pathway to give ribose 5-phosphate for nucleic acid synthesis and NADPH
- used for synthesis of structural polymers (e.g. chitin)
What are the 3 stages of glucose breakdown?
- glycolysis
- citric acid cycle
- electron transport chain
what phases are glycolysis seperated into?
Phase 1: preparatory phase
Phase 2: payoff phase
what is used in glycolysis?
- 1 glucose
- 2 ATP
- 2 NAD+
what is made in glycolysis?
- 2 pyruvate (various fates)
- 4 ATP
- 2 NADH - must be reoxidized to NAD+ in order for glycolysis to continue
What is the net gain of ATP in glycolysis?
2
generated 4 but used 2 in phase 1
Summary of glycolysis
Glucose + 2NAD+ + 2ATP + 2Pi -> 2 pyruvate + 2NADH + 2H+ + 2ATP
What are the fates of 2 pyruvate in hypoxic or anaerobic conditions?
- 2 ethanol + 2 CO2 (fermentation to ethanol in yeast)
- 2 lactate (muscles)
What is the fate of 2 pyruvate in aerobic conditions?
- 2 acetyl-CoA (release 2 CO2)
- go into citric acid cycle (kreb cycle)
- can be fully oxidised into 4CO2 and 4 H20
What are carbohydrates other than glucose transformed into when broken down? how
one of the glycolytic intermediates to enter glycolysis
via the feeder pathways of glycolysis
How can sucrose enter glycolysis?
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
How can dietary glycogen and starch enter glycolysis?
dk if need to know
can be broken down into simple glucose before being fed to the glycolytic pathway
glycogen into glucose 1 phosphate then glucose 6 phosphate
What is the major catabolic fate of G6P (glucose-6-phospahte) in most animal tissue?
is glycolysis to pyruvate
What are the fates of G6P (glucose-6-phosphate)
besides glycolysis and converted into pyruvate, it can go to the pentose phosphate pathway (PPP)
why is the pentose phosphate pathway (PPP) important?
- ribose-5-phosphate are made for the synthesis of nucleotides
- NADPH is produced for providing reducing power for biosynthetic reactions (e.g. making lipids and AA etc), important to when making biomolecules
how is NADPH produced in the pentose phosphate pathway?
conversion of G6P into ribose generates one NADPH
What does G6P depend on to decide whether to enter glycolysis or PPP?
- 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
What goes in glycolysis and PPP and what comes out?
glucose -> G6P (glucose w phosphate from ATP->ADP)
into glycolysis = 2 pyruvate + 2NADH + 2H+ + 2ATP
into PPP = ribose 5 phosphate and NADPH
What is gluconeogenesis?
synthesising glucose
what do some tissues (e.g. brain, nervous system, RBC etc) solely depend on for fuel?
glucose from blood
is supply of glucose from stores and diet enough?
may not be sufficient
What does the gluconeogenic pathway convert?
converts pyruvate to glucose
How do non carbohydrates precursors enter the gluconeogenesis pathway ?
they are converted to pyruvate or enter the pathway at later stages
What are the 3 major non-carbohydrate precursors?
- lactate
- AA
- glycerol - from fats
How is lactate readily converted to pyruvate?
- lactate dehydrogenase
- under hypoxic or anaerobic conditions
Where does gluconeogenesis take place in mammals?
mainly in the liver
What happens once glucose is made in the liver via glyconeogenesis?
glucose is supplied to other tissues via blood circulation
Are glycolysis and gluconeogenesis identical, just running in opposite directions?
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
Why is glycolysis and gluconeogenesis not identical and just running in opposite directions?
- 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).
How many irreversible steps in glycolysis?
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
How can be bypass the irreversible glycolytic reactions in glyconeogenesis?
find new enzymes
What are the 3 bypass reactions from pyruvate up
- conversion of pyruvate to phosphoenolpyruvate
What are the 3 irreversible glycolytic reactions?
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
Bypass 1: How to bypass the rxn of phosphoenolpyruvate to pyruvate
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
Bypass 2: How to bypass the rxn of fructose 6 phosphate into fructose 1,6 bisphosphate
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
Bypass 3: How to bypass the rxn of glucose to glucose 6 phosphate
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
