Glycolysis and Carbohydrates Flashcards
Why is the glycolysis pathway not in equilibrium (i.e. one sided) even if most reactions occur at equilibrium?
- we have a basal metabolic rate
- we consume energy all the time
- body is an open system (we lose energy all the time)
What is Gibbs Free Energy and what does ΔG and ΔG° indicate?
- Gibbs free energy is the indicator of spontaneity
- ΔG: is the true indication of the direction of a reaction
- ΔG°: indicates the nature of the reaction (theoretical)
How do we know when a reaction is endergonic, exergonic, reversible, and irreversible?
- ender: ΔG° is positive (requires energy)
- exer: ΔG° is negative (releases energy)
- rever: ΔG is zero or close to zero
- irrever: ΔG is much less that zero
How is glucose absorbed into the blood?
in the intestine:
- active transport: by Na+/glucose symport and Na+/K+ ATPase
- passive transport: glucose uniport
what pancreatic cell type uptakes glucose?
pancreatic B-cells
how/when does the pancreas uptake glucose and what is its role?
glucose goes in cells through Glut2 when [glucose] > 5.5mM and it releases insulin:
- no glucose: K(ATP) channel pumps K+ out, causing membrane hyper-polarization, which inhibits the Ca+ channel
- glucose: causes production of ATP in the cell, which inhibits the K(ATP) channel, which causes membrane depolarization, which opens the Ca+ channel, causing release of insulin in the blood
what is the role of insulin?
stimulate glucose uptake in adipose tissue and muscle, and stimulate glycogen synthesis in liver
Glut4 vs Glut2
Glut4:
- inducible by insulin
- not usually on surface but is recruited in the presence of insulin
- promotes glycogen synthesis (muscle) /lipogenesis (adipose tissue)
Glut2:
- non-inducible
- on liver
what is the role of the liver?
the keep glucose concentration at 5.5mM, and will do this by:
- fed-state: store glucose as glycogen
- fasting state: breaks down glycogen and gluconeogenesis
why are there so many glucose transporters?
- must be tailored to the needs of a tissue
- sugar specificity
- provides backup in case there is an issue
what is the ATP source during high-intensity exercise (at 6 secs, 10 secs, and 30-40 secs) and during prolonged duration?
- 6 secs: stored ATP in muscle
- 10 secs: from creatine ( P-creatine + ADP ➝ creatine + ATP)
- 30-40 secs: anaerobic glycolysis
- hours: aerobic glycolysis
What is the overview of the generation of ATP? What does each step produce and what controls each step?
- Glycolysis: generates 2 ATPs, 2 NADH, 2 Pyruvate out of 1 glucose, and does not require oxygen / stimulated by low ATP and inhibited by low NAD and high ATP
- Citric Acid Cycle: produces NADH and FADH2 out of acetyl-CoA / inhibited by high NADH
- Oxidative phosphorylation: produces ATP out of NADH, FADH2 and O2 / stimulated by low ATP, and inhibited by high ATP, low O2, and low NADH and FADH2
what is required for glycolysis to move forward? how is it replenished under different conditions (with and without O2)?
- NAD+ is required
- with O2, constantly replenished by oxidative phosphorylation
- without O2, can go through homolactic fermentation (production of lactic acid in muscle) or alcoholic fermentation in bacteria
What is the main source of ATP for cancer cells? What does this tell us?
glycolysis
- tells us that must use a lot of glucose since 17x less effective that oxidative phosphorylation
- they produce high amounts of pyruvate, acetyl-CoA, and lactic acid which is pumped out of the cancer cells by transporters
Which steps in glycolysis are the regulatory steps?
step 1 (glucose to G6P), step 3 (F6P to F1,6P), and step 10 (PEP to pyruvate)
Why is it important that glucose gets converted to glucose-6-phosphate?
- traps glucose in the cell (phosphorylated compounds do not transport readily)
- keeps the concentration of glucose in the cytoplasm low, which allows more glucose to be taken up passively
Compare hexokinase and glucokinase.
(catalyzed reaction, substrate specificity, tissue of prevalence, affinity constant, modulator, and metabolic consequence)
- catalyzed reaction: same reaction glucose + ATP → glucose-6-phosphate + ADP
- substrate specificity: hexoses vs glucose
- tissue of prevalence: all cell types vs liver
- affinity constant: ~0.1mM vs ~5mM
- modulator: glucose-6-phosphate vs glucokinase regulatory protein (GKRP)
- metabolic consequence: because hexokinase has a higher affinity for glucose, all cells will take up glucose first but it will saturate quickly, so the rest of the glucose will then be converted to glycogen in the liver by glucokinase
What is the role of glucokinase regulatory protein (GKRP)?
it is a binding partner that sequesters glucokinase in the nucleus when glucose levels are low and when glucose levels are high GKRP releases glucokinase to the cytoplasm
what does Phosphoglucose Isomerase do and what is it driven by?
equilibrium reaction ( glucose-6-phosphate ⇆ fructose-6-phosphate) that is driven by the abundance of substrates
what is the step that commits cells to metabolize glucose?
Conversion of fructose-6-phosphate into fructose-1,6-bisphosphate by PFK and ATP
what are the activators and inhibitors of PFK? How do these work?
- activators: AMP and F2,6P
- inhibitors: ATP, citrate
PFK has two states: T-state (inactive dimer) and R-state (active tetramer), AMP and F2,6P promotes R-state and ATP promotes T-state
Is ATP or AMP more potent at modulating PFK?
AMP
Role of Adenylate Kinase (ADK) and its biochemical consequence.
- it is an enzyme that regenerates ATP from two molecules of ADP ( 2 ATP ⇆ ATP + AMP )
- biochemical consequence: ADK keeps ATP abundant for exercise and also generates AMP, which activates PFK to create even more ATP
What principle allows for regulation of directionally in catabolic and anabolic pathways?
the pathway from the 1st to the 2nd metabolite must be different from the pathway from the 2nd to the 1st metabolite
Explain the PFK and FBPase cycle.
- F6P is converted to F1,6P by PFK1 by using 1 ATP and then can be converted to GAP and DHAP in the next step of glycolysis OR
- F1,6P can be converted back to F6P by FBPase (in gluconeogenesis)
- AMP and F2,6P is an activator of the glycolysis pathway and an inhibitor of the gluconeogenesis pathway
- at rest, both pathways are at equilibrium but when energy is needed (like during exercise) the glycolysis pathway is highly activated (through the AMP/F2,6P regulation)
Compare PFK1 and PFK2. (which reaction does it catalyze, role in glycolysis, type of activity, what are its modulators)
- catalyze: F6P→F1,6P vs F6P⇆F2,6P
- glycolysis: effector vs modulator
- activity: kinase only vs kinase and phosphatase
- modulators: ATP (inhibits) and AMP/F2,6P (stimulate) vs PKA
What are the two PFK2 isozymes?
liver isozyme and heart muscle isozyme
How is the PFK2 liver isozyme regulated?
when there is fasting or prolonged exercise,
- [blood glucose] decreases
- so [glucagon] increases
- causes increased cAMP
- which releases PKA
- the phosphatase side of PFK2 is phosphorylated
- decreased F2,6P
- activates gluconeogenesis in order to make more glucose
How is the PFK2 heart muscle isozyme regulated?
when there is stress:
- epinephrine is releases
- increase in cAMP
- which releases PKA
- the kinase side of PFK2 is phosphorylated
- increased F2,6P
- activates glycolysis
what is the role of aldose?
splits one 6 carbon sugar into two 3 carbon sugar (makes DHAP and GAP)
what is the role of triose phosphate isomerase?
interchanges GAP and DHAP
what is the concentration of GAP vs DHAP in cell and why does this occur?
DHAP»GAP since GAP is quickly consumed by glycolysis so to maintain equilibrium GAP is converted to DHAP
what are the roles of GAPDH and phosphoglycerate kinase?
They couple their reactions together to create NADH and ATP:
- reaction of GAPDH is endergonic and creates 1,3-bisphosphoglycerate
- reaction of phosphoglycerate kinase is exergonic
- overall reaction is exergonic
is 1,3-bisphosphoglycerate a high or low energy intermediate and what does this mean?
high energy, so it rapidly passed along/does not accumulate
what is the role of phosphoglycerate mutase?
transfers a phosphate from position 3 to position 2
what is the role of phosphoglycerate enolase?
dehydrates 2-phosphoglycerate to PEP, which is a high energy intermediate
what is the role of pyruvate kinase and how is it regulated?
- it generates 2 ATP
- allosterically: activated by F1,6P and inhibited by ATP
- hormonally: inhibited by glucagon (cAMP, PKA) / PKA phosphorylates the kinase
What is the major and the minor metabolism pathway for fructose?
major: in liver
minor: in muscle
Explain the liver fructose metabolic pathway.
- Fructokinase makes F1P (using ATP)
- F1P is separated into glyceraldehyde + DHAP by aldolase
Then two options:
1. glyceraldehyde made into GAP by glyceraldehyde kinase (using ATP) and DHAP made into GAP (normal glycolysis pathway)
2. glyceraldehyde made into glycerol by alcohol dehydrogenase, then made into glycerol-3-phosphate by glycerol kinase (using ATP), then can either be made into DHAP (by dehydrogenase) or triacylglycerol if needed
Explain the muscle fructose metabolic pathway.
- fructose made into F6P by hexokinase
- F6P made into F1,6P by PFK (normal glycolysis)
- F1,6P made into GAP + DHAP by aldolase
does fructokinase or hexokinase have a higher affinity for fructose?
fructokinase
How is fructose metabolism regulated?
- in muscle, regulated by normal glycolysis control since enters the pathway before the PFK point of control
- in liver, there is no control. all fructose enters glycolysis, but since the body doesn’t need energy, it will make fatty acids instead of entering the citric acid cycle
Explain the galactose metabolic pathway.
- galactose made into galactose-1-phosphate by galactokinase (using ATP)
- the made into G1P by a urityltransferase that takes glucose from UDP-glucose, which is then converted to UDP-galactose (UDP-glucose regenerated by UDP-galactose epimerase
- G1P converted to G6P by phosphoglucomutase
is galactose a substrate for hexokinase?
no
what is galactosemia? what are the different types?
- a rare autosomal recessive genetic disorder
- Type I: problem with urityltransferase
- Type II: problem with galactokinase
- Type III: problem with epimerase
Where is mannose found?
mostly in glycoproteins
Explain the mannose metabolic pathway.
since is a substrate for hexokinase,
- mannose converted to mannose-6-phosphate by hexokinase
- mannose-6-phosphate made into F6P by phosphomannose isomerase
- then enters glycolysis (PFK makes F1,6P)
