Glycolysis Flashcards
4 ways glucose can be used in cells
Synthesis of structural polymers
Storage as glycogen
Oxidation via PPP
Oxidation via glycolysis
Preparatory phase (general)
Steps 1-5
2 ATP = investment energy
Glucose splits into 2 3C molecules
(Glyceraldehyde-3-phosphates)
Payoff phase (general)
Steps 6-10
4ATP (net 2ATP)
2 NADH —> e-chain for reducing power
Hexokinase
Step 1
Phosphorylation
Glucose —> G6P (1 ATP used)
Irreversible
**this locks glucose inside the cell…and thus will bring glucose into the cell because the concentration of glucose will be lower inside than outside
G6P can used in glycolysis, PPP, or glycogen synthesis
Analog in liver = glucokinase (higher affinity for glucose)
Phosphohexose isomerase
Isomerization of an aldose into a ketose
G6P—>F6P
Reversible (so [subrate] dictates)
Mg2+ is required
phosphofructokinase-1 (PK1)
F6P —> F-1,6-BP (1 ATP used)
Committment step
Aldolase
Cleaves F1,6BP —> DHAP and GAP
DHAP will proceed to step 5
GAP will proceed to step 6 (payoff phase)
Reversible
Triose phosphate isomerase
DHAP GAP
Favors making GAP since GAP is being used in payoff phase
GAP dehydrogenase
GAP from step 4 is oxidized
NAD+ is reduced to NADH
Prodcut = 1,3-biphosphoglycerate
Very high energy product so rxn is readily reversible
Phosphoglycerate kinase
Named for the reverse reaction
1,3-BPG +ADP —> 3-PG +ATP
Coupled reaction to make formation of ATP favorable
Phosphoglycerate mutase
3-PG —> 2-PG
Reversible
Mg2+ needed
Enolase
2-PG —> phosphenolpyruvate (PEP)
PEP = top of free energy list
Largest -deltaG value (but this reaction has a +deltaG) —>
Reversible
Pyruvate kinase
Named for reversible reaction
PEP + ADP —> Pyruvate + ATP
Irreversible
Very regulated step
Mg2+ and K+ needed
Diseases associated with this step
Net reaction for glycolysis
Glucose + NAD+ + 2ADP + 2Pi —>
2 pyruvate + 2NADH + 2ATP + H2O
Importance of 1,3-bisphosphoglycerate
And
3-PG
From step 6 and 7 respectively
Both of them can be converted to 2,3-BPG
Compound in RBCs used for regulating O2 release from hemoglobin
High 2,3-BPG levels —> lower hemoglobin’s affinity of O2 —> O2 unloading
Glycolytic intermediates can be converted into…
Amino acids
Pyruvate can be converted into
Acetyl CoA —> enter citric acid cycle which those intermediates can also be turned into amino acids
Acetly -CoA can also be converted into FAs
DHAP (Step 4) can be convereted into
Glyceride3-phosphate with fatty acids
—> triglycerides
Metabolic acidosis
E-chain and the CAC require O2…so it patient does not have enough O2 circulating…pyruvate enters anaerobic glycolysis
Reduced to lactate (lactate dehydrogenase)
Since NADH —> NAD+ net energy is reduced
RBCs use anaerobic because they do not want to use the O2 they are transporting
Normal glucose concentration
5mM
GLUT1
Almost all tissues (most importantly the brain)
Km = 1-2mM
Not regulated
Almost always active
GLUT2
Liver and pancreas
Basolateral membrane on SI
Only active in high [blood glucose] = high Km
Removes excess blood glucose (liver stores it)
NOT regulated by insulin
GLUT4
Muscle and fat cells
Km = 5mM (at normal blood glucose its at 50% of Vmax)
Regulated by insulin…high glucose —>insulin—>GLUT4 brought to the membrane
GLUT5
Mucosal (apical) membrane of SI
Spermatoza
Fructose transport (does not transport glucose)
Main fructose transporter in the body
Mechanism for GLUT 2 in pancreatic beta inslet cells
- Increased blood glucose caused glucose to flow into beta cell via GLUT2
- Glucose undergoes glycolysis —> results in ATP formation, so the concentration of ATP in the beta cells increase
- This causes the closure of an ATP-gated K+ channel
- (Normally K+ concentration is much higher inside than outside…closing that channel results in less K+ being able to leave the cell) —> increase in K+ retention causes depolarization
- Stimulates voltage-gated Ca2+ channels to open
- Ca2+ moves down its concentration gradient into the cell
- Increased Ca2+ in the beta cell stimulates secretory vesicles (containing insulin) to undergo exocytosis
Sulfonylureas and meglitinides
Drugs that affect GLUT2
Mimic ATP…outisde of the ATP-gated K+ channels, closing the channels
Results in depolarization and released insulin
Diazoxide
Used to treat hypoglycemia associated with hyperinsulimia
Could be due to a insulin secreting tumor
Caused voltage K+ channels to remain open —> no insulin released
Mechanism of GLUT4 regulation
During fasting —> blood glucose is low…below the 5mM Km of GLUT4
So we don’t want GLUT4 activity b/c we need glucose available for our brain
Low insulin too —> so GLUT4 is sequestered in an inactive form inside the cell via endocytosis
High glucose —> high insulin —> insulin binds to insulin receptors to the cell membrane, stimulating a signlaing pathway that activates protein kinase B (PKB)
PKB causes exocytosis of GLUT4 to the membrane where it is active
Type I Diabetes
Lack of insulin produced by beta cells
Cells cannot take up glucose —> no energy
Adipocytes will mobilize triglycerides and released FAs into the bloodstream…live will take these up and can form energy from them in the form of ketones
Can accumulate —> acidosis —> diabetic ketoacidosis
Regulation of hexokinase (step 1 of glycolysis)
At normal glucose levels the enzyme is almost completely saturated and functioning maximally
Allosterically inhibited by the product of the reaction (G6P)
Regulation of glucokinase (hexokinase IV)
Much higher Km for glucose compared to the other hexokinase
In liver —> glycolysis does not occur in these cells until glucose levels are fucking high bro!
Not inhibited by G6P…so really controlled by its high Km and thus the [glucose]
It is regulated for F6P which is the product of step 2
When glucose is low…PK1 (step 3 enzyme is inactivated) so F6P builds up in the cytoplasm…thus keeps glucokinase inactive…also allows liver glucose to be available to replenish low glucose levels
Regulation of PK1
Mostly regulated by allosteric factors
Allosteric activation —> high levels of ADP/AMP and F2,6BP (aka low energy levels)
Allosteric inhibition —> high ATP and citrate
Non-allosteric activation —> increase [F6P]
Non-allosteric inhibition —> increase in acidity (H+ ion concentration)
Mechanism of allosteric regulation via F26BP on PK-1
F26BP is the most potent allosteric activator of PK1…it is producted by PFK-2 (other domain of this enzyme is FBPase-2)
Low glucose —> glucagon —> activation of PKA through cAMP pathway —> PKA phosyphorylates enzyme —> activates FBPase-2 domain —> Pk-1 deactivated via increase in Km —> GLUCONEOGENESIS
High glucose —> insulin —> activation of phosphoprotein phosphatase —> activation of PK-2 domain —> glycolysis
PK-1 allosteric inhibition via ATP
Increasing ATP shifts the kinetics curve of PK-1 activity such that Km is increased
ATP = means we have energy and do not need to make any more
Regulation on FBPase-2
Activity leads to gluconeogenesis
Inhibited allosterically by F26BP…what stimulates PK1
The regulation between PK1/2 and FBPase-2 helps ensure that glycolysis and gluconeogenesis are not occuring at the same time
Pyruvate kinase
PEP —> pyruvate (irreversible)
Makes ATP
Stimulated by the reation substrates (ADP and PEP) and F16BP
Inhibited by ATP, acetyl-CoA, long chain FAs, and alanine
In the liver…glucagon can start a signaling pathway via PLA which phosphorylates PK —> inactivating it (insulin would lead to dephosphorylation of PK in the liver)
2,3-biphosphoglycerate pathway in RBCs
Stimulates the release of O2 from hemoglobin
1,3BPG (substrate for step 7 gly) is converted into 2,3-BPG in red blood cells (BPG mutase)
Then 1,3BPG —> 3PG (BPG phosphatase)
3PG is the normal product of step 7…but the normal step was bypassed…therefore 2 fewer ATPs are produced
Mutase is stimulated when O2 needs to be unloaded from hemoglobin and enter tissues
Increase in mutase acvitity —> increase 23BPG levels
Clinical correlation to donating blood
When donated blood is stored the 23BPG in it are reduced
So blood transfused into someone has low 2,3BPG…
Which means the O2 has really really high affinity for hemoglobin and cannot unload enough into the tissues
Resolved within 24-48 hours
So if you transfuse too much blood —> serious issues since patient cannot oxygenate their tissues
Hemolytic anemia
RBCs do not have mitochondria…so cannot do oxidative phosphorylation…so all ATP comes from glycolysis
Too little ATP —> changes in shape and membrane in a RBCs
Common enzymes that get fucked
- G6P dehydrogenase
- pyruvate kinase
