Glycolysis Flashcards
Where does glycolysis occur
cytosol
Preparatory phase (5) vs payoff phase (5)
-
Preparatory phase:
- Investment of energy into the system from hydrolyzing 2 ATP
- Locks glucose into glycolytic pathway
- Split glucose into two, 3-carbon molecules
- Investment of energy into the system from hydrolyzing 2 ATP
Payoff phase: 4ATP and 2NADH are produced
First step - what is it? how is it possible? is it irreversible?
Glucose +ATP -hexokinase -> glucose-6- phosphate + ADP
Phosphorylation coupled to ATP hydrolysis.
Irreversible.
Glucokinase
IA lvier-specific hexokinase isoform with a higher Km
Importance of glucose phosphorylation in the first step
- Invests energy
-
Trap glucose in the cell
- Glucose-6-phosphate is too negative to leave the cell
- Decreases [glucose] in the cell so more unphosphorylated glucose can diffuse into the cell via transport channels
- Use glucose-6-phosphate in other cell pathways
- The negative phosphate charge can interact through weak ionic forces to stabilize the substrate in the active site of various enzymes
Substrate flux- what is it and what steps do it?
a reaction that can go in either direction is pushed in one direction because its product is rapidly used as a substrate for the next reaciton
Seen in step 2 (Isomerization of glucose-6-phosphate to fructose-6-phosphate) and step 5 (isomerization of DHAP into GAP)
Step 3 - what is it? how is it possible? is it irreversible?
Fructose-6-phosphate + ATP -PFK-1-> Fructose-1,6-bisphosphate + ADP
Coupled to ATP hydrolysis
Irreversible
Importance of fructose-6-phosphate phosphorylation
Invests ATP
Commitment step
PFK-1 is the most highly regulated glycolytic enzyme
Product has an axis of symmetry- cleavage in step 4 into DHAP and GAP molecules generates twice the E in the payoff phase
When are ATP molecules hydrolyzed?
steps 1 and 3
Step 6
GAP is oxidized, NAD reduced.
Glyceraldehyde-3-phosphate + Pi + NAD+
-GAP dehydrogenase->
1,3-bisphosphoglycerate + NADH + H+
- NADH will be used to donate e- to the ETC
- 1,3BPG is very high energy
Step 7
1,3BPG is hydrolyzed; substrate level phosphorylation
1,3BPG + ADP -phosphoglycerate kinase-> 3PG + ATP
- 1,3BPG’s phosphate bond is broken and the E is transferred to generate ATP
Which steps involves substrate level phosphorylation ?
7 & 10
7: 1,3BPG is hydrolyzed to 3PG to make an ATP
10: PEP is hydrolyzed to pyruvate to make na ATP
Step 10
PEP is hydrolyzed into pyruvate to make ATP.
PEP + ADP -pyruvate kinase-> pyruvate + ATP
Which steps of the payoff phase generated energy ?
Step 6- oxidation - NADH
Step 7 - sub lvl phos - ATP
Step 10 - sub lvl phos - ATP
End products of glycolysis of one glucose molecule
2 pyruvate
2 NADH
2 ATP (net, because Step1&3 used 2 ATP)
2 H2O
Bleeding can lower your hemoglobin and hematocrit, so your body is low oxygen. What do you expect to find in this patient’s blood test
High lactate
Acidic blood
COPD (poor gas exchange in the lungs) can cause ___ due to the increase in the partial pressure of CO2
Respiratory acidosis
What happens to pyruvate in aerobic vs anaerobic conditions?
Aerobic: pyruvate converted to acetyl-CoA for the citric acid cycle to make more NADH
Anaerobic: pyruvate converted to lactate by lactate dehydrogenase in a redox rxn; this oxidizes NADH from step6 into NAD+
Anaerobic glycolysis
One molecule of glucose –> 2 lactate + 2 net ATP
What causes metabolic acidosis?
Increased lactate (an acid) or ketone bodies in the blood
What is muscle cramping caused by? Why does massaging help it?
Lactate build up
Massaging sends more oxygenated blood there while removing CO2 and lactate –> lactate dehydrogenase rxn is reversed
Why do RBCs only do anaerobic glycolysis?
No mitochondria
They don’t want to use up the oxygen they’re carrying
Why does increased CO2 cause respiratory acidosis?
It’s converted by carbonic anhydrase into carbonic acid, which is how CO2 is carried in blood.
GLUT_ transporter is found in almost all tissues and is the basal transporter for glucose in most cells. Km is ~1-2
GLUT1
Low Km to make sure that even at lower glucose levels it’s still pretty active.
GLUT_ is the main transporter in muscle & fat cells. Regulated by insulin. It has Km of 5mM.
GLUT4
Insulin sensitive
GLUT_ doesn’t transport glucose, but transports fructose, such as in small intestine and sperm
5
GLUT_ is the main glucose transporter for the liver and pancreas. Also seen in the basoslateral membrane of small intestine. It has a high Km of 15-20mM and is __sensitive to insulin
Why?
GLUT2
Insensitive to insulin
High Km to ensure liver will only take up excessive glucose to store as glycogen and the pancreas will only secrete insulin when glucose is excessive.
GLUT2 & insulin secretion
- Excessive glucose enters B islet cells through GLUT2
- Glycolysis increases intracellular ATP
- ATP closes an ATP-sensitive K+ channel, depolarizing the cell.
- Depolarization opens a voltage-gated Ca++ channel.
- Increased intracellular Ca++ stimulates exocytosis of insulin-containing secretory vesicles
Sulfonylureas and Meglitinides
Anti-hyperglycemic drugs that stimulate insulin secretion by mimicking ATP to close K+ channels.
Diazoxide
drug that opens the K+ channel to maintain membrane polarization and inhibit insulin release
When blood glucose is low, what happens to GLUT4 in muscle & adipocytes?
Low glucose -> no insulin secreted by the pancreas
Without insulin signaling, GLUT4 becomes sequestered by endocytosis in the membrane of intracellular vesicles.
-> no glucose transported into muscle or fat
When well-fed, as glucose levels rise, the pancreas secretes insulin that binds to the insulin receptor on muscle & adipocyte plasma membranes, signaling through the ___ pathway to promote__
signal through the protein kinase B pathway to promote exocytosis of GLUT4 back to the plasma membrane to transport glucose into cells.
-> glucose is metabolized to pyruvate for ATP or glucose-6-phosphate can be converted to ribulose 5-phosphate for the pentose phosphate pathway
Type 1 Diabetes- impact on insulin & blood glucose
No insulin produced
Blood glucose is dramatically elevated
Why does type 1 diabetes cause ketoacidosis?
- Lack of insulin causes adipocytes to hydrolyze triglycerides, increasing fatty ACIDs in the blood.
- FA is taken up by liver hepatocytes to be
- metabolized by betaoxidation for energy or
- converted to ketone bodies (alternative fuel source fo rbrain), which are acidic
At normal physiologic blood glucose levels, hexokinase I is___. What about hexokinase IV (glucokinase)?
Hexokinase I is saturated, near Vmax. (Very low Km)
Glucokinase (hexokinase IV) is only active at much higher blood glucose levels.
Regulation of hexokinase I - III
Allosteric regulation - if glucose 6-phosphate builds up, this prevents further glucose phosphorylation by hexokinase I
In the fasting state, is glycolysis inactivated or activated?
Glycolysis is inactivated to favor glycogen breakdown and/or gluconeogenesis pathway.
Thus, glucokinase has to be turned off.
Glucokinase locks glucose in the cell, so it needs to be regulated when hepatocytes are performing
Gluconeogenesis or glycogen rbeadkwon
How is glucokinase regulated at low blood glucose?
- PFK-1 is inactivated at low blood glucose, so fructose-6-phosphate builds up
- Glucokinase sequestration in the nucleus
As blood glucose concentration rises in the well fed state, glucokianse can return to the cytosol.
Allosteric regulation of PFK-1- what are the inhibitors and activators?
PFK is inhibited by high ATP and citrate.
Activated by ADP/AMP and fructose 2,6-bisphosphate**
___is the enzyme of the commitment reaction for glycolysis, so it’s the most highly regulated glycolytic enzyme
PFK-1
PFK-_2_ catalyzes the production of ___ from fructose-6-phosphate. The enzyme that catalyzes the dephosphorylation of this product is ____.
PFK-2 makes fructose 2,6-bisphosphate
It’s broken down/dephosphorylated by fructose 2,6-bisphosphatase (FBPase-2) back to fructose-6-phosphate
Fructose 1,6 bisphosphate vs fructose 2,6 bisphosphate
Fructose 1,6 bisphosphate is the 3rd glycolytic intermediate, the product of the PFK-1 reaction
Fructose 2,6 bisphosphate is the most potnet allosteric activator of the PFK-1 enzyme
FBPase-1 catalyzes the dephosphorylation of ___, a glycolytic intermediate.
FBPase-2 catalyzes the dephosphorylation of ___, an allosteric activator of PFK-1
FBPase-1 dephosphorylates fructose 1,6-bisphosphate
FBPase-2 dephosphorylates fructose 2,6-bisphosphate
In the presence of F2,6BP, glycolysis is ___ and gluconeogenesis is ___.
F26BP –> glycolysis activated, gluconeogenesis inhibited
F26BP _creases the Km of PFK-1 and
_creases the Km of FBPase-1
decreases the Km of PFK-1
increases the Km of FBPase-1
___ and ___ are two domains of a bifuncitonal enzyme - what determines which functional domain will be active?
PFK-2 (glycolysis) and FBPase-2 (gluconeogenesis)
Glucagon (low blood glucose) activates FBPase2 -> FB2,6BP is inactivated -> can’t activate PFK-1 for glycolysis
Insulin (well-fed) -> phosphoprotein phosphatase -> activate PFK-2
What is 2,3-BPG
Promotes unloading of oxygen from hemoglobin in RBCs to be released into srurounding tissues
Pyruvate kinase (last enzyme) is stimulated by what molecules and inhibited by what molecules?
Stimulators:
- ADP + PEP (its substrates)
- Fructose 1,6-bisphosphate (feedforward activation)
Inhibitors:
- ATP, acetyl-CoA, long chain fatty acids (means high E)
- Alanine (from transamination of pyruvate)
- Glucagon (PKA signaling phosphorylates it)
How to bypass step 7, resulting in 2 less ATP per glucose
- Bisphosphoglycerate mutase turns 1,3BPG to 2,3-BPG in RBCs
- 2,3BPG is then hydrolyzed by bisphosphoglycerate phosophatase to 3PG
Results in 2 less ATP per glucose during glycolysis
Hemolytic anemia
RBCs have to depend on glycolysis for ATP
ATP deficiency changes the cell’s shape -> cell lysis (hemolysis) and phagocytosis of damaged cells
The most common enzyme deficiency causing hemolytic anemia
2: Pyruvate kinase
GLUCOSE 6-PHOSPHATE DEHYDROGENASE in the pentose phosphate pathway
What happens to 2,3BPG if you go to a higher altitude?
Higher altitude has more oxygen.
You would make more 2,3BPG to release that oxygen to the tissues
(Going to lower altitudes would hydrolyze 2,3BPG)
If too much 2,3-BPG is produced, you might get
hemolytic anemia
Because producing 2,3 BPG bypasses step 7 –> not enough ATP to support RBCs
