Carbohydrate metabolism Flashcards
What is the key common end product of metabolism?
Acetyl-CoA
How are carbohydrate used?
Carbs are used immediately as fuel, they are stored as glyocgen and fat
What cell has an absolute requirement for glucose?
Erythrocytes because they do not have mitochondria
What organ is the control point for glucose metabolism?
Liver
What happens after glucose absorption stops following a meal?
The liver returns stored glycogen to the blood as glucose to provide fuel. This is the “postabsorptive” phase of starvation and is short
How is glucose metabolized?
By glycolysis
Describe glucose obsorption
Glucose arrives in intenstinal lumen, is absorbed by intestinal mucousal cells via active transport, enters circulation and goes to liver & tissues
Where does glucose go first?
Liver
How is glucose absorbed by intestinal mucousal cells?
Sodium-GLucose transporter 1
What is SGLT1 dependent on?
Activity of Na+/K+-ATPase pump
Describe glucose uptake
Glucose is highly polar and cannot cross the cell membrane, so it is absorbed by the glucose transporter (GLUT)
What is GLUT?
A carrier-mediated glucose transport sysytem
Do all cells have at least 1 GLUT receptor?
Yes
Name GLUT receptors
GLUT1 (Low Km, basal transporter) GLUT2 (High Km) GLUT3 (Low Km, Basal transporter) GLUT4 (Insulin dependent, located in muscle and fat) GLUT5 (high affinity for fructose)
How does GLUT receptors work?
GLUT1 sits on the basal membrane, glucose binds to it. Insulin binds to an insulin receptor on the cell which upregulates GLUT4 and more glucose is absorbed
What is the end product of glycolysis?
Pyruvate
Where does glycolysis occur?
Anaerobic, in cytoplasm
How much energy from glycolysis is captured as ATP?
42%
What is step 1 of glycolysis?
Glucose -> Glucose 6-phosphate
Enzyme: hexokinase, glucokinase
Irreversible but not committed step
What is the difference between hexokinase and glucokinase?
Hexokinase is inhibited by larger concentrations of glucose 6-phosphate and hexokinase has a higher affinity for glucose than glucokinase
What is the concentration of glucokinase after a meal?
15mM
What is glucokinases major function?
Keep the brain supplied with glucose
What is glucose trapping?
When glucose -> glucose 6-phosphate, the molecule cannot diffuse out of the cell. G6P is a substrate for multiple pathways
WHat are the pathways of G6P?
Glycolysis, Glycogen synthesis, hexose monophosphate shunt (pentose shunt)
What is the 2nd step of glycolysis?
G6P to fructose 6-phosphate
Phosphoglucose isomerase
Freely reversible
What is the 3rd step of glycolysis?
F6P to Fructose 1,6-biphosphate
Phosphofructokinase 1
This is the committed step and rate limiting step of glycolysis
Irreversible
What factors increase PFK activation?
Upregulated by: ADP, AMP F6P Epinephrine Insulin Ammonium (NH4+)
What factors decrease PFK activation?
Citrate ATP Phosphocreatine Glucagon H+
What is step 4 of glycolysis?
F1,6BP -> Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde 3 phosphate (G3P)
Aldolase
Reversible
What is step 5 of glycolysis?
DHAP -> G3P
Triosphosphate isomerase
Equilibrium favors G3P over DHAP
Reversible
How many ATP are used in steps 1-5?
2
What is step 6 of glycolysis?
G3P -> 1,3 biphosphoglycerate
Glyceraldehyde 3-phosphate dehydrogenase
Reversible, produces NADH+ when hydrogen is replaced by high energy phosphate link (NAD+ -> NADH+)
What is step 7 of glycolysis?
1,3BPG -> 3-phosphoglycerate
Phosphoglycerate kinase
Reversible but makes ATP
Whatis unique about step 7 of glycolysis?
Eve though an ATP is made, it is a reversible reaction
How many ATP/NADH are made in steps 6 & 7?
2 ATP, 2 NADH+ because there are 2 G3P molecules
What is step 8 of glycolysis?
3-phosphoglycerate to 2-phosphoglycerate
Phosphoglycerate mutase
Reversible
What is step 9 of glycolysis?
2-phosphoglycerate to phosphoenolpyruvate (PEP)
Enolase
Reversible
Becomes a high energy compound from a low energy compound because of the double carbon bond that forms
What is step 10 of glycolysis?
PEP -> pyruvate
Pryruvate kinase
Creates (2) ATP
Irreversible
What are regulators of Pyruvate kinase?
Fructose 1,6 Biphosphate increases Pyruvate kinase in the liver
In the muscle, a drop in the ATP/ADP ratio will increase pyruvate kinase
High ATP will inhibit pyruvate kinase, phosphocreatine will inhibi pyruvate kinase
How many ATP is NADH+ equal to?
2.5
What is the net result of ATP from glycolysis?
7 ATP (2 ATP, 2 NADH) Total yield is 9 ATP (4 ATP, 2 NADH)
What makes glycolysis possible?
Energy coupling
What is the purpose of the electron transport system?
Aerobic metabolism, mass production of ATP
Where does 90% of the available metabolic energy in glucose remain?
In pyruvate
Where can pyruvate go?
Lactate, Acetyl-CoA, other Carb, Fat & Protein metabolism
What actions is lactate used for?
Anaerobic metabolism, sprinting
How does lactate produce ATP?
Quickly, but low quantity
What actions is Acetyl CoA used for?
Aerobic metabolism, endurance
How does Acetyl Coa produce ATP?
Slow, but high quantity
What organs rely heavily on anaerobic glycolysis?
Cornea, lens
Red blood cells
Cancer cells
Why is glycolysis essential?
Quickest way to provide energy from rest to full flgiht
Less dependent on blood supply and vasculature (cornea)
Reduce need to have bulky mitochondria in cornea, lens and blood
