Exam 3: Glycolysis Flashcards
Know that red blood cells, skeletal muscle cells, and brain cells are highly or exclusively dependent on glucose/glucose 6-phosphate as an energy source. And know why.
- RBCs : they lack mitochondria and therefore can only undergo glycolysis which requires glucose
- Brain: only glucose can cross the blood brain barrier.
- Skeletal muscle: during exercise anaerobic glycolysis occurs which produces ATP faster
Define Glycolysis
- A sequence of reactions that metabolizes one molecule of of glucose to 2 molecules of pyruvate and generates the net production of two molecules of ATP
- This is an anaerobic process
- Pyruvate can be completely oxidized under aerobic conditions generating more ATP
Which has a lower affinity for glucose, glucokinase or hexokinase?
glucokinase, found in the liver
When will the liver use glucose to produce ATP, instead of glucose storage?
When there is a high level of glucose, such as after a meal.
What is the reaction of glycolysis that reduces NAD+ to NADH?
Step 6: The coversion of Glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate by glyceraldehyde 3-phosphate dehydrogenase (oxidative-reduction rxn) (twice per each glucose molecule)
What is the significance of the oxidation-reducation rxn where NAD+ is reduced to NADH?
There is only a limited amount of NAD+ in the cell, the NADH formed by this reaction must be reoxidized to NAD+ for glycolysis to continue.
What are two mechanisms for reoxidizing NADH?
- NADH-linked conversion of pyruvate to lactate.
- Oxidation of NADH via the respiratory chain (requires shuttles)
What is the importance of the reduction of pyruvate to lactate in anaerobic glycolysis?
The formation of lactate is the major fate of pyruvate in lens and cornea of the eye, kidney medulla, testes, leukocytes and RBCs, b/c these are poorly vascularized and/or lack mitochondria for aerobic respiration
Also, in exercising skeletal muscle NADH production exceeds the oxidative capacity of the respiratory chain. This results in an elevated NADH/NAD+ ratio, favoring reduction of pyruvate to lactate. Therefore, during intense exercise, lactate accumulated in muscle, causing a drop in the intracellular pH, potentially resulting in cramps. Much of the lactate diffuses into the bloodstream and can be used by the liver to make glucose.
What are the three irreversible reaction of glycolysis?
- Glucose to Glucose 6-Phosphate via HEXOKINASE
- F6P to F1,6,BP via PFK1
- Phosphoenolpyruvate to Pyruvate via Pyruvate Kinase
What is considered the “committed step” of Glycolysis?
The conversion of F6P to F 1,6BP via phosphofructose kinase-1 (PFK1). Before this happened, glucose could go to the pentose phosphate pathway.
Where is glucokinase found?
in liver parenchymal cells and Beta-cells fo the pancrease.
What controls the regulation of PFK1 in muscle cells?
PFK-1 is inhibited allosterically by elevated levels of ATP, which act as an “energy-rich” signal inidicating an abundance of high energy compounds. Conversely, PFK-1 is activated allosterically by high concentractions of AMP, which signal that the cell’s energy stores are depleted.
What controls the regulation of PFK1 in liver cells?
- Fructose 2,6-bisposphate is the most potent activator of PFK1, it can activate the enzyme even with high ATP levels.
- F-2,6-BP is formed by PFK2.
- PFK2 can function as a kinase to produce F-2,6-BP and as a phosphatase that dephosphorylates F-2,6-BP back to F6P
- Inhibited by citrate
How is pyruvate kinase regulated in the Liver?
By allosteric effectors and covalent modification.
What reactions of Glycolysis require ATP?
- Step 1: Glucose to glucose 6-phosphate
- Step 3: F6P to F-1,6-BP