sakai-Regulation of Glycolysis/Gluconeogenesis Flashcards
What is the purpose of anaerobic or aerobic glycolysis in muscle? How is glycolysis regulated in exercising skeletal muscle?
The purpose of anaerobic glycolysis in muscle is to generate ATP by substrate level phosphorylation. Formation of lactate from pyruvate uses NADH and regenerates NAD+ in cytosol for glycolysis.
The purpose of aerobic glycolysis in muscle is to provide ATP by substrate level phosphorylation but also to generate pyruvate for the PDH complex and formation of acetyl CoA for the TCA cycle which will lead to oxidative phosphorylation of NADH and FADH2.
[The total degradation of glucose in this sequence can lead to about 38 ATP when the malate-aspartate shuttle is used and 36 ATP when the glycerophosphate shuttle is used.]
During muscle contraction, the ATP level decreases and the AMP level increases.
[PFK-1 is allosterically inhibited by ATP and is allosterically activated by AMP]
In addition, a possible ATP-inhibition of PFK-1 is overcome by fructose 2,6-bisphosphate which is formed by the bifunctional enzyme in the muscle.
[The bifunctional enzyme in skeletal muscle is always folded in a way that PFK-2 is active and BPase-2 is inactive, there is no phosphorylation site. High levels of fructose 6-P activate PFK-2 activity via substrate availability.]
[note: in the liver it is important to switch from glycolysis to gluconeogenesis after glucagon action which is performed by phosphorylation of the bifunctional enzyme. This is not needed for the muscle. The muscle can focus on its own energy metabolism]
What is the advantage of using glycogen degradation starting with glucose 1-P to form glucose 6-P in comparison to the hexokinase reaction?
The formation of glucose 6-P from glucose 1-P during glycogen degradation circumvents the hexokinase reaction, and this saves one ATP in the investment phase of glycolysis.
What is the product of glycogen phosphorylase?Which enzyme forms glucose 6-P needed for glycolysis in muscle?
Glycogen phosphorylase cleaves glycogen to glucose 1-P using inorganic phosphate instead of water (phosphorolytical cleavage).
Phosphoglucomutase forms glucose 6-P in a reversible reaction.
[The direction is determined by concentrations of glucose 1-P and glucose 6-P.]
[The fate of glucose 6-P is different in muscle than in the liver. In the muscle glucose 6-P is used for glycolysis, in the liver glucose 6-P is cleaved to free glucose which is released into the blood]
In the liver, which enzyme is the most controlling in the switch from glycolysis to gluconeogenesis and vice versa? Which metabolite plays an important role?
It is the bifunctional enzyme (PFK-2/BPase-2) that allows the switch and this enzyme is regulated in the liver by phosphorylation and dephosphorylation.
The dephosphorylated form is found at high insulin/glucagon ratio and generates fructose 2,6-bisphosphate with its PFK-2 activity.
The phosphorylation of the bifunctional enzyme by protein kinase A at low insulin/glucagon ratio leads to degradation of fructose 2,6-bisphosphate with the now active BPase-2 activity.
[Hepatic fructose 2,6-bisphosphate has an important role: some molecules activate the glycolytic enzyme PFK-1 and some molecules inhibit fructose 1,6-bisphosphatase which is a key enzyme of gluconeogenesis.]
Is the covalent modification for regulation of the hepatic bifunctional enzyme reversible?
The enzymes protein kinase A and protein phosphatase each catalyze an irreversible reaction.
The hepatic regulation, however, is reversible as the bifunctional enzyme switches back into the original conformation following phosphorylation or dephosphorylation.
Which hepatic enzymes act on glycerol, alanine, and lactate when these precursors are used for gluconeogenesis, respectively?
Glycerol kinase forms glycerol 3-P from free glycerol
Alanine aminotransferase (ALT) forms pyruvate from alanine
Lactate dehydrogenase (LDH) forms pyruvate from lactate
What is the function of hepatic glucokinase in relation to blood glucose levels?
Glucokinase reduces blood glucose at high blood glucose levels.
GLUT-2 allow the hepatic uptake of large amounts of glucose from the blood and
glucokinase can generate high levels of glucose 6-P inside the hepatocytes.
[Note, glucokinase is not product inhibited, its purpose is to trap as much blood glucose as glucose 6-P in the hepatocyte.
The Km of glucokinase for glucose is above the normal blood glucose level]
Describe the function of glucose 6-phosphatase in relation to blood glucose!
Glucose 6-phosphatase in the liver allows increase of blood glucose at low blood glucose levels.
[The cleavage of glucose 6-P allows the release of free glucose via GLUT-2 into the blood as the glucose flows with the concentration gradient.
Glucose 6-phosphatase cleaves glucose 6-P formed by gluconeogenesis or by glycogen degradation and both pathways generate the free glucose which can be released into the blood.]
Why is fatty acid degradation in the liver necessary for gluconeogenesis although the carbons of acetyl CoA cannot be used for gluconeogenesis?
Fatty acid degradation (-oxidation) provides the energy needed for gluconeogenesis. The generated acetyl CoA inhibits the PDH complex and save pyruvate for gluconeogenesis. In addition, pyruvate carboxylase needs acetyl CoA as absolute activator.
[Without an active pyruvate carboxylase, lactate and alanine cannot be used for gluconeogenesis. Pyruvate carboxylase needs biotin as prosthetic group.]
⦁ Which pathways are favored by glucagon in the liver?
Glucagon favors in the liver gluconeogenesis, glycogen degradation, fatty acid degradation and also ketone body synthesis.
⦁ Is the generation of NADH and FADH2 during fatty acid degradation needed for gluconeogenesis? Explain.
NADH and FADH2 are generated during -oxidation inside of mitochondria and they are used in the ETC/ oxidative phosphorylation for ATP formation. This energy is needed for gluconeogenesis and also for the general metabolism of the hepatocyte.
[note: starting with 2 pyruvates, 4 ATP and 2 GTP are also 2 cytosolic NADH are needed to form one molecule of glucose. This input of energy is normally not a problem, as the hepatocytes provide ATP by fatty acid degradation in this well oxidized tissue.]
⦁ Which enzyme leads to the release of fatty acids and of glycerol from fat cells?
Fat cells contain TAGs for storage of fatty acids. These precious stores for the whole body shall only be used under hormonal control when really needed.
Low blood levels of insulin and the action of epinephrine activate TAG lipolysis in fat cells.
[The lipase is named hormone-sensitive lipase (HSL) as its activity is regulated by hormones. HSL is only active when it is phosphorylated.]
⦁ Which enzyme inhibits gluconeogenesis in the liver?
The bifunctional enzyme in its dephosphorylated form (at high insulin/glucagon ratio) leads to fructose 2,6-bisphosphate which inhibits fructose 1,6-bisphosphatase.
⦁ Which enzymes of glycolysis are inhibited by phosphorylation in the liver?
Phosphorylation inhibits the PFK-2 activity of the bifunctional enzyme and also the activity of pyruvate kinase in the hepatocyte.
How is the ATP inhibition of PFK-1 in the liver overcome?
Fructose 2,6-bisP formed by PFK-2 activity of the bifunctional enzyme overcomes the inhibition of PFK-1 that takes place at normal ATP levels in the hepatocytes (high insulin/glucagon ratio).
