sakai-Gluconeogenesis and Regulation of Glycolysis / Gluconeogenesis Flashcards
What is the main pathway that releases glucose into the blood after about 24 hours of fasting? What is special about this pathway? Is it a reversal of glycolysis? Explain.
The pathway is gluconeogenesis. Glycogen stores of the liver are nearly depleted by that time in most individuals.
Glucoenogenesis is not a reversal of glycolysis. It uses however the hepatic enzymes that catalyze the reversible steps of glycolysis. This allows a rapid switch.
[ The pathway of gluconeogenesis circumvents the irreversible steps with a different enzyme (glucose 6-phosphatase, fructose 1,6-bisphosphatase, or need even two irreversible steps to form PEP from pyruvate catalyzed by pyruvate carboxylase and PEP carboxykinase.]
Describe the Cori-cycle. What is transported in blood? Compare it to the alanine-glucose cycle.
The Cori-cycle describes the release of lactate from exercising muscle (and always from RBC) into the blood and its uptake into the liver. Liver uses this lactate for gluconeogenesis and releases glucose into the blood where it can be taken up by muscle (and RBC).
The Alanine-glucose cycle describes the release of alanine from the muscle into the blood and the usage of alanine in the liver for gluconeogenesis.
[Alanine is formed in muscle by transamination using pyruvate and glutamate. In the liver, alanine is used to form pyruvate by transamination using alanine and -ketoglutarate]
What is the advantage of the Cori-cycle?
The Cori-cycle allows anaerobic glycolysis in skeletal muscle and release of lactate into the blood without leading to lactic acidosis.
Lactate does not accumulate in blood as it is used by the liver for gluconeogenesis
Which cells perform gluconeogenesis in the beginning of a fast, which cells add to this process during prolonged starvation?
At the beginning of a fast, the hepatocytes perform most of the gluconeogenesis. During prolonged starvation, the kidney cortex cells contribute.
[most of the glucose produced by kidney cortex cells is used in cells of the kidney medulla]
Lactate in the blood is taken up by the liver and the heart.
Describe the usage of lactate in the liver and compare it to the usage in the heart.
Lactate is taken up into the liver and heart and the respective lactate dehydrogenase forms pyuvate in both tissues. Both tissues are well oxygenated.
In the liver, the fate of pyruvate depends on the nutritional and hormonal status of the body.
When insulin is ruling after a meal (high insulin/glucagon ratio), pyruvate in the liver is substrate for PDH and the formed acetyl CoA joins the TCA cycle.
When glucagon is ruling during fasting (low insulin/glucagon ratio), pyruvate is substrate for pyruvate carboxylase and it is used for gluconeogenesis.
In the heart, there is only one purpose for lactate, pyruvate is formed and is substrate for the PDH and acetyl CoA is used for energy metabolism.
[The heart has a large demand for ATP due to its continuous pumping functions. The big advantage is that pyruvate is formed from lactate and not from glucose. Blood glucose is saved and blood lactate does not increase and does not lead to lactic acidosis]
[the heart takes up glucose via GLUT-4 and stores a small amount of glycogen. As the heart has normally always enough ATP, glycolysis and PFK-1 are inhibited by ATP. This saves glucose. The heart prefers degradation of fatty acids for its energy needs, the degradation of palmitate generates about 128 ATP.]
Which amino acids are mainly transported in the blood to the liver and can be used for the urea cycle and gluconeogenesis?
The amino acids found in the blood are mostly alanine and glutamine.
Alanine can be used for gluconeogenesis as it generates pyruvate in the liver.
Glutamine is also used for gluconeogenesis via glutamate and the formation of -ketoglutarate (an additional compound of the TCA cycle.)
Nitrogen transported to the liver is used for urea synthesis.
Alanine transports one nitrogen to the liver,
glutamine transports two nitrogens to the liver.
Which specific amino acid can be used for gluconeogenesis and inhibits at the same time hepatic glycolysis at the level of pyruvate kinase? Explain.
It is alanine that can be used for gluconeogenesis and inhibits at the same time allosterically the glycolytic enzyme pyruvate kinase in the hepatocyte.
High levels of alanine from the blood signal protein degradation in muscle during fasting (low insulin/glucagon ratio).
Under these conditions, in order to fulfill the alanine-glucose cycle, the hepatic glycolytic enzyme pyruvate kinase shall be allosterically inhibited by some alanine molecules, which saves PEP.
At this time the enzyme is also inhibited by covalent modification. It is phosphorylated (hepatocyte only) by protein kinase A.
Alanine itself is used to form pyruvate which is used for gluconeogenesis and is substrate for pyruvate carboxylase in mitochondria.
Which molecule is released during fasting from fat cells into the blood and can be used donating carbon atoms for gluconeogenesis in the liver?
Glycerol is released from fat cells after lipolysis of the stored triacylglycerols.
[ lipolysis in fat cells takes place after epinephrine and cortisol action and at low blood levels of insulin. It is performed by the phosphorylated hormone-sensitive lipase. TAGs are cleaved to free glycerol and three free fatty acids.
Epinephrine and cortisol in addition to glucagon lead in the liver to glycogen degradation (epinephrine, leads to phosphorylation of glycogen phosphorylase kianse via protein kinase A) and gluconeogenesis (cortisol, leads to enzyme induction of PEP carboxykinase)]
The liver contains the enzyme glycerol kinase which traps free glycerol as glycerol 3-P in the hepatocyte. Glycerol 3-P and NAD+ are used to form dihydroxyacetone phosphate (DHAP) which directly joins gluconeogenesis.
Do you need pyruvate carboxylase and PEP carboxykinase for gluconeogenesis starting with glycerol?
No, pyruvate carboxylase and PEP carboxykinase are needed starting with pyruvate, but they are not needed for gluconeogenesis starting with glycerol.
Which substrate is mostly used for gluconeogenesis during starvation and where does it come from?
Amino acids provide the main substrates for gluconeogenesis during starvation.
Low levels of insulin and raised cortisol levels lead to protein degradation in muscle.
The generated free amino acids are changed to mainly alanine and glutamine, which are released into the blood and are taken up by the liver.
Due to the large muscle mass, many amino acids can be provided for gluconeogenesis.
[note: glycerol and lactate are also used but they are not the main substrates for gluconeogenesis during starvation as long as amino acids are available.]
How many glucose molecules can be formed during gluconeogenesis starting with alanine and using only one alanine as substrate?
None, as two alanines are needed leading to two pyruvates.
One glyceraldehyde 3-P and one DHAP are needed to form fructose 1,6-bisphosphate.
NADH is needed directly in gluconeogenesis for the step catalyzed by glyceraldehyde 3-P dehydrogenase. On the other hand, in which other reactions related to gluconeogenesis is NAD+ needed and NADH formed?
Lactate dehydrogenase needs NAD+ in order to form pyruvate from lactate.
Malate dehydrogenase in cytosol needs malate and NAD+ in order to provide oxaloacetate for cytosolic PEP carboxykinase.
Glycerol 3-P dehydrogenase needs NAD+ for formation of DHAP.
[high levels of NADH reduce the usage of many precursors for gluconeogenesis, although one NADH is used in the pathway itself.]
Name the four irreversible reactions catalyzed by enzymes during gluconeogenesis starting with pyruvate.
Which enzyme needs biotin and performs carboxylation? Which enzyme uses GTP and performs phosphorylation and decarboxylation? Which enzyme is bound in the ER membrane?
The irreversible steps of gluconeogenesis are catalyzed by
Mitochondrial pyruvate carboxylase (biotin, ATP) uses pyruvate and forms oxaloacetate
Mitochondrial or cytosolic PEP carboxykinase uses oxaloacetate and GTP and forms PEP with decarboxylation
Fructose 1,6-bisphosphatase cleaves fructose 1,6-bisP to fructose 6-P and Pi
Glucose 6-phosphatase cleaves glucose 6-P to free glucose and Pi in the lumen of the ER. Transporters are needed to bring the substrate in and the products out of the ER.
Which enzyme forms oxaloacetate from pyruvate? Describe the reaction in detail.
It is pyruvate carboxylase which uses carbon dioxide and needs biotin and cleaves ATP to ADP and Pi and forms oxaloacetate from pyruvate.
What is the activator of pyruvate carboxylase? Why is it referred to as an absolute activator? Under which condition is pyruvate used for gluconeogenesis?
Acetyl CoA is the activator of pyruvate carboxylase. This activator is absolutely necessary, as without the allosteric activation by acetyl CoA, the enzyme is practically inactive. [This is very unusual, in most regulations an activation of an enzyme means further increase of activity.]
At insulin ruling (high insulin/glucagon ratio), pyruvate is used in hepatocytes to form acetyl CoA by the pyruvate dehydrogenase complex (PDH) in order to join the TCA cycle.
At glucagon ruling (low insulin/glucagon ratio), acetyl CoA, FADH2 and NADH are generated by mitochondrial -oxidation of fatty acids.
Pyruvate is saved for gluconeogenesis as the PDH complex is now inhibited by acetyl CoA and NADH.
[acetyl CoA and NADH inhibit PDH by product inhibition and in addition also activate PDH kinase which phosphorylates and inhibits PDH by covalent modification]
[The high level of acetyl CoA activates pyruvate carboxylase and allows the usage of pyuvate for gluconeogenesis. The formed oxaloacetate is changed to malate due to the high mitochondrial NADH levels and malate is transported into the cytosol for gluconeogenesis.]
