GLUCONEOGENESIS Flashcards
What are the sources of blood glucose in the fed and starved states?
exogenous (dietary glucose), total glucose, liver glycogen and glyconeogenesis
Where does gluconeogenesis occur?
Who? (what are the substrates for gluconeogenesis?)
Acetyl CoA does not give rise to net synthesis of glucose, why?
Site: Liver (90%), kidney (10%) during overnight fast
Kidneys account for 40% during a prolonged fast
Subcellular site: partly mitochondrial & partly cytosolic
Who????
•Intermediates of glycolysis and TCA cycle can be converted into glucose
•Carbon skeletons for glucose synthesis derived from
-Lactate (from exercising skeletal muscle & RBCs)
-Pyruvate
-Glycerol (from adipose tissue lipolysis)
-Glucogenic amino acids (all except leucine & lysine) from muscle
-Propionyl CoA (from oxidation of some a.a & odd chain fatty acids)
•Compounds giving rise to intermediates of glycolysis or TCA cycle are considered glucogenic
How does gluconeogenesis work? How many reactions of glycolysis are reversible and shared with gluconeogenesis? How many reactions are irreversible or key reactions? Draw out the steps of gluconeogenesis (key enzymes and shared enzymes)
Pyruvate carboxylase
Where is it used? What does it require? What type of enzyme is it? What is it activated me? How does malate shuttle come into play for oxaloacetate?
- Pyruvate carboxylase
- Mitochondrial enzyme, requires ATP, Biotin, CO2 (ABC)
- *-Irreversible reaction** (also serves as anaplerotic reaction)
- *-Allosterically activated by acetyl CoA** (from fatty acid oxidation)
- Oxaloacetate (OAA) cannot cross mitochondrial membrane
- OAA reduced to malate, transported to cytosol, re-oxidized to OAA (malate shuttle, involves mitochondrial & cytosolic malate dehydrogenases)
What does Phosphoenolpyruvate carboxykinase require? Where it is used? When does it act on the enzymes of glycolysis till?
- Mainly cytosolic (also present in mito), requires GTP, decarboxylates and phosphorylates OAA to PEP
- PEP is acted on enzymes of glycolysis till it forms fructose 1,6-BP
- Oxaloacetate can also be converted to PEP by mitochondrial PEPCK and transported to cytosol
- Oxaloacetate can also be transaminated to aspartate and transported to cytosol
- NADH produced by cytosolic MDH is used in the reduction of 1,3 BPG to Glyceraldehyde 3-P
- Pairing of carboxylation & decarboxylation drives the reaction towards formation of PEP
Where is fructose 1,6 Bisphosphatase located? What reaction of glycolysis does it bypass?
- Present in liver & kidney (cytosolic)
- Bypasses PFK-1 reaction of glycolysis, hydrolyzes phosphate on C-1
- Important regulated step
What does Glucose 6-phosphatase do ? Which reaction does it bypass? Where is it present? How is glucose 6-phosphatase transported?
What are Glucose 6-phosphatase & Glucose 6-P translocase involved in ? And how does it do it?
What does deficiencies of glucose 6-phosphate cause?
- Hydrolyzes glucose 6-P to glucose, bypasses HK/GK of glycolysis
- Present only in liver & kidney, free glucose released into blood only by these two organs
- ER membrane bound enzyme
- Glucose 6-P translocase( in green) moves G6P from cytosol into ER across ER membrane
- Glucose 6-phosphatase & Glucose 6-P translocase also involved in conversion of glycogen to glucose
- Deficiencies result in type Ia & Ib glycogen storage disease
Explain the cori cycle in the muscle and RBC
What does Epinephrine do and how does it relate to the cori cycle?
•Lactate released into blood by exercising skeletal muscle (due to lack of O2) & erythrocytes (lack of mitochondria), taken up by liver, converted to glucose & released into blood circulation
Epinephrine- Stimulates glycogenolysis -> ↑Glucose->↑pyruvate->↑lactate (under anaerobic conditions), ↓pH
causes muscle cramps, vasodilation improves blood supply to muscle, washes out lactate from muscle fibers
Occurs during exercise
Gluconeogenesis from amino acids: What happens to muscle protein during fasting?
What generates the pyruvate or intermediates of TCA cycle?
Is acetyl CoA an intermediate of the TCA cycle?
What generates acetyl-CoA?
What are the major transporter form of amino acids form the peripheral tissues to liver?
- During fasting, muscle protein degraded to a.a, transported to liver for gluconeogenesis
- Carbon skeletons of all amino acids except leucine & lysine on catabolism generate pyruvate or intermediates of TCA cycle -> Considered glucogenic
- Acetyl CoA; not an intermediate of TCA cycle
- Leucine & lysine generate acetyl CoA- ketogenic
- Alanine & glutamine; major transport form of amino acids from peripheral tissues to liver, major precursors for gluconeogenesis
Which are the main amino acids that can enter the TCA cycle for gluconeogenesis?
Which are the main amino acids that can enter the TCA cycle for gluconeogenesis?
Alanine-> pyruvate -> enter the TCA cycle-> oxaloacetate -> PEP-> glucose via glugoneogenesis
Glutamine -> Glutamate -> a-ketoglutarate -> oxaloacetate -> PEP-> glucose via glugoneogenesis
What is the glucose-alanine cycle?
Glucose-alanine cycle
During a fast, muscle protein is broken down to amino acids
↓
Amino groups eventually transferred to glutamate
↓
Reacts with pyruvate to form alanine
↓
Transported in blood to liver to form glucose
↓
Glucose released into blood by liver, used by muscle
Picture: Transaminiation – the transfer of NH4 groups from glutamate( amino acid to pyruvate( keto acid) . So in the muscle is broken down to amino acid( glutamate). glutamate undergoes transamination reaction to form alanine. Alanine enters the blood to the liver where alanine transfers its NH4 group to a-ketoglutarate to form pyruvate. Pyruvate can undergo gluconeogenesis to produce glucose.
Explain gluconeogenesis from glycerol
- Hydrolysis of stored TAG in adipose tissue (lipolysis) during a fast, releases glycerol into blood
- Glycerol kinase is present only in liver (adipose tissue cannot utilize glycerol)
- Fatty acids with even number of carbons produce acetyl CoA; do not give rise to net synthesis of glucose
- 2 glycerol is required to make two glucose
- Remove two hydrogen from the glycerol 3 p to DHAP
How is gluconeogenesis started from propionyl CoA. What forms propionyl- CoA? What vitamin is most important in this process and why? Draw the cycle
•Propionyl CoA derived from metabolism of odd chain fatty acids & amino acids like val, thr, ile, met
Deficiency of vitamin B12 results in accumulation of methylmalonic acid in blood & its excretion in urine (high yield area)
How is glucogneogenesis is regulated? Name and explain the 4 key enzymes that regulate it
- Glycolysis & gluconeogenesis are reciprocally regulated
- Some positive effectors of glycolysis are negative effectors of gluconeogenesis
- Glucagon & glucocorticoids while insulin ¯ gluconeogenesis
- Besides fasting, gluconeogenesis is stimulated during prolonged exercise & conditions of stress
- Availability of substrates stimulates gluconeogenesis
- 4 key enzymes are regulated
Pyruvate carboxylase
- activated by acetyl CoA (obligate allosteric activator)
- Inhibited by ADP
- Induced by glucocorticoids & glucagon, repressed by insulin
PEPCK
-Induced by glucocorticoids & glucagon, repressed by insulin
Fructose 1,6-bisphosphatase
- Point of reciprocal regulation of gluconeogenesis & glycolysis (along with PFK-1)
- Allosterically inhibited by fructose 2,6-bisphosphate (Fr 2,6BP) & AMP (low energy signal); both of these activate PFK-1 in glycolysis
- High ATP & low AMP levels (high energy signals) & low Fr 2,6BP levels stimulate gluconeogenesis
- Fr 2,6BP levels influenced by insulin/glucagon ratio
Glucose 6-phosphatase
-Induced by glucagon (during fasting) & glucocorticoids; repressed by insulin (in fed state)
Explain what happend in the liver during fasting state
Glucagon increases the transcription of PEPCK gene (induction) through the transcription factor CREB (cAMP response element binding protein)
Cortisol acts through zinc finger transcription factors
Inactivation of pyruvate kinase by glucagon diverts PEP into gluconeogenesis
Disorders of beta oxidation of fatty acids, less energy for gluconeogenesis during fasting & less acetyl CoA as activator for pyruvate carboxylase, results in fasting hypoglycemia
Acetyl CoA inhibits PDH so that the pyruvate does not become acetly CoA, rather it becomes oxaloacetate
What are PFK-2/FBP-2? Explain what happens to PFK-2/FBP-2 in well fed state and in fasting state
PFK-2/FBP-2, a bifunctional protein with two enzyme activities. Regulated by phosphorylation and dephosphorylation in the liver. The kinase is active in the dephosphorylated form whereas the phosphatase is active in the phosphorylated state.
in well-fed state -> high blood glucose -> inc insulin -> activates PFK-2 ( dephosphorylation) -> inc fructose 2,6 bisphosphate -> activates PFK-1 inhibits FBP-1-> inc glycolysis and dec gluconeogenesis
During starvation-> low blood glucose -> activate FBP-2 ( phosphorlaytion ) -> dec fructose 2,6 bisphosphate -> No activation of PFK-1 and no inhibtion of FBP-1-> dec glycolysis and inc gluconeogenesis
Reciprocal regulation of PFK-1 (glycolysis) & FBP-1 (gluconeogenesis) prevents futile cycling (Conversion of glucose to pyruvate followed by resynthesis of glucose from pyruvate)
Explain ethanol metabolisn and gluconegenesis
- NADH/NAD ratio inhibits fatty acid oxidation, accumulation of fatty acids which are converted to triacylglycerol (TAG)
- Acetaldehyde accumulation damages mitochondria, decreased oxidative phosphorylation, decreased ATP synthesis
- Decreased VLDL secretion results in accumulation of TAG in liver leading to fatty liver
- Alcohol DH isoenzymes with higher Vmax more common in oriental populations than in white Europeans, results in faster formation of acetaldehyde
- Oriental populations have low activity of aldehyde DH (high Km) compared to white Europeans, results in slower removal of acetaldehyde
- Accumulation of acetaldehyde leads to facial flushing, light-headedness, palpitations & nausea (unpleasant reactions to alcohol intake)
- People of who have higher activity of alcohol DH or low activity of aldehyde DH, less susceptible to alcohol dependence
