Gluconeogenesis and regulation Flashcards
Define Gluconeogenesis
Lactate from anaerobic metabolism of pyruvate can be converted back to glucose in the liver and kidney
What are the precursors of Gluconeogenesis?
Non-carbohydrate sources also provide precursors for gluconeogenesis
Most amino acids
Glycerol (via DHAP)
Metabolites which can be converted to pyruvate or oxaloacetate
What happens to the precursors formed in non-gluconeogenic tissues?
Transported to the liver
What takes place in the liver for the Cori cycle?
In the liver: Fatty acids are converted into ATP and Co2 + H2O is released. ATP is converted to glucose by lactate
What takes place in the muscle for the Cori cycle?
Muscle: Glucose is transported into the muscle, converting to glycogen and then back to glucose before producing ATP and lactate. Lactate then transported back to the liver.
Bypassing the irreversible steps to glycolysis: How is Oxaloacetate formed from pyruvate?
Requires 2 enzymes and 2 high energy phosphates
Pyruvate binds with HCO3
ATP is converted to ADP+Pi
Oxaloacetate is then formed
Bypassing the irreversible steps to glycolysis:
How is Phosphoenol pyruvate formed from Oxaloacetate?
GTP is converted to GDP
CO2 is lost
Forming Phosphoenolpyruvate
Highly endergonic
The energy supplied by decarboxylation helps drive the reaction
Additional energy supplied by GTP hydrolysis
What enzyme catalysis the conversion of Pyruvate to Oxaloacetate?
Pyruvate Carboxylase
What enzyme catalysis the conversion of Oxaloacetate to Phosphoenolpyruvate?
PEP carboxykinase
Bypassing the irreversible steps to glycolysis:
How is Fructose-1,6-bisphosphate converted to Fructose-6-phosphate?
F-1,6-bisphate is hydrolysed and Pi is lost
Hydrolysis of fructose-1,6-bisphosphate ester is thermodynamically favourable
Bypassing the irreversible steps to glycolysis:
How is Glucose-6-phosphate converted to Glucose?
G-6-phate is hydrolysed and Pi is lost
Hydrolysis of G-6-phate is thermodynamically favourable
Define Glucose-6-phosphatase
Membrane-bound enzyme localised on the smooth endoplasmic reticulum
Only present in liver and kidney
Has a high Km for Glucose-6-phosphate
Only active when substrate levels are high (substrate-level control)
Why is Gluconeogenesis regulated?
Meet major needs in the body:
1) ATP production
2) Production of precursors for other metabolic pathways
3) Conversion of pyruvate to glucose:
- To maintain blood [glucose] when alternative sources are exhausted
- Flux of metabolites through pathways must be tightly regulated
- Response to both intracellular and extracellular conditions
what 3 points are the pathways regulated?
1) Phosphorylation of glucose
2) Phosphorylation of fructose-6-phosphate
3) formation of pyruvate and ATP
All three steps are essentially irreversible under cellular conditions
What 3 different types of regulation controls enzyme activity?
Allosteric regulation: By substrate, products or effects (activators/inhibitors)
Covalent modification (phosphorylation)
Transcriptional activation
Regulation of Hexokinase: Whats the role of Hexokinase?
Effectively commits glucose to a fate within the cell
Glucose-6-phosphate can have a number of cellular fates
What inhibits hexokinase and how?
Glucose-6-phosphate is an allosteric inhibitor of hexokinase
Glucose-6-phosphate accumulates when all glucose-requiring pathways within the cell are full
Inhibits further glucose phosphorylation
How does the regulation of Glucokinase differ from Hexokinase?
Important because of its role in glucose homeostasis
Predominant isoform present in the liver
Glucokinase has a higher Km for glucose
Affinity is approximately 50-fold lower than hexokinase
The liver only takes up glucose when blood [glucose] is high
What regulates Glucokinase activity?
Fructose phosphates
Glucokinase is inhibited by?
Fructose-6-phosphate: Glucose-6-phosphate (a precursor of glycogenesis) does not inhibit.
What inhibits glucokinase?
Glucokinase regulatory protein:
GRP is present in liver cells
Binds to GK in the presence of fructose-6-phosphate
Glucose flux via glucokinase in the liver is usually low as a result
Why does Fructose-1-phosphate compete for bind sites on GRP?
Only present after a carbohydrate-rich meal
Fructose-1-phosphate relieves inhibitory effect of GRP on glucokinase
Liver, therefore, acts as a major control point in maintaining blood glucose levels
Beta-islet cells in the pancreas also contain glucokinase and GRP
Explain the regulation of glucose-6-phosphatase
High Km (substrate-level regulation)
The enzyme is only active when levels of glucose-6-phosphate
Levels only rise when all other pathways requiring glucose-6-phosphate are full
Formation of fructose-1,6-bisphosphate represents?
Regulation of phosphofructokinase-1 which is a committed step in glycolysis
What allosteric effectors does PFK-1 have?
ATP and Citrate
ADP
AMP
allosteric effectors of PFK-1: What do ATP and Citrate do?
Signal a high energy state within the cell
ATP is a potent inhibitor of PFK-1 (decreases affinity for fructose-6-phosphates)
Citrate enhances inhibitory effects
allosteric effectors of PFK-1: What do ADP and AMP do?
The potent stimulator of PFK-1
Represent a cell deficient in ATP
Activate PFK-1 by increasing affinity for fructose-6-phosphate
What activates PFK-1?
Fructose-2,6-bisphosphate
How is Fructose-2,6-bisphosphate formed?
Formed from fructose-6-phosphate by phosphofructokinase-2 (PFK-2)
Which increases affinity of PFK-1 for fructose-6-phosphate
Decreases inhibitory effect of ATP
What is PFK-2?
A bifunctional enzyme regulated by covalent modification
Tandem enzyme
A kinase activity (PFK-2) and fructose-2,6-bisphosphatase (FBPase-2) present on the same polypeptide
A phosphorylation domain controls the activity of both enzyme activities
Fructose-2,6-bisphosphate is also a potent allosteric inhibitor of?
Fructose-1,6-bisphosphatase
Covalent modifications regulate fructose-2,6-bisphosphate levels in response to hormonal signals
Fructose-1,6-bisphosphatase activity is also controlled by?
Allosteric effectors which prevent substrate cycling
Many activators of PFK-1 inhibit fructose-1,6-bisphosphatase
Inhibitors of PFK-1 activate fructose-1,6-bisphosphatase
Without regulation of fructose-6-phosphate to fructose-1,6-bisphosphate, what would happen?
The net result would be a futile cycle
Energy from ATP hydrolysis would be liberated as heat
Futile cycling through to be responsible for such conditions as malignant hyperthermia
Possibly important means of flux control during excessive exercise
Also an important factor in heat generation in insects
What isoenzymes exhibit a sigmoidal response to phosphoenolpyruvate (PEP)?
Liver, kidney, muscle and erythrocyte isoenzymes
Whats involved in the regulation of pyruvate kinase?
Allosterically activated by Fructose-1,6-bisphosphate
Allosterically inhibited by ATP
Regulation ensures full utilisation of glycolytic intermediates
Activity curve with fructose-1,6-bisphosphate is hyperbolic
Maximally active at physiological
High [ATP] lowers affinity for PEP
Alanine, acetyl-Coa and long-chain fatty acids are also inhibitors
pyruvate kinase isoenzymes present in?
Liver and intestinal cells are also regulated by covalent modification
Explain Phosphorylation by protein kinase A?
In response to the effect of glucagon
Results in a less active form of pyruvate kinase
Explain Transcriptional regulation
Levels of pyruvate kinase decrease during periods of starvation
Increase after introduction to a high carbohydrate diet
This is the result of changes in the rate of transcription of the pyruvate kinase gene
What is the fate of pyruvate?
ATP production from glyceraldehyde-3-phosphate in glycolysis -> reduction of NAD to form NADH
Cells only have limited amounts of NAD
If NAD is not regenerated, glycolysis will stop
Passing of reducing equivalents to electron transport chain (aerobic conditions)
Under anerobic conditions metabolism of pyruvate regenerates NAD
Activity changes in response to?
Concentration of substrate, product or different effector molecules
Why is Allosteric regulation important for control of metabolic pathways?
1) Small changes in [S] can result in large changes in activity: Helps to maintain homeostasis
2) Permits regulation by effectors totally dissimilar to substrates or products: End products of pathways can exert effects
3) Permits interaction of effectors formal several pathways
