Regulation of glycolysis and gluconeogenesis Flashcards
__ is regulated to maintain constant ATP levels
Glycolysis is regulated to maintain constant ATP levels
Glycolysis occurs if __ is high and __ is low
Gluconeogenesis occurs if __ is low
Glycolysis occurs if AMP is high and ATP is low.
Gluconeogenesis occurs if AMP is low
Which hormones regulate expression/function
of glycolytic enzymes
Glucagon, epinephrine and insulin
What are the isoforms of hexokinase
1, 2, 3 and 4
Where are hexokinase I &II found? What is their affinity for glucose ?
- Predominantly expressed in muscle
- High affinity for glucose (activity reaches maximum quickly)
Where is hexokinase IV found? What is it’s affinity for glucose
- Predominant in liver
- Lower affinity for glucose
What inhibits hexokinase IV
Inhibited by a regulatory protein (liver specific; GKRP
How’s hexokinase IV and glucose concentration related?
- Activity increases at high glucose concn
- Glu escapes glycolysis at low concn
High glucose concentration is required for max activity
Are hexokinase I, II and IV inhibited by
Hexokinase IV is not inhibited by G-6-P
Hexokinase II and III are allosterically inhibited by G-6-P, which is the product of hexokinase I and II (negative feedback_
How is hexokinase IV inhibited by GKRP?
GKRP (Glucokinase regulatory protein ) can take hexokinase away from the site of glycolysis (cytoplasm) into the nucleus. Occurs in the liver only
This translocation of Hex IV from cytoplasm to nucleus is stimulated by fructose 6-phsophate- downstream product of hexokinase IV->
If there’s a lot of glucose Hex IV will be drawn back to cytoplasm
What happens when blood glucose level raises?
when the blood glucose level rises, insulin acts in muscle to
(1) increase glucose transport into cells by conveying GLUT4 to the plasma membrane
(2) induce the synthesis of hexokinase, and
(3) activates glycogen synthase by covalent alteration
In which cells does glycolysis and gluconeogenesis occurs?
Glycolysis takes place in any cell.
Only few cells can do gluconeogenesis- hepatocytes, kidneys, intestinal epithelium
Which enzymes are used in liver when:
Blood glucose is low
Blood glucose is high
- High blood glucose – Hexokinase IV
- Low blood glucose – Glu-6-phosphatase
PFK-1 - Function Regulation: - Inhibitors - Stimulators
PFK-1 (Phosphofructokinase-1) converts phospho-6-fructose to fructose 1,6-bisphosphate
- High ATP allosterically inhibits PFK-1 (No ATP is needed = no need to break down glucose)
- High citrate increases ATP-inhibition
- High AMP or ADP relieve ATP-inhibition
FBPase-1
- Function
Regulation:
- Inhibitors
FBPase-1 (Fructose 1,6-Bisphosphatase) converts fructose 1,6-bisphosphate to fructose 6-phosphate
- High AMP allosterically inhibits FBPase-1 (Low energy inhibits FBPase as glucose doesn’t need to be made- it needs to be used )
What is an enzymes in gluconeogenesis that replaces PFK-1 in glycolysis?
FBPase-1
How are PFK-1 and FBPase-1 enzymes regulated?
They are regulated in the liver by enzymes FBPase-2 and PFK-2
PFK-2 uses ATP to add a phosphate group to fructose 6-phosphate making fructose 2,6-bisphosphate.
Fructose 2,6-bisphosphate is not a glycolytic intermediate- not used in glycolysis due to its asymmetry and is produced mainly in the liver. It is a allosteric activator of PFK-1 and inhibitor of FBPase-1
FBPase-2 removes a phosphate group from fructose 2,6-bisphosphate, turning it back into fructose 6-phosphate
PFK-1 and FBPase-1 vs PFK-2 and FBPase-2
- PFK-1 and FBPase-1 are encoded by two genes, PFKM and FBP1
- PFK-2/FBPase-2 is a bifunctional enzyme, expressed from single gene, PFKFB1
How is PFK-2/FBPase-2 regulated?
PFK-2/FBPase-2 is regulated by phosphorylation
Unphosphorylated-> PFK-2 activity -> increase in Fructose 2,6-bisphosphate
Phosphorylated-> FBP-2ase activity-> Fructose 2,6-bisphosphate is converted to Fructose 6-phosphate
How is phosphorylation of PFK-2/FBPase-2 regulated?
Insulin/ Xylulose 5-phosphate stimulates phospho-protein phosphatase which removes phosphate group and converting it to PFK-2
Glucagon/cAMP activates cAMP-dependent protein kinase that comes from GPCR- glucagon receptor. It phosphorylates the enzyme and induces FBP-2ase activity
What does high concentration of F26BP stimulates and inhibits?
High concentration of F26BP stimulates glycolysis and inhibits gluconeogenesis
Why does Xylulose 5-phosphate stimulates PFK-2 activity?
Xylulose 5-phosphate comes from phosphate pathway-> no more glucose is required for pentose phosphate pathway
Thus PFK-2 will stimulate the production of fructose 2,6-bisphosphate which activates PFK-1 and leads to glycolysis, breaking down glucose that is no longer needed
Does phosphorylation of pyruvate kinase activate or deactivate it
Deactivates
How’s pyruvate kinases regulated in the liver?
Glucagon activates cAMP-dependent protein kinase, which phosphorylates the pyruvate kinase L isozyme, inactivating it.
When the glucagon level drops, a protein phosphatase (PP) dephosphorylates pyruvate kinase, activating
What inhibits pyruvate kinase in all glycosidic cell, including liver?
Pyruvate kinase is allosterically inhibited by ATP, acetyl-CoA, and long-chain fatty acids.
Accumulation of alanine, which can be synthesized from pyruvate in one step, allosterically inhibits pyruvate kinase
What activates pyruvate kinase in all glycosidic cell, including liver?
Accumulation of fructose 1,6-bisphosphate triggers its activation.
Which step of glycolysis involves pyruvate kinase
The last step
Why should liver isoform of pyruvate kinase be regulated?
his mechanism prevents the liver from consuming glucose by glycolysis when blood glucose is low; instead, the liver exports glucose.
Are liver and muscle pyruvate kinases regulated hormonally and by phsophorylation?
The liver isozyme (L form) is regulated hormonally and is subject to further regulation by phosphorylation
The muscle isozyme (M form) is not affected by phosphorylation mechanism- muscles do not express glucagon receptor
What activates pyruvate kinase in all glycosidic cell, including liver?
High concentration of precursor/ substrates (e.g. glucose-6-phosphate and F16BP)
What are the 2 fates of pyruvate?
Acetyl CoA-> Citric acid cycle-> energy
Oxaloacetate-> gluconeogenesis -> glucose
What are the 2 ways of production of AcetylCoA?
acetyl-CoA can be produced either by fatty acid oxidation or by the pyruvate dehydrogenase complex from pyruvate
What is the effect of acetyl CoA on pyruvate carboxylase and pyruvate dehydrogenase?
Acetyl CoA stimulates pyruvate carboxylase and inhibits pyruvate dehydrogenase.
Reaction of pyruvate to acetyl CoA
By pyruvate dehydrogenase complex
Pyruvate-> Acetyl CoA + CO2
Reaction of pyruvate to oxaloacetate
By pyruvate carboxylase
Pyruvate-> Oxaloacetate
When is Acetyl CoA concentration is high
When liver has sufficient FA for β-oxidation- fatty acids are being oxidized in high amounts
Insulin __ expression of enzymes involved in glycolysis
Insulin __ expression of enzymes involved in gluconeogenesis
Effect of glucagon is the __
Insulin increases expression of enzymes involved in glycolysis
Insulin decreases expression of enzymes involved in gluconeogenesis
Effect of glucagon is the opposite
What are the roles of the following enzymes? What is the effect of insulin on these enzymes? Hexokinase II Hexokinase IV Phosphofructokinase-1 (PFK-1) PFK-2/FBPase-2 Pyruvate kinase
Essential for glycolysis, which consumes
glucose for energy
Insulin increases their expression
What are the roles of the following enzymes? What is the effect of insulin on these enzymes?
Glucose 6-phosphate dehydrogenase
6-Phosphogluconate dehydrogenase
Malic enzyme
Produce NADPH, which is essential for conversion of glucose to lipids. Involved in glycolysis
Insulin increases their expression
What are the roles of the following enzymes? What is the effect of insulin on these enzymes?
PEP carboxykinase
Glucose 6-phosphatase (catalytic subunit)
Essential for glucose production by gluconeogenesis
Insulin decreases their expression
What’s ChREBP?
What is it’s role
ChREBP (carbohydrate response element binding protein) is a transcription. It serves to coordinate the synthesis of enzymes needed for carbohydrate and fat synthesis
It serves to coordinate the synthesis of enzymes needed for carbohydrate and fat synthesis.
Where’s ChREBP expressed?
It is expressed primarily in liver, adipose tissue, and kidney.
What is the inactive state of ChREBP and where is it found?
ChREBP in its inactive state is phosphorylated, and is located in the cytosol.
Describe how does ChREBP regulate
When ChREBP in the cytosol of a hepatocyte is phosphorylated on a Ser and a Thr residue, it cannot enter the nucleus.
Dephosphorylation of p-Ser of ChREBP by protein phosphatase PP2A allows ChREBP to enter the nucleus, where a second dephosphorylation of P –Thr by nuclear PP2A activates ChREBP so that it can associate with its partner protein, Mlx.
ChREBP-Mlx now binds to the carbohydrate response element (ChoRE) in the promoter and stimulates transcription.
What does protein phosphatase PP2A do?
Dephosphorylates p-Ser of ChREBP, allowing it to enter the nucleus
What activates PP2A?
PP2A is allosterically activated by xylulose 5-phosphate, an intermediate in the pentose phosphate pathway.
What is the role of FOXO?
FOXO1 (forkhead box other) stimulates the synthesis of gluconeogenic enzymes and suppresses the synthesis of the enzymes of glycolysis, the pentose phosphate pathway, and triacylglycerol synthesis. Induces expression of PEP carboxykinase and Glucose 6-phosphatase
Describe functioning of FOXO1
Insulin activates the signaling cascade, leading to activation of protein kinase B (PKB).
FOXO1 in the cytosol is phosphorylated by PKB, and the phosphorylated transcription factor is tagged by the attachment of ubiquitin for degradation by proteasomes.
FOXO1 that remains unphosphorylated or is dephosphorylated can enter the nucleus, bind to a response element, and trigger transcription of the associated genes.
Insulin therefore has the effect of turning off the expression of these genes, which include PEP carboxykinase and glucose 6-phosphatase.
What does FOXO1 suppress?
Transcription of enzymes of glycolysis, pentose phosphate pathway and FA synthesis
What’s the effect of insulin on gluconegenisis enzymes?
Insulin activates a signaling cascade that degrades FOXOQ and therefore has the effect of turning off the expression of these genes, which include PEP carboxykinase and glucose 6-phosphatase.
