Regulation of Glycolysis/Gluconeogenesis

Question 1

Organismal level regulatory mechanisms

Answer 1

1) Opposite pathways not favored simultaneously
2) Maximizes product utilization
3) Partitioning metabolites between alternative pathways
4) Use best fuel for need
5) Slow down synthetic pathways when product accumulates

Question 2

Glyceraldehyde-3-phosphate conversion fates

Answer 2

To Dihydroxyacetone phosphate –> gluconeogenesis

To pyruvate –> glycolysis

To fructose-6-phosphate –> pentose phosphate pathway

*all depending on cell’s relative needs

Question 3

Regulation of Glycolysis Big Players

Answer 3

Enzymes: Hexokinase, PFK-1, Pyruvate Kinase (Steps 1,3 10)
Hormones: Insulin, glucagon, epinephrine
Products/Precursors: AMP/ATP

Question 4

Hexokinase I/II role, location, action rate and inhibition

Answer 4

Phosphorylation of Glucose –> G6P
Predominate in muscle
High affinity for glucose (reaches Vmax very quickly)
Inhibited allosterically by G6P

Question 5

Hexokinase IV role, location, action rate and inhibition

Answer 5

Phosphorylation of Glucose –> G6P
Predominates in liver
Lower affinity for glucose than I/II (slow build to Vmax)
- high [Glucose] increases hexokinase activity
- low [Glucose] lower activity, glucose escapes glycolysis
inhibited by regulatory protein GKRP (glucokinase regulatory protein)

Question 6

When there is high [glucose] in liver
When there is low [glucose] in liver

Answer 6

High –> upregulation of hexokinase IV gene transcription
Low –> upregulation of glucose-6-phosphatase gene transcription

Question 7

Regulation of PFK-1 stimulators/inhibitors
How stimulators and inhibitors work

Answer 7

Stimulators: ADP, AMP
Inhibitors: ATP, Citrate

High ATP allosterically inhibits PFK-1 affinity
High citrate enforces ATP allosteric inhibition
High AMP/ADP relieves allosteric inhibition
(glycolysis when ATP is high and AMP is low)

Question 8

Regulation of Fructose-1,6-bisphosphatase (FBPase-1)

Answer 8

Stimulators: none directly
Inhibitors: high AMP

High AMP allosterically inhibits FBPase-1
(gluconeogenesis when AMP is low)

Question 9

Fructose-2,6-bisphosphate as an allosteric regulator

Answer 9

NOT a glycolytic intermediate !
Occurs in liver

Allosterically activates PFK-1: Fructose 6 phosphate –> Fructose 1,6-bisphosphate

Inhibitor of FBPase-1: NO F1,6BP –> Fructose-6-phosphate

Question 10

Fructose-2,6-bisphosphate formation
Enzyme, hormone, location in body

Answer 10

Fructose-6-phosphate –> Fructose 2,6-bisphosphate
Enzyme: PFK-2
Hormone: insulin
Function: acts as allosteric activator of PFK-1 and inhibits allosterically FBPase-1

Fructose 2,6-bisphosphate –> Fructose-6-phosphate
Enzyme: FBPase-2
Hormone: glucagon (turns off inhibition of FBPase-1)

Liver

Question 11

PFK-2/FBPase-2 gene

Answer 11

unusual gene that encodes for a bifunctional enzyme from single gene PFKFB1
whereas PFK-1 and FBPase-1 encoded by separate genes

Question 12

Regulation of PFK-2/FBPase-2 enzyme in the liver

Answer 12

Insulin activates phosphoprotein phosphatase via dephosphorylation
–> activating PFK-2 activity/inactivating FBPase-2 activity
[F26BP] ↑ –> glycolysis

Glucagon activates cAMP dependent protein kinase via phosphorylation
–> activating FBPase-2 activity/inactivating PFK-2 activity
[F26BP] ↓ –> gluconeogenesis

Question 13

Function of xylulose-5-phosphate in the liver

Answer 13

Xylulose-5-phosphate stimulates insulin pathway following high carb meal
phosphoprotein phosphatase stimulated to dephosphorylate PFK-2/FBPase-2 enzyme, so [F26BP] ↑

Xylulose-5-phosphate is from pentose phosphate pathway - increases as glucose enters glycolytic/PPP pathways

Question 14

Regulation of pyruvate kinase in the liver

Answer 14

Inactive when phosphorylated
Glucagon activates PKA which phosphorylates liver isoform pyruvate kinase

When [Glucagon] drops, protein phosphatase activates liver pyruvate kinase

Question 15

Regulation of pyruvate kinase in all glycolytic tissues

Answer 15

↑ [Fructose-1,6-bisphosphate] –> stimulates pyruvate kinase

↑ [ATP], [Acetyl-CoA], [Long chain FA] - inhibits pyruvate kinase

↑ [Alanine] from transamination of pyruvate - inhibits pyruvate kinase

Question 16

Regulatory response of high [acetyl-coA] in the liver

Answer 16

Pyruvate synthesis regulation:

↑ [Acetyl coA] in liver inhibits pyruvate dehydrogenase complex (TCA and beta oxidation)

↑ [Acetyl coA] in liver stimulates pyruvate carboxylase (pyruvate –> oxaloacetate) for gluconeogenesis pathway

Question 17

Insulin stimulates expression of these enzymes

Answer 17

To ↑ Glycolysis: Hexokinase II, Hexokinase IV, PFK-1, PFK-2/FBPase-2, pyruvate kinase

To ↑ NADPH production: G6P dehydrogenase, 6-phosphogluconate dehydrogenase and malic enzyme

Question 18

ChREBP stands for
ChoRE stands for

Answer 18

ChREBP: carbohydrate response element binding protein
ChoRE: carbohydrate response element (promoter gene)

Question 19

ChREBP location and function

Answer 19

ChREBP phosphorylated on serine and threonine is inactive
Inactive ChREBP found in cytosol of liver, adipose and kidney

↑ [Xylulose-5-phosphate] activates protein phosphatase 2A, dephosphorylating ChREBP
1) Dephosphorylation of serine allows entry into nucleus
2) Dephosphorylation of threonine allows binding to ChoRE with Mlx
3) leads to ↑ glycolysis and fatty acid synthesis

Question 20

FOXO-1 function

Answer 20

suppresses transcription of glycolysis, PPP and FA synthesis enzymes

Stimulates gluconeogenesis enzyme transcription
ex. PEP carboxykinase and glucose-6-phosphatase

Insulin activates PKB which phosphorylates FOXO-1, inactivating and ubiquitination for degradation
- glucagon does the opposite

Question 21

General transcription factor level regulation of glucose utilization

Answer 21

Coordination of many transcription factors on promoter region leads to regulation of glucose utilization

Mutation in 1 transcription factor can lead to a type of type II diabetes

Question 22

what is GKRP

Answer 22

glucokinase regulatory protein
inhibits hexokinase IV in liver activity when [glucose] are low

Question 23

NADPH is essential for

Answer 23

conversion of glucose to lipids (electron donor)
reduction of glutathione