6 - Metabolic Regulation 2

Question 1

What are the 5 mechanisms of homeostatic regulation?

Answer 1

Gene expression and compartmentation (within cells / tissues)

Induction of enzymes (gene expression)

Covalent modification

Allosteric effectors

Substrate availability

Question 2

How do allosteric regulators work and how do activators and inhibitors affect the rate?

Answer 2

Binding of allosteric regulators (at a site other than the site) affect substrate binding

Allosteric activators: lower Km and increase observed rate
Allosteric inhibitors: increase Km and decrease observed rate

Question 3

What are the 4 allosteric regulators and how do they effect the metabolic pathways?

Answer 3

1) Fructose-2,6-BP!
- Key regulator of glycolysis
- Allosteric ‘activator’ of phosphofructokinase
- Allosteric inhibitor of fructose-bisphosphatase
- produced when F-6-P is high

2) Citrate
- Allosterically increase production of malonyl-CoA
- Negative regulator of PFK
- produced at higher levels under a glycolytic and lipogenic state

3) Malonyl coA
- Key regulator of fatty acid synthesis/oxidation
- Allosteric inhibitor of CPT1 (or CAT1)

4) Acetyl coA
- Regulator of pyruvate dehydrogenase (indirect) and pyruvate carboxylase
- Prevents conversion of pyruvate derived from cori and glc:ala cycle into acetyl coA
- Key substrates for gluconeogenesis

(High citrate promote fatty acid synthesis, the first of these steps produces malonyl co-A which inhibits CPT-1 and prevents fatty acid oxidation)

Question 4

Which alloesteric regulators are involved in high insulin levels and at high glucagon level?

Answer 4

High Insulin (Well-Fed) - Fructose-2,6-BP, Malonyl coA, Citrate

High Glucagon (Fasting) - Acetyl coA

Question 5

What are the 4 Principles of Metabolic Regulation and how does the PFK test show this?

Answer 5

The change of activity alters both substrate and product levels and hence the rates of neighbouring steps.

The relative change in the rate of an enzyme divided by the relative change in substrate concentration is called the elasticity of that step.

In reality metabolite concentrations hardly change.

Control has moved from the step catalysed by phosphofructokinase to other parts of the pathway.

Question 6

How does Covalent Modification work and how does insulin and glucagon affect this?

Answer 6

Glucagon -> phosphorylation by protein kinase

Insulin -> Dephosphorylation by phosphoprotein phosphatase

Glucagon receptor is a G-protein coupled receptor
Insulin receptor is a RTK

Question 7

Describe Control of liver glucose metabolism by glucagon (example of covalent modification)

Answer 7

bifunctional enzyme

slide 12

Question 8

Describe Regulation of Fatty acid synthesis (example of covalent modification)

Answer 8

Acetyl coenzyme A carboxylase (ACC)

Slide 13

Question 9

List 6 key enzymes regulated by covalent modification by insulin:glucagon. For each enzyme state the reaction being controlled and whether the enzyme is activated or inactivated in terms of its phosphorylation status in response to insulin:glucagon
!!!

Answer 9

Glycogen phosphorylase
Glycogen synthase
6-Phosphofructo-2-kinase/ Fructose-2,6-bisphosphatase
Pyruvate kinase
Pyruvate dehydrogenase
Acetyl CoA carboxylase

slide 18

Question 10

How is Control of gluconeogenesis different with and without glucagon, and how is this related to phosphorylation?

Answer 10

Without - Control strength distributed along pathway
- Note the negative control by pyruvate kinase

With - Control strength increased at the first committed
step of gluconeogeneis!
- No negative effect of pyruvate kinase

Question 11

Explain how these covalent modifications control adaptations to nutrient supply/demand in the starve-feed cycle !!!

Answer 11

Think

Question 12

What are the Long-term metabolic adaptations to the ‘well-fed’ state?

Answer 12

1) High glucose –> carbohydrate-response-element-binding protein (ChREBP)
Mediated by a protein phosphatase in response to glucose

ChREBP –> Increase expression of lipogeneic genes (CarRE: carbohydrate response element)

opposed by glucagon

2)Insulin –> sterol-response-element-binding protein (SREBP-1c)
and insulin –> Inactivation of forkhead transcription factor (mediated by PKB/Akt)

results in decreased expression of gluconeogenic genes and those for fatty acid oxidation

Question 13

What are Long-term metabolic adaptations to the ‘fasting’ state?

Answer 13

> Glucagon –> adenylate cyclase –> cyclic AMP –> protein kinase A

• -> glucagonic genes
• -> inhibit lipogenic genes

Question 14

How does PPAR signalling modulates adiposity and diet-induced inflammation?

Answer 14

Known to be activated by a range of fatty acids and their eicosanoid derivatives (‘adopted orphan receptors’)

Increases pre-adipocyte differentiation

Increase TAG export/storage!!

Question 15

Explain how long-term adaptations to the well-fed and fasting states lead to changes in gene expression of enzymes in key metabolic pathways

Answer 15

Think!