Regulation of carbohydrate metabolism

Question 1

Where does the regulation of glycolysis occur?

Answer 1

Regulation occurs primarily at the level of glucose transport into the cell, PFK-1 and pyruvate kinase (in the liver)

Question 2

What is the purpose of glycolysis?

Answer 2

Metabolizes glucose to produce
- energy in the form of ATP by substrate level and
oxidative phosphorylation
- glycerol-3-phosphate for fat synthesis
- acetyl CoA for fat and cholesterol synthesis
- amino acids

Question 3

Where does glycolysis occur?

Answer 3

Glycolysis occurs in all tissues, particularly important for energy in brain and Rbc’s and also in contracting skeletal muscle. Rbc’s account for 10% of the bodies total usage.

Question 4

What is gluconeogenesis?

Answer 4

De novo glucose synthesis from non-carbohydrate precursors e.g.

• lactate from glycolysis
• amino acids from protein breakdown
• glycerol (but NOT fatty acids) from fat metabolism

Question 5

Where does gluconeogenesis occur?

Answer 5

In the liver

Question 6

What is the purpose of gluconeogenesis?

Answer 6

Maintains blood glucose during fasting, starvation or when glycogen reserves are depleted to preserve glucose-dependent cerebral function and red blood cell metabolism

Question 7

How does gluconeogenesis differ from the conceptual reversal of glycolysis?

Answer 7

Not a simple reversal of glycolysis, has unique enzymes to overcome energetically unfavourable reactions and introduce points of control

Question 8

What does gluconeogenesis require? State where the requirements are provided from

Answer 8

Requires both a source of energy for biosynthesis and a source of carbon for formation of glucose molecules
Energy is provided by metabolism of fatty acids released from adipose tissue
Carbon skeletons are provided by lactate, amino acids or glycerol released from TGs by lipolysis in adipose tissue

Question 9

Why can only the liver carry out gluconeogenesis?

Answer 9

Irreversible steps in glycolysis overcome by expression of specific gluconeogenic enzymes. These are also sites of regulation by hormones
Liver cells are only ones which express these specific enzymes

Question 10

What does the gluconeogenesis pathway start with?

Answer 10

Oxaloacetate which is then converted to pyruvate

Question 11

How is glycolysis regulated?

Answer 11

6-phosphofructo-1-kinase (PFK-1) is subject to energy-dependent allosteric regulation by ATP, AMP and H+

Question 12

What happens if lots of ATP is present in glycolysis?

Answer 12

ATP inhibits - sign of high energy levels in muscle. Prevents glucose being utilised by glycolysis when ATP is available. Co-ordinates glycolysis with glycogen breakdown via phosphorylase

Question 13

What happens if lots of AMP is present in glycolysis?

Answer 13

AMP (present when ATP is depleted e.g. during muscle contraction or anoxia) leads to activation. Competes with ATP. Increases glycolysis and energy production. Co-ordinates glycolysis with glycogen breakdown via phosphorylase

Question 14

Describe the regulation of PFK-1 by H+ ions

Answer 14

H+ increased during anoxia or anaerobic muscle contraction as a result of lactic acid production

Inhibits glycolysis to prevent cellular pH falling too low and damaging the cellular machinery

In heart can be overcome by high AMP resulting in cellular damage and chest pains experienced in heart attacks and angina

Question 15

Describe the allosteric regulation by Fru-6-P,

Fru-2,6-P2 and citrate of PFK-1

Answer 15

Fru-6-P activates - sign of high rates of glucose entry or glycogen breakdown. Stimulates glycolysis to allow utilisation for energy production or fat synthesis.

Fru-2,6-P2 is also a signal of high rates of glucose entry or glycogen breakdown and leads to activation. Most potent allosteric activator known. Stimulates glycolysis to allow utilisation for energy production or fat synthesis

Citrate inhibits. Signals TCA cycle overload (more acetyl CoA than can be oxidised) or fatty acid oxidation (e.g. starvation) and the need to conserve glucose by inhibition of glycolysis

Question 16

Which nutrient is the most potent allosteric activator?

Answer 16

Fru-2,6-P2

Question 17

How is Fructose 2,6 bisphosphate synthesized?

Answer 17

Synthesized from F-6-P by the enzyme 6-PHOSPHOFRUCTO-2-KINASE
(PFK-2)

Question 18

What is fructose-2,6-bisphosphate an inhibitor of?

Answer 18

Potent inhibitor of fructose-1,6-bisphosphatase

Question 19

What inhibits glycolysis?

Answer 19

Presence of sufficient energy (ATP)
Fatty acid oxidation (i.e. citrate) indicating the need for glucose ‘sparing’
H+ ions (lots of lactate)

Question 20

What activates glycolysis?

Answer 20

Low levels of energy (AMP)

Lots of glucose or its metabolites

Question 21

Does lots of available glucose always signal the need for glycolysis?

Answer 21

Not in the liver since you want to export it

Question 22

Describe the action of fructose-2.6-bisphosphate in the liver

Answer 22

In liver, not only have to control glycolysis at the level of PFK-1, but also the reverse reaction of gluconeogenesis at F-1,6,BPase to allow reciprocal control of the two reactions.

Neither PFK-1 nor F-1,6-BPase are directly controlled by hormones through phosphorylation but by level of F-2,6-BP which IS affected by hormones

In liver PFK-2 and F-2,6-BPase are a single tandem enzyme with two active sites, Phosphorylation inhibits PFK-2 and stimulates F-2,6-BPase = F-2,6-P2

Question 23

Describe the regulation of gluconeogenesis

Answer 23

Major sites of regulation occur at the reactions where glycolysis and gluconeogenesis use different enzymes

Stimulated in the short term by glucagon and adrenaline by changes in protein phosphorylation or mobilisation of fatty acids and production of acetyl CoA

Long term stimulation occurs through enzyme induction by glucagon, glucocorticoids and thyroid hormones

Inhibited acutely by insulin via dephosphorylation and suppression of lipolysis and in the long term by suppression of gluconeogenic enzymes

Increased fatty acid oxidation leads to increase in acetyl CoA – an allosteric activator of pyruvate carboxylase and inhibitor of pyruvate dehydrogenase – so favours gluconeogenesis over glycolysis

Increased glucagon inhibits PFK-2 activity and stimulates F-2,6-BPase by phosphorylation (via cAMP-dependent protein kinase) resulting in a fall in F-2,6-BP

Decreased F-2,6-BP levels reduces activation of PFK-1 (inhibits glycolysis) and relieves inhibition of F-1,6-BPase (stimulates gluconeogenesis)

Question 24

Describe the urea cycle

Answer 24

To use amino acids as a source of carbon skeletons for glucose production, must first be transaminated to lose their ammonia.

Ammonia is toxic to cells, so must be eliminated from the body. Converted to urea in the liver, then passed out into the bloodstream and excreted by the kidneys

Fumarate is converted to oxaloacetate in the cytoplasm thereby generating substrate for gluconeogenesis
Increased rates of gluconeogenesis are always coupled with increased rates of urea synthesis