Regulation of carbohydrate metabolism Flashcards
What does glycoloysis produce?
Metabolizes glucose to produce:
- energy in the form of ATP by substrate level phosphorylation
- glycerol-3-phosphate for fat synthesis
- pyruvate for conversion to acetyl CoA for TCA cycle or fat and cholesterol synthesis
- amino acids
Where does regulation of glycoloysis occur?
•Regulation of glycolysis occurs primarily at the level of glucose transport into the cell, PFK-1 and pyruvate kinase (in the liver)
How is metabolism regulated?
- Glycogenolysis and glycogen synthesis are reciprocally regulated to avoid futile cycling of glucose
- Regulatory mechanisms are allosteric
Regulatory mechanisms are also hormonal
Why is it important for carbohydrate metabolism to be regulated?
- Glucose generated from glycogenolysis must be directed into the appropriate biological pathway (tissue specific)
- The opposing processes of glycolysis and gluconeogenesis must be reciprocally regulated
Which steps of glycoloysis differ from gluconeogenesis?
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. The irreversible steps (shown in red) are where the pathway differs from gluconeogenesis.
What is gluconeogenesis?
•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
•Not a simple reversal of glycolysis, has unique enzymes to overcome energetically unfavourable reactions and introduce points of control
Where does gluconeogenesis occur?
•Occurs in liver (and kidney)
Why is gluconeogenesis important?
•Maintains blood glucose during fasting, starvation or when glycogen reserves are depleted to preserve glucose-dependent cerebral function and red blood cell metabolism
What are the 2 requirements for gluconeogenesis?
What is the role of the urea cycle?
- Increased rates of gluconeogenesis are always coupled with increased rates of urea synthesis
- To use amino acids as a source of carbon skeletons for glucose production, must first be transaminated to lose their ammonia.
What happens in the urea cycle?
•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
NH3 + CO2 + 2H2O + 3ATP + aspartate -> urea + fumarate + 2ADP + AMP + 2Pi + PPi
•Fumarate is converted to oxaloacetate in the cytoplasm thereby generating a substrate for gluconeogenesis
What are the checkpoints in glycoloysis and gluconeogenesis?
How is glycoloysis regulated?
PFK-1 is subject to energy-dependent allosteric regulation by ATP, AMP and H+
- 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
- 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
When is H+ increased?
•H+ increased during anoxia or anaerobic muscle contraction as a result of lactic acid production
How does increased H+ regulate PFK-1?
- 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
How is PFK-1 regulated by nutrients?
PFK-1 is also subject to allosteric regulation by Fru-6-P, Fru-2,6-BP and citrate
- 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-BP 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
Where is PFK1, PFK-2 & F-2,6-BPase found?
All cells
Where is F-1,6,BPase found?
Liver and kidney
Complete the diagram on an extra level of control in glycoloysis
What is the Most potent allosteric activator of PFK-1?
Fructose 2,6 bisphosphate
How is fructose 2,6 bisphosphate produced?
What is the role of fructose 2,6 bisphosphate?
Most potent allosteric activator of PFK-1. Potent inhibitor of fructose-1,6-bisphosphatase
Not involved in metabolic pathways: acts solely to re-inforce allosteric control on PFK-1
What is glycolosysis inhibited by?
- Presence of sufficient energy (ATP)
- Fatty acid oxidation (i.e. citrate) indicating the need for glucose ‘sparing’
- H+ ions (lots of lactate)
What is glycoloysis activated by?
- Low levels of energy (AMP)
- Lots of glucose or its metabolites
Does lots of available glucose always signal the need for glycolysis
Not in the liver
What is the difference in the source of glucose usage in the muscle and the liver?
Muscle uses glucose and glycogen for energy production by increasing F-2,6-BP and stimulating glycolysis
Liver uses glucose produced via gluconeogenesis and glycogen to maintain blood glucose so glycolysis is inhibited
What is different about Fructose-2,6-bisphosphate in liver?
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.
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 = decreased F-2,6-BP
How are PFK-1 and F-1,6-BPase controlled in the liver?
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
How is gluconeogenesis activated?
- 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)
How is gluconeogenesis hormonally controlled?
- 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
