Lecture 8 Flashcards
Describe 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
Occurs in liver (and kidney)
Maintains blood glucose during fasting, starvation or when glycogen reserves are depleted to preserve glucose-dependent cerebral function and red blood cell metabolism
Not a simple reversal of glycolysis, has unique enzymes to overcome energetically unfavourable reactions and introduce points of control
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
What are the three irreversible steps in gluconeogeneis
Glucose->G-6-P
F-6-P->F-1,6-P2
PEP->Pyruvate
What are the four irreversible steps for the gluconeogenic pathway
Pyruvate->Oxaloacetate using pyruvate carboxylase
Oxaloacetate->PEP
F-1,6-P2->F-6-P
G-6-P->Glucose
Describe the regulation of glycolysis
6-phosphofructo-1-kinase (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
Describe the regulation of PFK-1 by H+ ions
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
Describe the regulation of PFK-1 by nutrients
PFK-1 is also subject to allosteric regulation by Fru-6-P,
Fru-2,6-P2 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.
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
Discuss fructose-2,6-bisphosphate
Synthesized from F-6-P by the enzyme 6-PHOSPHOFRUCTO-2-KINASE
(PFK-2)
Most potent allosteric activator of 6-phosphofructo-1-kinase
(PFK-1). Potent inhibitor of fructose-1,6-bisphosphatase
Not involved in metabolic pathways: acts solely to re-inforce allosteric control on PFK-1
Activated by: F-6-P - Increased glucose concentrations
- Increased glycogen breakdown (muscle)
AMP - Increased contraction (low ATP)
Inhibited by: Citrate - Increased fatty acid oxidation (TCA cycle overload)
Discuss the affect of fructose-2,6-bisphosphate in the 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.
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 = decrease F-2,6-P2
Discuss the regulation of gluconeogenesis
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
How is gluconeogeneis controlled
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)
Discuss the urea cycle
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
NH3 + CO2 + 2H2O + 3ATP + aspartate->urea + fumarate + 2ADP + AMP + 2Pi + PPi
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
