TOPIC A: REGULATION OF METABOLISM Flashcards
What is homeostasis?
The maintenance of a dynamic steady state by regulatory mechanisms that compensate for changes in external circumstances
What is glucose homeostasis?
The entry of glucose into the blood is equal to/ balanced with the uptake of glucose from the blood
- concentration of blood glucose levels should be maintained at approx. 5mM
How does the body achieve glucose homeostasis?
Regulation of:
- glycolysis/gluconeogenesis
- glycogen metabolism
Intracellular control:
- allosteric regulation
- covalent modification
Hormones:
- insulin
- glucagon
- epinephrine
What is glycolysis?
The breakdown of 1 molecule of glucose into 2 molecules of pyruvate while releasing ATP and NADH (energy)
It occurs in the cytosol of most eukaryotic cells however because it does not require oxygen it can occur in both anaerobic and aerobic cells
It is a major source of energy in mammalian cells
What is the equation for the reaction of glycolysis?
2 NAD+ + 2 ADP + 2Pi –> 2 pyruvate + 2 NADH + 2 ATP + 2 H2O
- the pyruvate produced can be used either in the citric acid cycle (when oxygen is present) or in fermentation (anaerobic conditions)
What are the two phases of glycolysis?
preparatory phase: phosphorylation of glucose and conversion to 2 molecules of glyceraldehyde 3-phosphate
- energy dependent
- is thought of like an investment
- requires 2 ATP molecules
payoff phase: Oxidative conversion of glyceraldehyde 3-phosphate to pyruvate and the coupled formation of ATP and NADH
- produces energy
- 4 ATP produced (net = 2)
When is glycolysis active?
- ATP is low
- ADP is high (up to 5 fold)
- AMP is relatively high (up to 20 fold)
- NADH to NAD+ ratio is low
- occurs when cells need energy
What is gluconeogenesis?
The generation of glucose from non-carbohydrate pre-cursors such as pyruvate or oxaloacetate
- most likely to occur during times of fast or when there is low blood glucose levels
- it is NOT simply the reverse of glycolysis
- however, 7 of the 10 enzymatic reactions are shared with glycolysis
- mainly takes place in the liver
- Glycolysis and gluconeogenesis are reciprocally regulated so that both processes don’t occur simulatenously in a futile cycle (energy saving)
What is the equation for the reaction of gluconeogenesis?
2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 4H2O –> glucose +
4 ADP + 2 GDP + 6 Pi + 2 NAD+
What occurs in bypass reaction one of gluconeogenesis/ futile cycle 3?
The conversion of 2-phosphoenol-pyruvate into pyruvate and the opposite in gluconeogenesis
In the first bypass, pyruvate is converted to PEP via oxaloacetate in two steps catalysed by pyruvate carboxylase (which uses ATP) and PEP carboxykinase (which uses GTP)
- 2 reactions required
- the first reaction occurs in the mitochondria
- the second reaction occurs in the cytosol OR the mitochondria
what happens in bypass reaction two of gluconeogenesis/futile cycle 2?
Fructose 1,6-bisphosphate is hydrolysed by fructose 1,6-bisphosphate-1, generating fructose-6-phosphate
- FBPase-1 removes a phosphate group from fructose 1,6-bisphosphate to produce fructose-6-phosphate
what happens in bypass reaction three of gluconeogenesis/ futile cycle 1?
Glucose-6-phosphate is hydrolised by glucose-6-phosphatase to produce glucose
when does gluconeogenesis occur?
- ATP is high
- ADP is low
- AMP is low
- NADH to NAD+ ratio is high
What is glycogen?
glycogen is a polysaccharide of glucose
- primarily stored in the liver (but can also be found in skeletal muscle too)
- branched molecule made up of glucose-residue chains
- protein core - glycogenein
- each chain has 12-14 residues
What is glycogenesis?
The addition of glucose molecules to glycogen
- occurs when blood glucose levels are high
- conversion of glucose to UDP-glucose prior to glycogen formation
