Regulation of Metabolic Pathways Flashcards
What is steady state and how can it be maintained?
In steady state a cell forms and consumes intermediates at equal rates. Flux (rate of metabolite flow) can be modulated by changes in enzymatic activity of amount of enzymes.
Name some factors that affect enzyme activity
Extracellular signals (transcrition changes), binding of regulatory proteins, protein degradation, allosteric regulation, (de)phosphorylation.
How are adenine nucleotides (ATP/ADP) important for metabolic regulation?
ATP is a compound important for a vast amount of reactions. A steady concentration of ATP is important. The [ATP]/[AMP] is an important factor of metabolic regulation. AMP is a much more sensitive indication of energy in a cell, because the relative increase in [AMP] is much bigger than the corresponding decrease in [ATP].
What are the different hexokinases and their differences?
Hexokinases are responisble for glucose entry to the glycolytic pathway by catalyzing glucose to Glu6P. There are four isozymes: I, II, III, IV. I, II and III are allosterically inhibited by Glu6P.
I and II are the hexokinases in muscle, and hava a high affinity for glucose (Half saturated at low concentration, normally acts at or neat Vmax).
Hexokinase IV is the isozyme of livercells. The glucokinase has a higher Km than normal blood concentration (more activity at higher concentrations than other isozymes).
Glucokinase is not inhibited by its product Glu6P.
Glucokinase is reversibly inhibited by a regulatory protein, and the binding is regulated by the competing compunds glucose and Glu6P.
Shortly explain the regulation of hexokinase IV
Hexokinase IV (glucokinase) is regulated by the concentrations of glucose and Fru6P (substrate and product). Fru6P facilitates the binding of a regulatory protein, and glucose facilitates the dissociation of the protein. High glucose concentration activates glucokines by relieving it of the inhibition. Low glucose concentration leads to the facilitation of inhibition by Fru6P, the regulatory protein anchors the enzyme in the nucleus.
Signals for need of greater energy production leads to increased transcription of the glucokinase gene (produces more enzymes).
Explain the functions of Phosphofructokinase-1 (PFK-1) and Fructose 1,6-phosphatase (FBPase)
PFK-1 catalyzes the irreversible reaction commiting glucose to glycolysis (the reaction fructose 6-phosphate to fructose 1,6-biphosphate).
FBPase catalyzes the conversion of fructose 1,6-biphosphate to fructose 6-phosphate in gluconeogenesis.
Explain the reciprocally regulation of PFK-1 and FBPase
PFK-1 is allosterically inhibited by ATP, which signals that there isn’t a need for more energy (glycolysis), ADP and AMP work allosterically to relieve the inhibition. PFK-1 is also allosterically inhibited by citrate, which increases the inhibitory effect of ATP.
FBPase is allosterically inhibited by AMP, decreasing the production of glucose when energy is low.
AMP inhibits formation of glucose from fructose 1,6-biphosphate, simultaneously relieving inhibition of PFK-1, increasing flow through glycolysis (energy production).
Fructose 2,6-phosphate allosterically regulates PFK-1 and FBPase.
It increases the affinity of PFK-1 for its substrate and reduces the affinity for its inhibitors (increasing flow throug glycolysis).
It reduces the affintiy of FBPase for its substrate, slowing gluconeogenesis.
Explain the regulation of Fructose 2,6-phosphate levels
Phosphofructokinase 2 (PFK-2) catalyzes the formation of fructose 2,6-biphosphate.
Fructose 2,6-biphosphatase 2 (FBPase-2) catalyzes the breakdown of fructose biphosphate.
Glucagon indirectly results in phosphorylation of the bifuntional protein PFK-2/FBPase-2 by cAMP-dependent protein kinase, enhancing FBPase activity and inhibiting PFK-2 activity, resulting in decrease amount of Fru 1,6-P. This inhibits glycolysis and stimuates guluconeogenesis.
Insulin has the opposite effect. Resulting in dephosphorylation of the bifunctional protein, lowering activity of FBPase and increasing PFK-2 activity, increasing concentration of Fru 1,6P and flux through glycolysis, and inhibiting gluconeogenesis.
Explain the regulation of pyruvate kinase
Pyruvate kinase catalyzes the reaction from phophoenolpyruvate (PEP) to pyruvate.
The enzyme is allosterically inhibited by compunds that signal abundant energy, such as ATP, acetyl-CoA, and long-chain fatty acids. This decreases energy consumption (no pyruvate = no citric acid cycle).
