Control of Glycolysis Flashcards
Which 3 enzymes in glycolysis operate far from equilibrium?
1) Hexokinase
2) Phosphofructokinase
3) Pyruvate kinase
Which enzymes in metabolic pathways are potential sites of control in the pathways?
Those that catalyze essentially irreversible reactions.
Which enzymes in glycolysis catalyze irreversible reactions and are, therefore, potential sites of control in the glycolysis pathway?
1) Hexokinase
2) Phosphofructokinase
3) Pyruvate kinase
Under what conditions does glycolysis operate continuously in most tissues?
Steady state conditions
What allows control of irreversible reactions (and, therefore, control of the pathway) in metabolic pathways?
1) Reversible binding of allosteric effectors
2) Covalent modification
Which enzymes in glycolysis are regulated simply based on relative concentrations of substrate and product?
All that are reversible (7 of the 10)
In what ways is phosphofructokinase like hemoglobin?
It is a tetramer with R and T conformations
Which enzyme is the most important regulatory enzyme in glycolysis?
Phosphofructokinase
Is phosphofructokinase a more important enzyme in liver or in muscle?
In muscle
Which molecule inhibits phosphofructokinase? What kind of inhibitor is it?
ATP; allosteric
What molecules reverse the inhibitory effect of ATP on phosphofructokinase and activate the enzyme?
AMP, ADP, and fructose 2, 6 phosphate (book says only AMP—???)
True or false: ATP functions as both a substrate and an inhibitor for phosphofructokinase?
True
How does a decrease in pH affect the activity of phosphofructokinase? Why?
It inhibits it. The pH would drop when lactic acid builds up as the muscle functions anaerobically; the inhibition of ATP protects the muscle from damage due to continuing anaerobic activity.
Which site or sites on phosphofructokinase can bind ATP in either the R or T state?
Only the substrate site
Which site or sites on phosphofructokinase can bind ATP only in the T state?
The inhibitor site
Which state must phosphofructokinase be in to bind fructose-6-phosphate?
The R state
How does the binding of ATP during time of high concentration of ATP affect the conformation of phosphofructokinase?
ATP binds to the inhibitor site, shifting the RT equilibrium to the T state.
How does an increase in concentration of ATP affect the curve of the graph of PFK activity vs. concentration of F6P?
It makes the curve more sigmoidal, shifting it to the right (KM is increased).
How does an increase in concentration of AMP affect the curve of the graph of PFK activity vs. concentration of F6P?
It shifts the curve to the left, making it less sigmoidal (KM is decreased)
When is PFK near max activity?
When there is a low concentration of ATP or no inhibitors
Why does an increased concentration of AMP shift the binding curve of PFK to the left?
Binds to the R state PFK, shifting the TR equilibrium toward R.
True or false: Direct allosteric control of PFK by ATP appears to be all that’s needed to control glycolysis flux. Why?
False; because flux through glycolysis varies by 100x or more, while concentration of ATP only varies by +/- 10%.
What causes the concentration of ATP to not vary more than +/- 10%?
The buffering activity of two enzymes: 1) creatine kinase and 2) adenylate kinase.
How does creatine kinase work as a buffer?
Catalyzes the equilibrium reaction between ATP + creatine creatine-P + ADP
How does adenylate kinase work as a buffer for ATP concentration?
Catalyzes the equilibrium reaction of 2 ADP ATP + AMP
Where does most of the ADP in muscle come from?
ATP hydrolysis during muscle contractions
What is the relative concentration of ATP to ADP and AMP in muscle?
[ATP]= 50x[AMP] and 10x[ADP]
What does a 10% decrease in ATP affect AMP and ADP due to the affects of adenylate cyclase?
100% increase in ADP and 400% increase in AMP
How can a forward and reverse reaction both be favorable?
They must each be catalyzed by a different enzyme.
True or false: it is possible for both a forward and reverse reaction to be catalyzed by a single enzyme and both be favorable simultaneously.
False
What enzyme catalyzes the reverse reaction of the glycolysis step catalyzed by phosphofructokinase?
Fructose-1,6-bisphosphatase
What price does muscle pay for being able to switch from resting to maximum activity quickly?
Substrate cycling (which uses 1 molecule of ATP every time the cycle is used)
What are two ways that muscle produces body heat?
1) Thermogenesis through substrate cycling
2) Shivering
How is non-shivering thermogenesis produced?
By substrate cycling in liver and muscle
What stimulates non-shivering thermogenesis?
Thyroid hormone
Why do chronically obese people tend to have lower metabolic rates?
They have lower rates of non-shivering thermogenesis (from substrate cycling) due to lower thyroid production.
