11. Regulation Of Protein Function Flashcards
What are isoenzymes?
Different forms of the same enzyme. They catalyse the same reaction but have different kinetic properties such as Km.
Hexokinase and glucokinase are examples of this.
What is product inhibition?
When the accumulation of the product of a reaction inhibits the forward reaction.
E.g Glucose-6-phosphate inhibits glucokinase activity.
What type of relationship between rate and substrate concentration do allosteric enzymes show?
Sigmoid relationship.
This is different to the rectangular hyperbola seen for simple enzymes.
What is meant by an allosteric enzyme?
Allosteric enzymes are enzymes that change their conformational ensemble upon binding of an effector, which results in an apparent change in binding affinity at a different ligand binding site.
What are the 2 different conformations that multi-subunit enzymes exist in?
T state- low affinity
R state - high affinity
What is the difference between allosteric activators and inhibitors?
They both bind to the enzyme at a site away from the active site and cause a conformational change
Allosteric activators increase the proportion of enzyme in the R state
Inhibitors increase the proportion of the enzyme in the T state
What are the key 2 enzymes involved phosphorylation and dephosphorylation?
Protein kinases- transfer the terminal phosphate from ATP to the -OH groups of Ser, Thr,Try.
Protein phosphotases - reverse the effects by catalysing the hydrolytic removal of phosphoryl groups from proteins.
Why is protein phosphorylation effective in altering enzyme activity?
It can completely change the enzyme conformation as it adds 2 negative charges and a phosphoryl group that can make H-bonds.
Glycogen breakdown and synthesis are reciprocally regulated. What does this mean?
It means that the signals that initiate the breakdown of glycogen will also inhibit glycogen synthesis.
How can enzymes be regulated in the long term?
- Change in the rate of protein synthesis - enzyme induction/repression
- Change in the rate of protein degradation - ubiquitin-proteasome pathway
What are allosteric activators and inhibitors for the phosphofructokinase enzyme in glycolysis?
Activators - AMP and fructose-2,6-bisphosphate
Inhibitors- ATP, Citrate, H+
What 3 amino acids do tyrosine kinases phosphorylate?
Serine, Threonine and Tyrosine
What enzyme reverses phosphorylation?
Protein phosphotases
How does the addition of a phosphate group alter the properties of an enzyme?
It adds 2 negative charges to the protein, which can cause conformational change.
Phosphoryl group can make additional hydrogen bonds
How are inactive zymogens activated?
Proteolytic cleavage
Give an example of enzymes which are synthesised as zymogens.
Digestive enzymes
Which digestive enzyme is the ‘master regulator’ of further activation of enzymes?
Trypsin
How does trypsin act as the master regulator of digestive enzymes?
It activates other zymogens, for example chymotrypsinogen to chymotyrpsin. Proelastase becomes elastase and prolipase becomes lipase.
How is trypsin activity controlled?
Pancreatic trypsin inhibitor binds to the enzyme tightly and stops its activity.
What is alpha1-antitrypsin?
A plasma protein that inhibits a range of proteases
How does a deficiency in alpha1-antitrypsin affect the lungs?
It is responsible for inhibiting elastase in the lungs, in a1at deficiency, elastase is not inhibited and it breaks down elastin, destroying alveoli lungs and can cause emphysema.