Lecture 11 Flashcards
What are some methods of enzyme regulation?
Methods of enzyme regulation are allosteric effects, covalent modification and proteolytic cleavage.
How many enzymes need to be regulated to control a metabolic pathway?
in metabolic pathways one or two key regulatory enzymes are regulated, these are gatekeepers and they determine the overall rate of the metabolic pathway.
What are the key properties of allosteric enzymes? How can they have their function regulated?
allosteric enzymes properties: typically display cooperativity as seen in a sigmoidal plot of substrate concentration to velocity. They are a multiunit enzyme with subunits which interact. For these the activity can be regulated by binding of molecules (allosteric regulators) to allosteric sites on the enzyme which can inhibit or activate the enzyme, these regulatory effects are achieved by conformational changes in the protein when the effector molecules bind.
These are the enzymes which are typically found as regulatory enzymes.
What is the difference between positive and negative regulators?
Positive regulators stimulate enzyme activity, negative regulators (inhibitors) slow or stop the enzyme activity.
How do allosteric enzymes detect conditions in cells?
They have many different allosteric sites for allosteric regulators, these allow it to respond (e.g via feedback) to specific factors occuring in the cell. Different concentrations of the regulators determine the likelihood of binding and are reversible (can come on and off).
What is feedback inhibition? Give an example.
Feedback inhibition is when a molecule somewhere within a metabolic pathway (typically towards the end) signals to an enzyme earlier in the pathway that there has been enough produced. An example is CTP production (DNA base) inhibiting the aspartate to carbonyl aspartate enzyme.
What changes to the graph of reaction speed for an enzyme vs substrate concentration when an activator or inhibitor is present?
The shape of a graph of enzyme activity when affected by an inhibitor or activator changes to being less sigmoidal and more like the michaelis menton plot.
What are the main properties of covalent modification of enzymes?
Covalent modification of enzymes is: mostly reversible, the most common type is phosphorylation/dephosphorylation and involves sidechains of Ser, Thr or Tyr (R-OH group), the enzymes involved are phosphatases (remove phosphate) or kinases (add phosphate). Regulation is caused by the conformational change. Sometimes the phosphorylation down regulates, sometimes it up regulates, depending on the enzyme.
This is a very common way to regulate enzyme function and the proteins which do the adding or removal can be very specific or very broad with the enzymes they act on.
How can regulation of two linked pathways be done due to phosphorylation?
One pathways may be turned on by phosphorylation while the other is turned off.
Are enzymes limited to only one type of control? Give an example.
Enzymes aren’t limited to one type of controle (e.g glycogen phosphorylase is regulated by both phosphorylation and allosteric effects).
What is a zymogen? What is this good for? Give an example.
Some enzymes are synthesized as zymogens, this means they are inactive precursor molecules which can become activated by cleavage of one or more peptide bonds (changing the conformation). These are good for when the protein is wanted to be available to act when required but you don’t want it to be acting all the time (e.g fibrinogen becoming activated to fibrin by specific protease enzymes).
Another example is many pancreatic digestive enzymes.
What are isoenzymes? How does their concentration vary? Give a good example.
Isoenzymes/ isozymes are one of multiple forms in which a multi subunit enzyme may exist within an organism, they differ in their physical, kinetic and immunological properties due too their different subunits (produced by different genes). These different isozymes will be present in different concentrations within different locations (this is due to different gene activation). They still do the same reactions but do so in slightly different ways. A good example is lactate dehydrogenase, in which the different subunits will want to make the reaction go in different ways.
