Enzyme Regulation Flashcards
What are the 3 levels of regulation of enzyme activity within a cell?
Allosteric regulation
Covalent modification
Regulation of concentration of enzymes
What is the difference between an allosteric inhibitor and an allosteric activator of an enzyme?
They are small molecules that can bind to enzymes, activators stimulate enzyme activity whilst inhibitors prevent activity
What is feedback inhibition and what type of molecules undergo this?
Allosteric inhibitors, it is when they bind to an enzyme and the product of this pathway feeds back into an earlier step in the pathway and inhibits this activity
What is feedforward activation and what type of molecules under go this?
Allosteric activators, it is when a substrate is able to activate an enzyme further down in the pathway so that its substrates are used the the whole pathway is pulled through
What is the rate-limiting enzyme?
The enzyme in a pathway which has the lowest max enzyme activity and therefore is the reason for any limitations in enzyme activity rate
It is the regulatory enzyme of the pathway where allosteric feedback inhibitors and feedforward activators act
It is often the drug target
Give an example of feedback inhibition
When there is plenty of fatty acid, it feeds back into Acetyl-CoA carboxylase to reduce production, this leads to a build up of citrate which then feeds back and inhibits glycolysis
What is the benefit of feedback inhibition?
Prevents excess products being made and wasted
Describe the mechanism of allosteric effector binding
The allosteric effector non-covalently binds to the allosteric site on the enzyme and causes a conformational change which alters the active site, making it either easier or harder for the substrate to bind, when the effector dissociates from the enzyme, the shape of it returns to its normal state
If allosteric effectors only alter the affinity of an enzyme, what effect does it have on the Km and Vmax? (for inhibitor and activator)
Inhibitor increases Km
Activator decreases Km
Vmax remains the same
What does it mean for regulatory enzymes to be multimeric?
They have multiple copies of a single polypeptide chain/ subunit each with their own active site (multiple active sites per enzyme)
What is cooperativity?
When a substrate binds to one active site on an enzyme, causing a change in conformational shape in the other subunits making it easier for other substrates to bind
It allows an enzyme to respond to a small change in substrate concentration
How does an allosteric activator affect the enzyme activity - [substrate] graph?
Reduces the [S] for a specific activity
Reduces Km
Reduces [S] required for Vmax
How does an allosteric inhibitor affect the enzyme activity - [substrate] graph?
Increased Km
Increases [S] required to reach Vmax
Why is the shape of the enzyme activity - [substrate] graph altered for regulatory enzymes?
Regulatory enzymes experience cooperativity
What is the rate limiting enzyme of glycolysis?
PFK-1
Describe how ATP is a feedback inhibitor for glycolysis
When ATP is plentiful, it allosterically binds and inhibits PFK-1 which is the regulatory enzyme by reducing the binding of its substrate, F-6-P
What is the allosteric activator of glycolysis?
AMP, it allosterically binds to PFK-1 and opens up its active site making it easier for F-^-P to bind
How are inactive precursor enzymes activated?
Through proteolytic cleavage, e.g chymotrypsinogen is a precursor for protease and is activated by proteolytic cleavage in the gut
Describe reversible protein phosphorylation
3 amino acids are able to be phosphorylated and form a covalent phosphoester bond, the added negative group causes a change in protein shape = change in active site shape = change in catalytic activity
Which enzymes are normally inhibited and which are normally activated by protein phosphorylation?
Enzymes involved in synthesis are inhibited and enzymes involved in degradation are activated
How does covalent modification affect Vmax and Km (for activation and inhibition)?
Activation = lowers Km and increases Vmax Inhibition = increases Km and decreases Vmax
What enzyme reverses protein phosphorylation?
Protein phosphatase
Give an example of covalent modification activation of an enzyme
Glycogen phosphorylase, it is activated by phosphorylation of a specific kinase in response to hormonal stimulation
Give an example of an enzyme that is inhibited by covalent modification
Acetyl-CoA Carboxylase in liver cells when stopping fatty acid synthesis
