Enzyme Regulation

Question 1

What are the 3 levels of regulation of enzyme activity within a cell?

Answer 1

Allosteric regulation
Covalent modification
Regulation of concentration of enzymes

Question 2

What is the difference between an allosteric inhibitor and an allosteric activator of an enzyme?

Answer 2

They are small molecules that can bind to enzymes, activators stimulate enzyme activity whilst inhibitors prevent activity

Question 3

What is feedback inhibition and what type of molecules undergo this?

Answer 3

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

Question 4

What is feedforward activation and what type of molecules under go this?

Answer 4

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

Question 5

What is the rate-limiting enzyme?

Answer 5

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

Question 6

Give an example of feedback inhibition

Answer 6

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

Question 7

What is the benefit of feedback inhibition?

Answer 7

Prevents excess products being made and wasted

Question 8

Describe the mechanism of allosteric effector binding

Answer 8

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

Question 9

If allosteric effectors only alter the affinity of an enzyme, what effect does it have on the Km and Vmax? (for inhibitor and activator)

Answer 9

Inhibitor increases Km
Activator decreases Km
Vmax remains the same

Question 10

What does it mean for regulatory enzymes to be multimeric?

Answer 10

They have multiple copies of a single polypeptide chain/ subunit each with their own active site (multiple active sites per enzyme)

Question 11

What is cooperativity?

Answer 11

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

Question 12

How does an allosteric activator affect the enzyme activity - [substrate] graph?

Answer 12

Reduces the [S] for a specific activity
Reduces Km
Reduces [S] required for Vmax

Question 13

How does an allosteric inhibitor affect the enzyme activity - [substrate] graph?

Answer 13

Increased Km

Increases [S] required to reach Vmax

Question 14

Why is the shape of the enzyme activity - [substrate] graph altered for regulatory enzymes?

Answer 14

Regulatory enzymes experience cooperativity

Question 15

What is the rate limiting enzyme of glycolysis?

Answer 15

PFK-1

Question 16

Describe how ATP is a feedback inhibitor for glycolysis

Answer 16

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

Question 17

What is the allosteric activator of glycolysis?

Answer 17

AMP, it allosterically binds to PFK-1 and opens up its active site making it easier for F-^-P to bind

Question 18

How are inactive precursor enzymes activated?

Answer 18

Through proteolytic cleavage, e.g chymotrypsinogen is a precursor for protease and is activated by proteolytic cleavage in the gut

Question 19

Describe reversible protein phosphorylation

Answer 19

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

Question 20

Which enzymes are normally inhibited and which are normally activated by protein phosphorylation?

Answer 20

Enzymes involved in synthesis are inhibited and enzymes involved in degradation are activated

Question 21

How does covalent modification affect Vmax and Km (for activation and inhibition)?

Answer 21

Activation = lowers Km and increases Vmax
Inhibition = increases Km and decreases Vmax

Question 22

What enzyme reverses protein phosphorylation?

Answer 22

Protein phosphatase

Question 23

Give an example of covalent modification activation of an enzyme

Answer 23

Glycogen phosphorylase, it is activated by phosphorylation of a specific kinase in response to hormonal stimulation

Question 24

Give an example of an enzyme that is inhibited by covalent modification

Answer 24

Acetyl-CoA Carboxylase in liver cells when stopping fatty acid synthesis

Question 25

What are the 4 ways that enzyme concentration can be controlled in mammals?

Answer 25

Control of gene transcription (steroid hormones)
Control of stability of mRNA
Control of protein synthesis
Control of protein degradation

Question 26

Give an example of when the concentration of an enzyme effects the activity

Answer 26

Cholesterol synthesis rate is dependent on the concentration of HMG-CoA and is evident as when this concentration is high, lots of cholesterol is synthesised

Question 27

What is the TATA box?

Answer 27

A DNA sequence that indicates where a genetic sequence can be read and decoded, it is found on promoters, the binding site for the tata binding protein

Question 28

What allows RNA polymerase II to bind to DNA in order for transcription to occur?

Answer 28

General transcription factors (GTFs) bind to the DNA which can then interact with RNA polymerase II to allow transcription to occur

Question 29

Describe the process of GTFs allowing the binding of RNA polymerase II to DNA

Answer 29

TFIID binds to the TATA box on the promoter using its TATA-binding protein. TFIID becomes activated. TFIIA then binds to help stabilise TFIID. TFIIB and TFIIH then join the complex, this is the rate controlling step. RNA polymerase then binds to TFIIB. The tail of RNA polymerase II interacts with TFIID. TFIIE and TFIIF binds to RNA polymerase II, F unwinds the DNA in front of the RNA and E hydrolyses ATP to provide energy. TFIIH is then activated which phosphorylates the tail end of RNA so that it is released from TFIID and transcription can occur

Question 30

What are the 2 roles of TFIID?

Answer 30

To act as the foundation for RNA polymerase II

To remove histone proteins in the promoter region

Question 31

What are TAFs?

Answer 31

Other subunits on TFIID, TATA-binding protein-associated factors
When response elements bind to it, binding of TFIID to the TATA box can be enhanced
They determine if TFIID remains bound to the promotor and are co-activators for the binding of the TATA box and the other TFIIs

Question 32

What is the role of specific transcription factors (response elements)?

Answer 32

They can bind upstream to the promotor to enhance the rate of transcription

Question 33

How do response elements enhance transcription?

Answer 33

They interact with TBP on TFIID using a TAF so that the DNA loops back on itself which enhances the transcription complex