Lecture 4. Biological Regulation of Enzyme Activity

Question 1

What are enzymes regulated by?

Answer 1

Relevant metabolites
Regulatory proteins
Reversible covalent modification (e.g phosphorylation)
Controlled proteolysis

Question 2

Which enzyme is affected by ‘product feedback inhibition’?

Answer 2

Usually the first unique enzyme in a biosynthetic pathway

Question 3

What is an example of regulation by relevant metabolites?

Answer 3

Regulation by ATP/ADP/AMP of enzymes of central metabolic pathways

Question 4

How many enzymes are involved with catalysing the conversion of threonine?

Answer 4

5

Question 5

What is threonine converted into?

Answer 5

Isoleucine

Question 6

What is E1?

Answer 6

Threonine deaminase

Question 7

What specifically inhibits E1and how?

Answer 7

Isoleucine (the end product). Binds at a distinct regulatory site; hence ‘allosteric’ regulation

Question 8

Proteins that act as inhibitors to other proteins work in what ratio?

Answer 8

1:1

Question 9

What is antitrypsin?

Answer 9

A serpin - serine protease inhibitor

Question 10

What does antitrypsin inhibit?

Answer 10

Elastase in the lungs, allowing the lungs to stay elastic

Question 11

How do some regulatory proteins mediate control of enzymes?

Answer 11

By interpreting other signals

Question 12

What is calmodulin and what does it do?

Answer 12

Calmodulin is a calcium binding protein which when activated, binds to many enzymes and other proteins, modifying their activities

Question 13

What is an example of regulation by reversible covalent modification?

Answer 13

Phosphorylation

Question 14

In phosphorylation, where does the phosphate group attach to in the enzyme?

Answer 14

-OH groups

Question 15

How can a phosphate be removed from an enzyme?

Answer 15

Hydrolysis

Question 16

What are Ras proteins?

Answer 16

Ubiquitous small GTPases which split GTP

Question 17

What are RAFs?

Answer 17

Serine/Threonine kinases

Question 18

What is mutated in about 20-25% of human cancers?

Answer 18

RAS

Question 19

What is mutated in 80% malignant melanomas?

Answer 19

BRAF - most of the mutations are in the kinase domain and lead to elevated or constitutive activity

Question 20

What does ubiquination do?

Answer 20

Control of cell cycle and involved in plant hormone signalling, development and gene expression

Question 21

How are inactive enzymes activated?

Answer 21

A specific signal leads to their activation by cleavage of
specific peptide bond(s)

Question 22

What are examples of enzymes made in an inactive form?

Answer 22

Mammalian digestive enzymes

Question 23

Are enzymes rigid structures?

Answer 23

No

Question 24

What can some enzymes convert between?

Answer 24

Alternative conformations

Question 25

What can alternative conformations be exploited for?

Answer 25

Enhancing specificity ('induced fit') and regulation (co-operativity and allosteric phenomena)

Question 26

What is the classic example of allosteric regulation?

Answer 26

Aspartate transcarbamoylase (ATCase)

Question 27

What does ATCase catalyse?

Answer 27

The conversion of aspartic acid and carbamoyl-phosphate to carbamoyl-aspartate

Question 28

What is the product of the first unique step in synthesis of pyrimidines?

Answer 28

Cytidine trisphosphate (CTP)

Question 29

What allosterically inhibits ATCase?

Answer 29

CTP

Question 30

What is PALA used for and how does it work?

Answer 30

PALA binds at the active site and structurally mimics the ‘transition state’ on the reaction pathway, acting as a competitive inhibitor. It allows us to capture the protein structure in flagrante and shows that there is a conversion to an alternative subunit arrangement

Question 31

What is the enzyme's T state?

Answer 31

Tense

Question 32

What is the enzyme's R state?

Answer 32

Relaxed - favoured by substrate binding

Question 33

Which state of ATCase is favoured by CTP binding?

Answer 33

T state (less active)

Question 34

Which state of ATCase is favoured by substrate binding?

Answer 34

R state (more active)

Question 35

When bound to S or an allosteric activator (e.g ATP), which way does equilibrium shift in ATCase?

Answer 35

Towards the R state

Question 36

When bound to an inhibitor (e.g CTP), which way does equilibrium shift in ATCase?

Answer 36

Towards the T state