Lecture 4. Biological Regulation of Enzyme Activity Flashcards
What are enzymes regulated by?
Relevant metabolites
Regulatory proteins
Reversible covalent modification (e.g phosphorylation)
Controlled proteolysis
Which enzyme is affected by ‘product feedback inhibition’?
Usually the first unique enzyme in a biosynthetic pathway
What is an example of regulation by relevant metabolites?
Regulation by ATP/ADP/AMP of enzymes of central metabolic pathways
How many enzymes are involved with catalysing the conversion of threonine?
5
What is threonine converted into?
Isoleucine
What is E1?
Threonine deaminase
What specifically inhibits E1and how?
Isoleucine (the end product). Binds at a distinct regulatory site; hence ‘allosteric’ regulation
Proteins that act as inhibitors to other proteins work in what ratio?
1:1
What is antitrypsin?
A serpin - serine protease inhibitor
What does antitrypsin inhibit?
Elastase in the lungs, allowing the lungs to stay elastic
How do some regulatory proteins mediate control of enzymes?
By interpreting other signals
What is calmodulin and what does it do?
Calmodulin is a calcium binding protein which when activated, binds to many enzymes and other proteins, modifying their activities
What is an example of regulation by reversible covalent modification?
Phosphorylation
In phosphorylation, where does the phosphate group attach to in the enzyme?
-OH groups
How can a phosphate be removed from an enzyme?
Hydrolysis
What are Ras proteins?
Ubiquitous small GTPases which split GTP
What are RAFs?
Serine/Threonine kinases
What is mutated in about 20-25% of human cancers?
RAS
What is mutated in 80% malignant melanomas?
BRAF - most of the mutations are in the kinase domain and lead to elevated or constitutive activity
What does ubiquination do?
Control of cell cycle and involved in plant hormone signalling, development and gene expression
How are inactive enzymes activated?
A specific signal leads to their activation by cleavage of
specific peptide bond(s)
What are examples of enzymes made in an inactive form?
Mammalian digestive enzymes
Are enzymes rigid structures?
No
What can some enzymes convert between?
Alternative conformations
What can alternative conformations be exploited for?
Enhancing specificity (‘induced fit’) and regulation (co-operativity and allosteric phenomena)
What is the classic example of allosteric regulation?
Aspartate transcarbamoylase (ATCase)
What does ATCase catalyse?
The conversion of aspartic acid and carbamoyl-phosphate to carbamoyl-aspartate
What is the product of the first unique step in synthesis of pyrimidines?
Cytidine trisphosphate (CTP)
What allosterically inhibits ATCase?
CTP
What is PALA used for and how does it work?
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
What is the enzyme’s T state?
Tense
What is the enzyme’s R state?
Relaxed - favoured by substrate binding
Which state of ATCase is favoured by CTP binding?
T state (less active)
Which state of ATCase is favoured by substrate binding?
R state (more active)
When bound to S or an allosteric activator (e.g ATP), which way does equilibrium shift in ATCase?
Towards the R state
When bound to an inhibitor (e.g CTP), which way does equilibrium shift in ATCase?
Towards the T state
