Protein control: (leyland) regulation of protein function 2

Question 1

Haemoglobin exists in what 2 states? What does this mean with regard to Michaelis-Menten kinetics?

Answer 1

Haemoglobin exists in a high and a low affinity state
-> allosterically regulated enzyme (undergoes structural change via O2 binding)

This means it does not follow the hyberbolic shaped Michaelis-Menten kinetics curve, and instead exhibits a sigmoidal shaped reaction curve

Question 2

Role of allosteric activators and inhibitors?

Answer 2

Activators: increase proportion of enzyme R state (high affinity, high activity “relaxed”)

Inhibitors: increase proportion of enzyme T state (low affinity, low activity “tense”)

Question 3

Why is phosphorylation an effective means of regulation? 4 reasons

Answer 3

1) Large free energy of phosphorylation
- provides energy to allow shift in conformation

2) Addition of 2 negative charges
- disrupts existing electrostatic charges, may allow new ones to form

3) Phosphoryl group can form 3 or more hydrogen bonds
- allows specific interactions with new hydrogen bond donors

4) Kinetics of phosphorylation can be adjusted
- may be from a few seconds to several hours

Question 4

Purpose of a ‘kinase cascade’?

Answer 4

Signal amplification -> quick mass phosphorylation of many target molecules

Question 5

What is a pseudosubstate region/sequence?

Answer 5

Any substance that mimics the substrate of an enzyme and thus inhibits its activity.

Question 6

What is a consensus sequence? Example?

Answer 6

Calculated order of most frequent residues, either nucleotide or amino acid, found at each position in a sequence alignment

e.g. PKA; P-R-X-S-I

Question 7

Comparison of allosteric modulation vs covalent modification

Answer 7

Covalent involves enzymes in binding of effector, allosteric does not

Covalent = strong molecular interactions, allosteric = weak (non-covalent) interactions

Both reversible

Allosteric fast (msec-sec)
Covalent slow (sec-hours)

Question 8

Properties of regulation via partial proteolysis?

Answer 8

Highly specific process - breaking a peptide bond in the zymogen catalysed by an enzyme (Part of polypeptide sequence is cleaved off -> protein becomes active)

Irreversible

Cleavage does not need ATP

Can be used to activate intracellular and extracellular proteins

Question 9

Examples of processes controlled by partial proteolysis? Common factor?

Answer 9

Digestive enzymes

blood clotting

Hormone secretion

Complement activation

Programmed cell death

Common factor: high level of controlled needed

Question 10

Zymogen suffix?

Answer 10

“ogen” e.g. Tripsinogen -> tripsin