Lecture 13 Allostery

Question 1

What are the different mechanisms of enzyme regulation?

Answer 1

• Allostery
• Multiple Isozymes (same reaction, different protein)
• Covalent modification
• Proteolytic activation
• Altering amount of enzyme (regulated synthesis and degradation)

Question 2

What are allosteric enzymes?

Answer 2

Allosteric enzyme have distinct regulatory sites and multiple active sites

• The binding of small molecules to regulatory sites controls enzyme activity
• These small regulatory molecules are often products of the pathway in which the enzyme functions
• Allosteric enzymes often show cooperativity, binding to one site affects others

Question 3

What is an example of an allosteric enzyme?

Answer 3

Aspartate Transcarbamoylase

Question 4

What is Aspartate Transcarbamoylase?

Answer 4

Sometimes called Aspartate Carbamoyl Transferease

• First step in build up for the synthesis of pyrimidine nucleotides (CTP, UTP and TTP)
• Catalyses the condensation of Carbomoyl phosphate and Aspartate to give N-Carbomoylaspartate (anabolic pathway)

Question 5

What type of kinetics does Aspartate Transcarbamoylase follow?

Answer 5

Shows sigmoidal kinetics

• ATCase does not show Michelis-Menten kinetics
• this is because of the way it is regulated through allosertic enzymes*

Question 6

What is Pyrimidine (and Purine) Biosynthesis essential for?

Answer 6

essential for the production of DNA

Question 7

What is tightly regulated in Pyrimidine (and Purine) Biosynthesis?

Answer 7

Pathways & the multiple enzymes in both pathways, must be very tightly regulated, otherwise DNA synthesis will be impaired

Question 8

What occurs during this feedback inhibition, how is pyrimidine biosynthesis allosterically regulated?

Answer 8

Allosterically regulated by end product of purine biosynthetic pathway, CTP

• Low CTP, high ATCase activity
• High CTP, low ATCase activity

–> Feedback Inhibition

Question 10

What is Haemoglobin?

Answer 10

• Main oxygen-carrying protein
• Found in red blood cells
• Binds Oxygen in lungs and releases it in tissues
• Also binds CO₂ in tissues for release in lungs

Question 12

What is the chemical structure of haemoglobin?

Answer 12

• Haemoglobin is tetrameric protein
  
  • 2 identical alpha subunits
  • 2 identical beta subunits
• Structure: α₂β₂ configuration
• Each globin subunit contains a Haem group which binds O₂ (4 O₂ in total)
• Haemcofactor: Tetrapyrrole ring molecule in association with an Fe²⁺ ion

Question 13

Label:

Answer 13

Question 14

What protein is this?

Answer 14

Myoglobin

Question 15

What is the most efficient oxygen carrier, Tetrameric Haemoglobin or monomeric protein myoglobin?

Answer 15

Tetrameric Haemoglobinis, due to cooperative binding to O₂ to Haemoglobin

Question 16

How was the molecular mechanism for O₂ binding to Haem was elucidated?

Answer 16

using Myoglobin, as the mechanism for Myoglobin are the same as it occurs in a single Haemoglobin subunit

Question 17

How does oxygen bind to haem?

Answer 17

• Fe²⁺ lies outside plane of tetrapyrrole ring
• On binding Fe²⁺ moves into plane of ring

Question 18

When oxygen binds to haem, what is used to stabilize it?

Answer 18

A distal Histidine residue helps to stabilize the bound O₂