Mechanism Of Lysozyme Flashcards

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1
Q

What is a lysozyme?

A

It is a part of an organism defence against bacterial invasion

It is part of the innate immune system (the immune system you are born with)

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2
Q

Where can you find lysozyme?

A

This is an enzyme, isolated in tears and bodily fluids and in the whites of eggs

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3
Q

Why do you study lysozyme?

A

It was the first enzyme structure to have its 3-D structure determined
Highlights the need to use many different experiments to elucidate mechanism
Brings together many concept- structure, substrate binding, use of isotopes, general acid and base hydrolysis, mechanistic ideas, TS stabilisation and strain

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4
Q

What is glycoside (acetal) hydrolysis?

A

What is added and OR is the leaving group
An acetal forms a hemiacetal
It is catalysed by glycosyl hydrolyse which chops up sugars

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5
Q

What is an important role of glycosyl hydrolase? (Lysozyme)

A

It chops up sugars
Importantly it breaks down peptidoglycan in the cell wall of bacteria
The breaks down the structural stability
Breaks glycosidic bond

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6
Q

What provides a bacteria cell wall with protection?

A

Polymers of sugars are cross linked by peptides which provides protection for bacteria cell wall

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7
Q

What is the use of 18OH2?

A

Isotopic labelling experiment

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8
Q

What is the isotopic labelling experiment?

A

Treat peptide chain with 18OH2 in the presence of lysozyme
Water acts as a nucleophile as it is in the product
Retention of configuration
There is no inversion
(See mechanism)

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9
Q

How can you tell the difference between beta and alpha using NMR?

A

The couple contain between H1 and H2 has an angular dependence
The trans diaxial angle (180) gives a large j1,2 8Hz- this is beta
The angle (90) gives a small j1,2 2Hz- this is alpha

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10
Q

What does the pH profile look like.

A

Classic bell shaped
Maximum activity at pH 5
Most active at pH5
If you j crease or decrease pH from 5 the rate drops
It goes there must be 2 residues involved in catalysis with pka at 4 and 6
The deprotonated residues are not as good in catalysis

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11
Q

How many sugars can lysozyme bind to?

A

Lysozyme has 6 subsides in its enzyme activate site

These all contribute to catalysis- it wants to bind to 6 sugars which gives the max rate

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12
Q

Outline information relating to mechanism of lysozyme

A

Mechanism involved retention of anomeric configuration
6 sub sites contribute to catalysis
Cleavage between the D and E sugars
PH profile shows dependence of two groups with pkas of 4 and 6

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13
Q

Why is the mechanism of acetal hydrolysis not inversion?

A

SN2 mechanism proved wrong
Product has has net inversion of stereochemistry (back face attack) which is not the product
See mechanism

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14
Q

What is the shielded SN1 mechanism of lysozyme ?

A

Shielded SN1 was proved wrong
Mechanism results in net retention of configuration, not a single SN2 type displacement
See mechanism

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15
Q

What is the correct mechanism of lysozyme?

A

Double displacement
Two subsequential SN2 reactions
Product with net retention of stereochemistry
Use the side chain to accelerate reactions
Covalent intermediate
See mechanism

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16
Q

What are the chemical challenges of the lysozyme mechanism?

A

Unstable glycosyl cation- OR is not a good leaving group

Unlikely alkoxide ion

17
Q

What are the chemical challenges of the lysozyme mechanism overcome?

A

To stabilise the alkoxide ion, use an acid catalyst to make it neutral and create and better leaving group
Nature uses COOH (general acid catalysis)

18
Q

How does pH affect the acid catalyst for mechanism?

A

When the acid catalyst is deproonated, the rate is slower

When pH gets above the pka of the residue, it deprotonated the acid

19
Q

What are the two residues involved in the catalysis?

A

Asp52 sits below
Glu35 sits above (pk6)- this is the catalytic acid as it is protonated
Identified by crystallography
Below pk4, asp52 is protonated

20
Q

What provides the fastest rate (structure of the residues)

A

For fastest rate
- glu35 is protonated and asp52 is deprotonated which acts as the catalytic nucleophile
Asp52 pka is 4 (becomes deprotonated
Glu35 pka6 (becomes deprotonated)
Therefore fastest rate between 4 and 6 pH

21
Q

How does asp52 act as a nucleophile?

A

Position below ring to attack- close
Held in correct orientation for attack by enzyme
Proximity and orientation increase rate of attack

22
Q

How does the enzyme change the starting material to increase the rate?

A

Also enzyme strains the starting material to increase reaction rate- release of strain drives the reaction forwards

23
Q

Outline the overall mechanism of generic double displacement

A

COO- residues nucleophile (asp) attacks from below and acid COOH (glu) attacks from above, SN2- HOR leaving group
Covalent intermediate with inversion of configuration - SN2 (base attack)
Overall retention of configuration
See mechanism

24
Q

Describe the reaction coordinate of enzymes

A
Enzyme substrate complex
Enzyme destabilised the starting material 1st SN2
Enzyme intermediate 
2nd SN2
Enzyme product complex is formed
25
Q

Describe the TS of the generic double displacement intermediate

A
Extra O in the ring
Add partially positive charge to the carbon
1) sp2 hybridisation
2) positive charge
See diagram
26
Q

What should inhibitors mimic?

A

They should mimic the transition state as enzymes bind to these the best

27
Q

Outline lysozyme catalystis in a nutshell- summary

A

Acid base catalysis- glu35 as an acid then base
Covalent catalysis- asp52 as nucleophile
Stress and strain- substrate distortion- the enzyme distorts the substrate towards the TS conformation
Inhibition by transition state mimics