Molecular Interactions in Cell Events

8/8/2019 Molecular Interactions in Cell Events

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Molecular Interactions in Cell

Events

Advanced Higher Biology


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Catalysis

Speed up chemical reactions between a

million to a trillion times

Carbonic anhydrase is said to convert 36

million molecules per minute

Enzymes can only speed up a chemical

reaction that could take place anyway


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Case Study: Hydrolysis of Maltose

This reaction can happen

without an enzyme if:

-Maltose and water molecule

collide at the right speed to

provide sufficient energy

-water molecule has to be in

the correct orientation

-water molecule hits the

glycosidic bond at just the

right angle

All this isnt impossible but will

occur infrequently.

Enzymes makes sure this

happens more frequently!

O

HH

Hydrolysis of

maltose

Hydrolysis breaks the 1,4 glycosidic bond

Addition of water provides atoms lost in the

dehydration reaction


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Proteases

Proteases

hydrolyse peptide

bonds

Liberate amino

acids

Digestive system


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ATPases

Hydrolyse ATP into ADP and Pi.

Energy released is used to power a particularreaction

OR

Provide a phosphate to phosphorylate aparticular moelcule


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Nucleases

Nuclease enzymes also take part in a hydrolysis

reaction

Break phosphodiester bonds in RNA and DNA to

separate nucleotides form one another

Endonucleases (restriction enzymes) recogniseand cut at particular base sequences


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Kinases

Add phosphates to molecules

P

hosphate is negative so it will change theshape of the enzyme

Phosphorylation may activate or deactivate(depends on enzyme)


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Specificity of enzyme activity

related to induced fit


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Induced Fit Theory

A change in the conformation of an

enzyme in response to substrate binding

that renders the enzyme catalytically

active.


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1. Active site lined with many amino acid R groups

2. Charged groups in active site complement charged

groups on substrate3. Bind in correct position

4. Enzyme structure altered

5. Enzyme folds round substrate bringing catalytic Rgroups of enzyme closer to substrate reactive groups

6. Enzyme flexes putting substrate under stress andcomplex goes through several unstable intermediateforms (transitional state)

7. Bonds are made and broken and electrons are movedwith catalytic R groups acting as go-between

8. Enzyme-substrate complex formed

9. Product diffuses away and enzyme returns to originalshape


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Inhibition of enzymes

Decrease rate of reaction

1. Competitive

2. Non-competitive


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Competitive Inhibitors

Reaction

Rate

No inhibitor

Low

Vmax

High

Competitive inhibitor

Low HighSubstrate

Concentration

Low concentration of substrate relative to inhibitor = inhibitor will enter active

site first

As substrate concentration increases reaction rate increases as more chance

substrate will enter first

At very high substrate concentrations Vmax will be reached as inhibitor willhardly ever enter first


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Case Study: Succinate Dehydrogenase

Inhibition is

reversible as

malonate issmaller so can

come out of

active site.


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Case Study:

Sulphonamide

Binds permanently to bacterial enzyme

Enzyme is the start of the folic acidsynthesis pathway

Bacterium unable to make folic acid and

dies Humans not affected as do not synthesise

our own folic acid


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Non-competitive inhibition

No inhibitor

Low

VmaxHigh

Non-competitive

inhibitor

Low HighSubstrate

Concentration

Enzyme is non-functional when non-competitive inhibitor is present

Substrate cannot enter active site as the shape of the cleft has changed

Inhibitor lowers enzyme concentration so Vmax also lower

Reaction

Rate


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Summary of Inhibition


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Allosteric effects

Allosteric enzymes have at least 2 binding sites

Small molecules can bind far from the active site

an so affect active site shape-enzyme r-groups reshuffle and make newbonds

Shifting of amino acids result in smalladjustments throughout the rest of the enzymeincluding conformation of the active site


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Positive Modulators (activators)


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Negative Modulator (inhibitor)


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Summary of Allosteric Enzymes


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What to do now

Read p66 Covalent Modification to p67

Role of end-product inhibition in control of

metabolic pathways and make notes

You will be doing a practical next week

based on the information above

Molecular Interactions in Cell Events

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