Molecular Interactions in Cell events (i) Catalysis (ii) The Sodium-Potassium Pump (iii) Cell...
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Transcript of Molecular Interactions in Cell events (i) Catalysis (ii) The Sodium-Potassium Pump (iii) Cell...
Molecular Interactions in Cell events
(i) Catalysis
(ii) The Sodium-Potassium Pump
(iii) Cell Signalling
What caused this?
That’s clever
Bombardier Beetles Oxygen gas formed
during break down of H2O2 forces out water and other chemicals
Reaction releases a lot of heat so the water comes out as steam
A reminder of their importance Catalase breaks down 5 million molecules of
H2O2 per minute at 0oC, to protect cells.
It would take 300 years to break down the same number of molecules using iron as a catalyst
Chemical Reactions
Synthesis (anabolic) Condensation reactions
Removal of water to form a bond
Degradation (catabolic) Hydrolysis reactions
Addition of water to break a bond
Enzymes
Proteases Hydrolyse peptide bonds break down proteins into amino acids
Nucleases Hydrolyse phosphodiester bonds Break down nucleic acids into nucleotides
ATPases Hydrolyse ATP Break ATP into ADP and Pi with the release of
energy
Enzymes continued….
Kinases Catalyse the transfer of a phosphate group onto a
molecule such as a carbohydrate or a protein
Specificity of enzymes
Compare the two diagrams
Think about your knowledge of proteins tertiary structure? Why do you think the induced fit model is favoured?
Specificity of enzymes
Induced-fit model When substrate combines with the enzyme it
causes a change in shape of the active site The change in shape results in an optimal fit for
the substrate-enzyme interaction Once the product diffuses away, the enzyme
returns to its original shape
What analogy could be applied to this model?
What do the following have in common? Arsenic Nerve Gases
TabunSarin
Mercury
Cyanide DDT Lead Cadmium
Cyanide
Cyanide is found is a gas (sometimes liquid) Used / found in
House fires Apricot stones Suicide pills Gas chambers (both US and Nazi Germany) Stock piled by US and Soviet Union in 50’s and 60’s Mining Photography Electroplating
Cyanide
Binds to iron atom in the enzyme cytochrome C oxidase
This changes the shape of the enzyme Knowing how this works has important
applications for Detection of poisoning Treatment
Control of enzyme activity - Inhibitors Competitive Inhibitors
Decrease the rate of reaction Inhibitor is similar in structure and electrical
charge to substrate It binds to the active site An increase in the substrate can result in an
increase of product formation (inhibitor is out competed)
Competitive inhibition can be reversible or irreversible (depending on mechanism of binding)
Inhibitors cont..
Competitive inhibitors cont…
Inhibitors cont…
Non-competitive inhibitors Decrease the rate of reaction Inhibitors have no similarity to the substrate Inhibitor binds to part of the enzyme (other than
the active site) distorting the shape of the enzyme Increase in substrate concentration does not
increase product formation Can also be reversible
Inhibitors cont…
Non-competitive inhibition cont….
Control of enzymes – Enzyme modulators Allosteric enzymes
Allosteric enzymes have at least one other binding site than the active site (called an allosteric site)
Allosteric enzymes have 2 forms – active and inactive
When a substance binds to an allosteric site it changes the shape of the active site.
Positive modulation The modulator changes the active site so the enzyme
becomes active (substrate fits) Positive modulators are activators
Enzyme modulators cont…
Allosteric enzymes cont… Negative modulation
The modulator changes the active site so the enzyme becomes inactive
Negative modulators are inhibitors
Enzyme modulators cont….
Allosteric enzymes cont…
Control of enzymes – Covalent modifications
Addition, modification or removal of a variety of chemical groups
Changes the shape of the enzyme Phosphorylation and dephosphorylation
Kinase enzymes add phosphate Phosphatase enzymes remove phosphate Some enzymes are activated by phosphorylation,
others are inactivated (and vice versa for dephosphorylation)
Covalent modifications cont… Proteolytic cleavage
Conversion of an inactive enzyme to an active one Example Trypsinogen – Trypsin Trypsinogen is synthesised in the Pancreas Activation occurs when trypsinogen has amino acids
removed in the duodenum by another protease enzyme This changes the trypsinogen into the active form trypsin Trypsin then helps to activate more trypsinogen molecules
Back to cyanide
What type of inhibition is demonstrated by cyanide in the inhibition of cytochrome oxidase?
Enzyme inhibition is often how drugs work – targeting enzymes specific to other organisms, not humans
Control of metabolic pathways End product inhibition
Chemical reactions are normally organised into metabolic pathways with enzymes controlling each chemical reaction
The end-product can act as a negative modulator, binding to the first enzyme preventing the metabolic pathway from proceeding because intermediary substrates are not produced
This is a process of negative feedback
Learning Activities
Read and take notes from DART pg 61-68 Scholar 6.3 and 6.4 Check out
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter8/animations.html#
Find examples for each type of enzyme control Use flash cards to remember the enzymes and their reactions Draw posters of each type of enzyme control Use the information on ‘end-product inhibition in respiration’ to
demonstrate the principle of negative feedback ‘Enzymes’ worksheet ‘Enzyme cofactors and inhibitors’ worksheets Advanced Higher Biology Questions