Unit 1Cell and Molecular

Biology

Section 6

Catalysis

Chemical Reactions (Revision)

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

Kinases Catalyse the transfer of a phosphate group onto a molecule

such as a carbohydrate or a protein

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

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

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

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)

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

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

Negative modulation The modulator changes the active site so the enzyme

becomes inactive Negative modulators are inhibitors

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)

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

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

Activity

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