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Enzymes Notes
1. Enzymes
Definitions
- Product = molecules produced from substrate molecules, by an enzyme-catalysed reaction
- Substrate = molecule that is altered by an enzyme-catalysed reaction
Biological catalysts – basic concept
- Enzymes are biological catalysts because: 1. Inside living organisms, 2. they speed up metabolic reactions
and remain unchanged at the end of them, able to be used again
- Small amount of catalyst can catalyse reaction of large number of substrate molecules into products
- The number of reactions that an enzyme molecule can catalyse per second = the turnover number
Enzymes vs chemical catalysts
- Catalyst = chemical that speeds up the rate of a reaction and remains unchanged and reusable at the end
of the reaction
- Chemical catalysts usually need:
a) Very high temperatures
b) Increased pressures
c) Extremes of pH
- Enzymes are able to increase metabolic reactions by up to 1012 times at lower temperatures with:
a) Lower temperature
b) Normal pressure
c) Neutral pH
(conditions that sustain life)
- Enzymes are more specific than chemical catalysts
- Enzymes don’t produce unwanted by-products
- Enzymes rarely make mistakes
- The cells where they are made/act can regulate their production/activity to fit the needs of the cell at the
time
How enzyme structure determines its function
- May need help from cofactors to catalyse some reactions
- Instructions for making enzymes are encoded in genes so if a mutation occurs in the gene which alters the
sequence of amino acids in the protein, it may alter the enzymes tertiary structure and prevent it from
functioning
- If an enzyme that catalyses a metabolic enzyme is deficient, a metabolic disorder results
- Enzymes catalyse formation of structural components e.g. collagen in bone/cartilage/blood-vessel
walls/joints so genetic problems with enzyme formation can damage these
Active site
- Active site = indented area on the surface of an enzyme molecule with a shape that is complementary to
the shape of the substrate molecule
- Only 6-10 amino acids involved in the active site
- Tertiary structure is important as its shape must be complementary to the shape of the substrate
- Each enzyme is therefore highly specific and can only catalyse a reaction involving the particular substrate
- Shape of active site, hence its ability to catalyse a reaction, can be altered by changes in temperature in
pH – as these affect the bonds that hold the tertiary structure together
Intracellular enzymes
- Intracellular = Inside the cell
- Up to 1000 metabolic reactions occurring at once in any cell, each catalysed by a different enzyme
- Some are part of a metabolic pathway (metabolism = the chemical reactions that take place inside living
cells/organisms)
- Each metabolic pathway is one of a series of consecutive reactions, every step catalysed by a specific
enzyme to produce a specific product
- Various reactants and products that become reactants act as substrates for specific enzymes
- The reactants, intermediates, and products are called metabolites
, Types of metabolic pathways
- Catabolic = metabolites are broken down to smaller molecules and release energy
- Anabolic = energy is used to synthesise larger molecules from smaller ones
- Respiration and photosynthesis are examples of complex metabolic pathways using many enzymes
Extracellular enzymes
- Extracellular = outside the cell
2. Cofactors
Definition
- A substance that has to be present to ensure that an enzyme-catalysed reaction takes place at the
appropriate rate
- Some cofactors are prosthetic groups and part of the enzyme’s structure
- Others – mineral ion cofactors and organic coenzymes – form temporary association with the enzyme
Prosthetic groups
- Prosthetic group = cofactor that is permanently bound by covalent bonds to an enzyme molecule
Carbonic anhydrase enzyme
- Found in red blood cells, zinc ion permanently bound as a prosthetic group to the active site
- Catalyses the interconversion of CO2 and H2O to carbonic acid which then breaks down to protons and
hydrogencarbonate ions
- The reaction can occur in either direction depending on the concentration of substrate or product
molecules
- Amylase digests starch to maltose and only functions if chloride ions are present
Cofactors – temporary binding
- Some enzymes work better in the presence of ions that are temporarily bound to them – also cofactors
- In enzyme-catalysed reactions, the enzyme and substrate molecules temporarily bind to form an enzyme-
substrate complex
- The presence of certain ions that may temporarily bind to either the substrate or enzyme may ease the
formation of the ES complexes, therefore increasing the rate of enzyme-catalysed reactions
- Some cofactors act as co-substrates: they and the substrate together form the correct shape to bind to
the active site
- Some cofactors change the charge distribution on the surface of the substrate or active site and make
temporary bonds in the ES complex easier to form
Coenzymes
- Small organic non-protein molecules that bind temporarily to the active site of enzyme molecules, either
just before or simultaneously with the substrate
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