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The role of enzymes in catalysing reactions that affect metabolism at a cellular and whole organism level
To include the idea that enzymes affect both structure and function
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They
interact with substrate molecules causing them to react at faster rates without the need for harsh environmental conditions
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The 3D shape usually has hydrophobic amino acid R-groups in the centre and hydrophilic amino acid Rgroups around the outside
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This means they are proteins that are used in metabolism to alter the rate of chemical
reaction, lowering the activation energy and providing an alternative route for reaction but aren’t used up
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This involves the synthesis of large polymer-based components
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The
chemical reactions required for growth are anabolic (building up) reactions, which are catalysed by enzymes
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Energy is released by large organic molecules (like
glucose, obtained from digestion of larger organic molecules like starch) in metabolic pathways consisting of many catabolic
(breaking down) reactions which are also catalysed by enzymes
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Metabolism is the sum of all the reactions and is
controlled and order imposed by enzymes
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Catalase is an intercellular enzyme that catalyses the breakdown of hydrogen peroxide into oxygen and water quickly,
preventing its accumulation
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Amylase is an extracellular enzyme that breaks down starch (amylose) polymers into maltose
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It’s released in saliva into the mouth, and in pancreatic juice into the small intestine
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Maltase is present in the small intestine
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Trypsin is an extracellular protease (type of enzyme) that catalyses the digestion of proteins into smaller peptides which are
broken down further by other proteases
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Amino acids which are produced by the action of proteases, are then absorbed into the bloodstream through
the cells lining the digestive system
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Enzyme function is related to shape
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Each enzyme has a specific active site maintained by a very specific overall tertiary structure
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Activation energy is the amount of energy that must be applied for a reaction to proceed
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Different reactions require different levels of activation energy
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The active site of an enzyme is complementary to shape of a specific substrate molecule
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When the substrate is bound to the active site an enzyme-substrate is formed
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The product(s) are then released, leaving the enzyme unchanged and able to take
part in subsequent reactions
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The R-groups within the
active site of the enzyme will also interact with the substrate, forming temporary bonds
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More recently, evidence suggests the active site of the enzyme actually changes the shape slightly as the substrate enters
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As a substrate molecule collides with an enzyme’s active site, the enzyme molecule changes shape slightly
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The substrate fits into place called an enzyme-substrate complex
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This destabilises the substrate molecule, so the reaction
occurs more easily
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The enzyme is now able to catalyse the same reaction with another substrate
molecule
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It
shows people how it works
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An enzyme can catalyse a reaction only if a substrate collides with the active site so that an enzymesubstrate complex is formed
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An enzyme-product complex is the intermediate structure in which product molecules are bound to an
enzyme molecule
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PH is a measure of the hydrogen ion concentration
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Hydrogen bonds and ionic bonds between amino acid R-groups, resulting from interactions
between polar and charged R-groups present on the amino acids forming the primary structure,
hold the tertiary structure
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Hydrogen ions
can interfere with the hydrogen bonds and ionic bonds, altering the structure
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The
optimum pH is the pH value at which the rate of an enzyme-controlled reaction is at its maximum
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When the pH changes slightly, the active site is altered
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This is renaturation
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The enzyme has denatured
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It
doesn’t change the primary structure of an enzyme
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The increase in kinetic energy causes the molecules to vibrate faster
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As heat is increased, more and more bonds are broken as there is more strain
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If enough of the bonds are broken, the whole
tertiary structure will unravel and the enzyme will stop working – it denatures
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Only a slight change in the shape of an active site
means it will no longer be complementary to the substrate
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The temperature coefficient, Q₁₀ of a reaction is a measure of how much the rate of a reaction increases with a 10°C rise in
temperature
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Enzymes adapted to the cold tend to have more flexible structures, particularly at the active site, making them less stable than
enzymes that work at higher temperatures
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Enzymes adapted to hot environments are more stable than other enzymes due to the increased number of bonds,
particularly hydrogen bonds and sulphur bridges, in their tertiary structures
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As the enzyme concentration increases, more active sites become available
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If the enzyme concentration increases further, a point will be reached where all the substrate
molecules are occupying enzyme active sites
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This is a limiting factor
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Enzyme concentrations in cells are usually maintained at a relatively low level
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As the concentration of substrates increases, collisions between enzyme and substrate
molecules occur more often
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The reaction rate increases
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At this point, all the enzyme molecules present are forming enzyme-substrate complexes
as fast as possible
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Enzyme concentration can be a limiting factor at this point because the active sites are all used up so decrease in number
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At this point no more enzyme-substrate complexes can be formed
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Practical investigations into the effects of pH, temperature, enzyme concentration and substrate concentration on
enzyme activity
Serial dilutions
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Insert the delivery tube
into the open end of the measuring cylinder under water
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Quickly add catalase and insert the rubber bungs and side-arm delivery tubes
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Repeat the steps with the other pHs
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As the pH moves towards pH 7, the volume of gas collected increases
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As it is digested by protease e, the milk changes from
white to clear because the casein protein is being broken down into amino acids and therefore loses the ability to make milk
appear white or opaque
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Heat protease and the milk
at different temperatures in a water bath e
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0⁰C, 20⁰C, 40⁰C, 60⁰C and 80⁰C for 5 minutes
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Record the time and repeat the steps for the other
temperatures
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The time decreases as the temperature
continues to increase
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Soak filter paper in catalase enzyme for
five minutes
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Repeat for the other concentrations and record results
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Eventually increasing the
concentration has no effect on the time taken for the discs to rise to the top
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Add sucrose solution to each test tube and place in a water bath at 35-40⁰C for 2 minutes
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Add sucrase to the sucrose solution and leave the tubes in the water bath for 10 minutes
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Add Benedict’s solution to each of the
sucrose/enzyme test tubes and measure how long it takes for the solution to turn cloudy green
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Whatever the variables being investigated, all other variables must be kept constant
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More accurate results are obtained from more
repeats
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Use accurately measured volumes/mass of enzyme and substrate in solutions
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The need for coenzymes, cofactors and prosthetic groups in some enzyme-controlled reactions
To include Cl¯ as a cofactor for amylase, Zn²⁺ as a prosthetic group for carbonic anhydrase and vitamins as a source
of coenzymes
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Some cofactors are a part of the enzyme (prosthetic groups); others affect the enzyme on a temporary basis (coenzymes and
inorganic ion cofactors)
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Coenzymes are small, organic, non-protein molecules, derived from vitamins, which bind for a short period to the active site
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Coenzymes take part in the reaction and changed but
aren’t recycled back like substrates
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Prosthetic groups are coenzymes that have permanent parts in an enzyme molecule
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g
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The enzyme carbonic anhydrase contains a zinc-based prosthetic group
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This enables carbon
dioxide to be transported in the blood
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Ions
may combine with either the enzyme or the substrate
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The enzyme amylase will function only if chloride ions are present
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To include competitive and non-competitive and reversible and non-reversible inhibitors with reference to the action
of metabolic poisons and some medicinal drugs, and the role of product inhibition and inactive precursors in
metabolic pathways
An enzyme inhibitor is any substance or molecule that slows down the rate
of an enzyme-controlled reaction by affecting the enzyme molecule in some
way – they inactivate enzymes
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The enzyme is inhibited (cannot carry out its function)
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Substrate and inhibitor molecules present in a solution will
compete with each other to bind to the active sites of the enzymes
catalysing the reaction, reducing the number of enzyme-substrate
complexes formed and slowing down the rate of reaction
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However there are some
exceptions, like aspirin
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If the substrate concentration is increased enough there will be more substrate that inhibitor that the original
Vmax can still be reached
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High
blood cholesterol levels result in heart disease
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Non-competitive inhibitors bind to the enzyme at a location other than the active site – allosteric site
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This results in the active site no longer
having a complementary shape to the substrate so it is unable to bind to the enzyme
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The inhibitor
doesn’t compete with the substrate for the active site
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Increasing the enzyme/substrate concentration will not overcome the effect of a non-competitive inhibitor
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Organophosphates used as insecticides and herbicides irreversibly inhibit the enzyme acetyl cholinesterase, an enzyme
necessary for nerve impulse transmission
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Protein pump inhibitors (PPIs) are used to treat long-term indigestion
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They reduce the production of excess acid which, if left untreated, can lead to
the formation of stomach ulcers
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It is enzyme inhibition that occurs when the
product of a reaction acts as an inhibitor to the enzyme that produces it
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Excess products aren’t made or wasted
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Two phosphate groups are added
to the glucose molecule
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This enzyme is competitively inhibited by ATP
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Metabolic poisons may be enzyme inhibitors as they inhibit the action of enzymes involved in metabolic processes, which
disturbs an organism
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If this enzyme is inhibited, ATP cannot be made since oxygen use is decreased
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This is potentially fatal
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Infection by viruses can be
treated by Inhibitors to the viral enzyme protease, often competitive Inhibitors
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Penicillin works by inhibiting a bacterial enzyme that is responsible for forming
cross-links in bacteria cell walls
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The same enzyme may act as both a poison and a drug, depending on the amount of inhibitor and its location
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Precursor enzymes often need to undergo a change in
their tertiary structure, particularly to the active site, to
be activated
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Before the cofactor is added, the precursor
protein is called an apoenzyme
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Sometimes the change in tertiary structure is brought about by the action of another enzyme, such as protease, which cleaves
certain bonds in the molecule
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These types of precursor enzymes are called zymogens or proenzymes