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Enzymes
This is a document that consists of notes for the topic, Enzymes, as part of the AS/A level OCR Biology A course
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Enzymes = Biological Catalysts (substance that speeds up a reaction without being used upin the reaction itself)
Enzymes = Globular proteins:
● Compact, water soluble
● Form when proteins fold into their tertiary structure - hydrophobic R-groups on amino
acid are kept away from aqueous environment
● Hydrophilic R-groups are on the outside of protein - soluble in water
Chemical reactions required for growth are anabolic (building up) reactions
Energy is released from large organic molecules in metabolic pathways consisting of many
catabolic (breaking down) reactions
Metabolism - Sum of all the chemical reactions and reaction pathways happening in a cell or
an organism
Mechanism of enzyme action
The specificity of an enzyme is determined by shape of its active site - specific substrate has
to fit into the active site (complementary fit)
Specificity = relationship between an enzyme + only type of molecule that fits into active site
Active sites are composed of a small number of amino acids within a polypeptide
Features of active site + type of substrate that it accepts is determined by the R groups of
these amino acids
Some have nonpolar R groups > provide hydrophobic interior to active site
Some have polar R groups > form temporary ionic bonds with substrate molecules
During enzyme-catalysed reaction, the substrate fits into the active site of enzyme. Enzyme
functions by combining with substrate to form enzyme-substrate complex
Activation Energy (AE) - a certain amount of energy needed to be supplied to chemicals
before a reaction starts
(Enzymes can find alternative pathways with AE so the reaction can happen)
Enzymes reduce the amount of AE that’s needed, so reactions can take place at lower
temperatures. This speeds up the rate of reaction
Formation of enzyme-substrate complex lowers AE. Why?
● If 2 substrates need to be joined, attaching to enzyme holds them close together,
reducing any repulsion between the molecules so they can bond more easily
● If an enzyme is catalysing a breakdown reaction, fitting into the active site puts a
strain of bonds in the substrate. Causes substrate to break up more easily
Lock-and-Key hypothesis - idea that enzyme active site is complementary in shape to
substrate in the same way as a key and lock. Interactions between the R groups within the
active site and substrate stabilise the enzyme-substrate complex. Substrate is altered and
forms a product (Enzyme-product complex)
, Induced-fit hypothesis - the active site of an enzyme changes shape during binding of a
substrate molecule, and this puts strain on substrate molecule contributing to reaction
● Initial interaction between enzyme + substrate = weak, but these interactions rapidly
induce changes in the enzyme’s tertiary structure that strengthen binding, putting
strain on molecule
● This weakens a particular bond or bonds in substrate, therefore lowering the
activation energy for the reaction
Enzyme action can be intercellular (inside cells) or extracellular (outside cells)
Example of intercellular: Catalase
Hydrogen Peroxide (H₂O₂) is a toxic product of many metabolic pathways. If it is left to build
up, it can kill cells
Catalase can break this down to Oxygen and Water quickly
Example of extracellular: Amylase and Trypsin
Both found in the digestive system
Amylase catalyses hydrolysis of starch into maltose into the mouth
Amylase produced in salivary glands and pancreas - released in saliva into the mouth and
pancreatic juice into small intestine
Trypsin is a protease - catalyses hydrolysis of peptide bonds - turning big polypeptides into
smaller ones (which then get broken down into amino acids by other proteases)
Produced in pancreas and released with the pancreatic juice into small intestine, where it
acts on proteins
The amino acids that are produced by action of proteases are absorbed by cells lining the
digestive system and then absorbed into bloodstream
Factors affecting enzyme activity
Enzymes are proteins. If they have high temperatures and pH - weak bonds that hold tertiary
structure together break
Temperature: Higher temperature = more kinetic energy = molecules move faster - enzymes
more likely to collide with substrate - energy of collisions increase - each collision likely to
result in reaction
Too high and the reaction stops:
● The vibration of molecules can break some
bonds that hold enzyme in shape
● Active site changes shape so substrate no
longer fits into enzyme
● Enzyme = denatured
Temperature coefficient (Q10) - shows how much the rate of a reaction changes when the
temperature is raised by 10°C
Most enzyme-controlled reactions have a Q10 value of around 2 - rate doubles
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