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2.1.18 Enzymes are Globular Proteins
· All enzymes are very similar:
­ They are globular proteins ­ generally soluble in water.
­ They act as catalysts.
­ They are specific.
­ They have an active site.
­ Their activity is affected by temperature and pH.
· Enzymes are quite large molecules ­ their tertiary structure must be maintained in a very specific way.
· Each enzyme has a very specific, individual shape for the active site ­ so the reaction an enzyme can catalyse is also very
specific and individual.
· Loads of chemical reactions are catalysed by enzymes ­ metabolism is `enzyme-driven'.
· In a chemical reaction that is catalysed by an enzyme, substrate is turned into product.
Maltose ----MALTASE----> Glucose + Glucose
Substrate Enzyme Products…read more

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2.1.19 Where Enzymes Work Best
Enzymes + Endotherms
· Endothermic animals are able to maintain their internal body temperature independently of the environment ­ this allows
them to live in very extreme places.
· Regulating body temperature means that enzymes can function at near-optimum temperature inside the organism.
· This ability comes at a very high energy cost to the organism ­ birds + mammals need a lot more food than similarly sized
reptiles.
· But the advantages of having enzyme activity at continuous + optimum level have allowed these animals to survive very well.
Digestion
· Digestion involves breaking down larger molecules into their subunits by breaking bonds ­ this is catalysed by different
enzymes.
· Some organisms secrete enzymes outside themselves onto the food source.
· Other organisms have an internal digestive system ­ as food passes through the digestive system, various enzymes are mixed
with it in order to digest the nutrients it contains.
· Many enzymes in digestive systems are extracellular ­ they are released from the cells that make them, onto food within the
digestive system spaces.
· Many enzymes are found inside cells or attached to cell membranes ­ intracellular.…read more

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2.1.20 Enzyme Action
· Activation energy is the amount of energy that must be applied for a reaction to proceed. Different reactions require
different levels of activation energy.
· Enzymes work by reducing the amount of activation energy required ­ this means that reactions can proceed quickly at
temperatures much lower than boiling point.
Active Sites
· An enzyme has a specifically shaped active site ­ complementary to the shape of the substrate molecule involved in the
reaction.
· The term `lock-and-key' is used to describe how enzymes work ­ the substrate FITS into the active site LOCK. The substrate
is then held in place so the reaction can go ahead.
· The `induced-fit' hypothesis believes that changes in the shape of the enzyme occur as the substrate binds to the active site.
Charges on the amino acids in the active site also contribute to holding the substrate so the reaction can occur:
­ As a substrate molecule collides with an enzyme's active site, the enzyme molecule changes shape slightly.
­ This makes the active site fit more closely around the substrate. The substrate is held because oppositely charged groups on the substrate
and the active site are found near to each other. This is an `enzyme-substrate complex'.
­ The change in the enzyme shape places a strain on the substrate molecule which destabilises the molecule ­ reaction can occur more easily.
­ A product is produced, an `enzyme-product complex'.
­ The product is a different shape to the substrate molecule.
­ The product now longer fits into the active site so it moves away.
­ The enzyme is now able to catalyse the same reaction with another substrate molecule.…read more

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2.1.21 Enzymes + Temperature
· An enzyme can only catalyse a reaction if a substrate molecule collides with the active site of the enzymes so an enzyme-
substrate complex is formed ­ this happens due to random movements of molecules.
· If the kinetic energy of enzyme + substrate is increased by heating, there will be an increased number of collisions = increased
rate of reaction = more product formed.
· Heat also makes molecules vibrate ­ this puts a strain on the bonds that hold molecules together.
· In enzymes, the vibrations can break the weaker bonds which hold the tertiary structure in place (the active site).
· As heat increases, more and more bonds are broken = rate of reaction decreases because active site's shape will break down.
· If enough bonds are broken, the whole tertiary structure will unravel and the enzyme will stop working = denaturation (not
reversible).
Speeding up + Slowing down
· Increasing the temperature increases the rate of reaction at first but as the temperature increases further, the rate of reaction
decreases ­ eventually the enzyme will stop working.
· The temperature that gives the maximum rate of reaction is the enzyme's optimum temperature.
· Many enzymes have an optimum temperature somewhere between 40-50°c but there are some organisms for which these
enzymes would be useless.…read more

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2.1.22 Enzymes + pH
· pH can interfere with the hydrogen bonds and ionic bonds holding an enzyme's tertiary structure in place ­ changing the pH
can alter the tertiary structure of the enzyme + so the active site + so the rate of an enzyme-controlled reaction.
· All enzymes have their own optimum pH ­ for most it is about pH 7.
· Changes to pH, even very slightly, will result in a fall in the reaction rate because the shape of the active shape will be
changed.…read more

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