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Summary Enzymes

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Compiled and Summarised notes all based on the chapter Enzymes and units falling under it, with diagrams included

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  • June 10, 2023
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AS Bio notes Raida 12D

Enzymes

3.1 What is an enzyme?
> Enzyme: a protein that acts as a biological catalyst in a chemical reaction by reducing
activation energy
 An enzyme is a biological catalyst; because all enzymes are proteins
 Catalyst because it speeds up a chemical reaction but remains unchanged at the end of
the reaction
 Enzymes are globular proteins, they fold up into precise shapes
 Almost all metabolic reactions which take place in living organisms are catalysed by
enzymes
 Many enzyme names end in -ase; for example, amylase and ATPase


 Intracellular and extracellular enzymes:
 Not all enzymes work inside cells, those that do are described as intracellular
 Enzymes that are secreted by cells and catalyse reactions outside cells are described as
extracellular
 Digestive enzymes in the gut are extracellular enzymes
 Some organisms secrete enzymes out their bodies, fungi, for example, often do this in
order to digest the food on which they are growing

3.2 Mode of action of enzymes
> Active site: area on an enzyme where the substrate can bind
> Lock-and-key hypothesis: the substrate is a complementary shape to the active site of the
enzyme, and fits exactly into the site; the enzyme shows specificity for the substrate
> Induced-fit hypothesis: the substrate is a complementary shape to the active site of the
enzyme, but not an exact fit - the enzyme, or sometimes the substrate, can change shape
slightly to ensure a perfect fit but is still described as showing specificity


 The lock-and-key hypothesis and the induced-fit hypothesis:
 Like all globular proteins, enzyme molecules are coiled into a precise three-dimensional
shape
 Hydrophilic R groups on the outside of the molecule make them soluble in the water in
the cytoplasm
 Enzyme molecules have an Active site
 The active site of an enzyme is a region to which another molecule/s can bind
 This molecule is the substrate of the enzyme
 The shape of the active site allows the substrate to fit perfectly
 The idea that the enzyme has a particular shape into which the substrate fits exactly is
known as the Lock-and-key hypothesis
(The substrate is the key whose shape fits the lock of the enzyme)
 The substrate is held in place by temporary bonds which form between the substrate
and some of the R groups of the enzyme’s amino acids
 This combined structure is the Enzyme-substrate complex
 Each enzyme will act on only one type of substrate molecule

,  This is because the shape of the active site will only allow one shape of molecule to fit
 The enzyme is said to be specific for this substrate (enzyme is showing specificity)




 Induced-fit hypothesis is the same as the lock-and-key hypothesis, but adds the idea
that the enzyme, and sometimes the substrate, can change shape slightly as the
substrate molecule enters the enzyme, in order to ensure a perfect fit (this makes the
catalysis even more efficient)
 An enzyme may catalyse a reaction in which the substrate molecule is split into two or
more molecules
 Alternatively, it may catalyse the joining together of two molecules (ex. Making a
dipeptide from two amino acids)
 Enzyme-product complex is briefly formed before the release of the product
 When the reaction is complete, the product/s leave the active site
 The enzyme is unchanged by this process, so it is now available to receive another
substrate molecule
 The rate of the overall reaction can be high




The example of lysozyme:

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