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AS BIOLOGY CAMBRIDGE T-3 Enzymes Notes + Classified p

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Cambridge AS Biology topics 1,2,3 notes, common misconceptions & many mindmps and diagrams

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Cambridge AS Biology Notes
Topic 3: Enzymes

,Rodayna Elnaggar (+201021214991)

3.1 Mode of action of enzymes

Key terms:

● Enzyme definition: a globular protein produced by a living organism that acts as a biological catalyst in a chemical
reaction by reducing activation energy, by providing an alternative pathway for the reaction.

● Enzymes are globular proteins with a tertiary structure (globular is in chapter 2) → hydrophilic R groups are to the
outside so making enzymes soluble.

● Active site: an area on the enzyme molecule where the substrate can bind.

● Intracellular enzymes: work inside the cell eg. lysosomes & peroxisomes contain intracellular enzymes.
● Extracellular enzymes: enzymes secreted by the cell & catalyze reactions outside cells eg. digestive enzymes in the
gut.

What is specificity?
The shape of the active site will only allow 1 shape of molecule or a particular substrate to fit.

How the specificity of the enzyme is determined:
● Due to the primary structure of the protein determining the sequence of amino acids and thus the type of R groups
and bonding involved between the amino acids in a polypeptide chain.

● The bonding and interactions between R groups determine the overall tertiary structure of the enzyme, giving the
enzyme a specific 3D active site shape.

2

,Rodayna Elnaggar (+201021214991)

Lock-and-key hypothesis
Definition: the substrate is a complementary shape to the active site of the enzyme & fits exactly into the site; the enzyme
shows specificity for the substrate

Approaching a 4 mark ppq:
1. Enzymes have a specific shape of active site complementary to the substrate and allows the substrate to fit perfectly.
2.Random movement of enzyme & substrate brings substrate to active site
3. Substrate fits in the active site and is held by temporary hydrogen bonds which form between substrate & some of the R
groups of enzyme’s amino acids.
4. Enzyme substrate complex (ESC) forms, this adds stress to substrate, activation energy is therefore reduced.
5. The molecule is either broken (catabolic reaction) or new bonds are formed (anabolic).
6. Product will no longer fit in the active site so is released & enzyme remains unchanged.

Induced-fit hypothesis
Definition: the substrate is a complementary shape to the active site of the enzyme but not an exact fit- the enzyme, or
sometimes substrate, CAN CHANGE SHAPE SLIGHTLY to ensure a perfect fit → but still shows specificity.

Approaching a 4 mark PPQ:

1. The substrate is partially complementary to the active site, random movement of enzyme & substrate brings
substrate to active site.
2. Interaction between the r groups of the enzyme & substrate causes the active site to change its shape SLIGHTLY &
to carry out conformational change & enzyme molds and folds around the substrate.
3. R groups of amino acids in the active site interact with substrate, strong binding of substrate to the active site adds
stress on the substrate and lowers the activation energy.
4. Active site and the substrate are now said to be
complementary allowing a better fit & forming an
enzyme substrate complex.
5. This interaction can cause the break of substrate
apart (catabolic) or allow the formation of new bond
between molecules (anabolic)
6. Products then leave the active site & enzyme
remains unchanged.

3

, Rodayna Elnaggar (+201021214991)

PPQ F/M 2019

Enzyme examples:

Lysozyme Found in tears & saliva
Natural defense against bacteria → breaks the
polysaccharide chains that form the cell walls of
bacteria

Catalase Converts hydrogen peroxide into water & oxygen

How enzymes lower the activation energy of a reaction?
● Enzymes work by lowering the activation energy of a reaction and in doing so they provide an alternative
energy pathway with a lower activation energy.
● By bringing substrate close to active site and is held together by r group interactions of the enzyme
● This influences the stability of bonds in the reactants & destabilization of bonds in the substrate makes
it more reactive by putting stress/ strain on the substrate making it easier to react.

4

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