Summary AQA Paper 1 Biological molecules - 3.1.4.2 Enzymes

Enzymes : The active site’s shape is specific and
: Enzymes increase the rate of chemical unique
reactions without being used up - the specific folds and bonding in the
tertiary structure of the protein
determines the shape.
: catalyse metabolic reactions by
lowering the activation energy of the : enzymes can only attach to substrates
reaction that are complementary in shape to the
active site
: The activation energy is the minimum
energy required for a reaction to take - very specific in what reactions it
place catalyses

– reducing the activation energy means
that more particles can collide : There are two models to explain how
successfully and react this occurs: lock and key model and
: reactions can take place at a lower induced fit model
temperature than they could without an
enzyme
Lock and Key model
- the rate of the reaction thus massively
increases : the enzyme active site is a fixed shape
that is perfectly complementary to the
substrate
: Enzymes are tertiary structure - substrate binds to the active site to
proteins form an enzyme-substrate complex
: During catalysation, a substrate fits - causes the substrate molecules to be
into the enzyme’s active site closer together to reduce repulsion
- binding to the active site to form an between the molecules
enzyme-substrate complex. - can bond more easily and during
- enzyme-substrate complex breakdown

- fitting into the active site puts strain
on bonds in the substrate so it breaks
: Only a small part of the enzyme down more easily
attaches to the substrate – this is called
the active site - bonds are weaker

- complementary in shape to the : products are then released and the
substrate active site is ready to be reused



: The substrate fits into the active site

- complementary in shape to the shape
of the active site

- can easily bind to it

, : If temperature gets too high beyond
the optimum, then the enzymes
denature

- high temperature breaking hydrogen,
ionic, disulfide bonds holding the amino
acids together in the tertiary structure

- the amino acids vibrate more, causing
Induced fit
some of the bonds holding it in its
: the active site and substrate do not tertiary structure to break
have to be perfectly complementary
- the active site to change shape and
: the active site moulds around the enzyme-substrate complexes
substrate to make it perfectly
- no longer form as active site no longer
complementary to each other
complementary to the substrate
: “moulding” then places strain and
- rate of reaction drops to 0
tension on the bonds in the substrate

- making the bonds slightly weaker

- so requires less energy to break the
bonds in the substrate

- so, activation energy is lowered in the
reaction

: the active site then returns to its
original shape after the substrate is
released

: This is the modernly accepted theory (2) pH
out of the two : optimum pH where the rate of reaction
is the highest

There are 5 factors that affect rate : If the pH is too high or too low, there is
of enzyme-controlled reactions: an imbalance of charge

(1) Temperature - interferes with the charges in the
bonds between amino acids in the
: as temperature increases, rate of active site
enzyme activity/reaction increases
: H+ and OH- ions in alkalis and acids
- kinetic energy of particles increases disrupt ionic and hydrogen bonds that
- increasing frequency of successful hold the enzyme’s tertiary structure in
collisions between enzyme and its specific shape
substrate : interference in charge causes the
- more enzyme-substrate complexes bonds that hold the tertiary structure in
form per second place to break

- reaction happens faster - causing a change in the active site
shape