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Enzymes
The total sum of all chemical reactions, metabolism
ü Anabolic reactions: synthesis of molecules and need energy to form bonds
...

E
...
photosynthesis use H2O and CO2 to produce sugar – needs energy (from
sun)
ü Catabolic reactions: breakdown of molecules and release energy
...

E
...
respiration: sugar + O2 -> CO2 + H2O + energy
ΔG = G

products

–G

reactants

If G is positive; reaction is endergonic
...

Activation energy: initial input of energy to start the reaction
...

Energy Carrier Molecules:
1
...
Energy carrier molecules: capture energy from energy-releasing reactions
3
...
In
energy releasing reactions (e
...
respiration and photosynthesis) some energy is
transferred to electrons which are captured by electron carriers (e
...
NAD and FAD)
...
g
...


1 | P a g e


Catalysts:
1
...
Not used up and not permanently altered

Properties of Enzymes:
1
...
They speed up chemical reactions at temperatures suitable
for living organisms
...
Specific: they catalyse a single reaction that involves 1 or 2 specific
molecules
...
g
...

3
...

E + S ↔ ES ↔ EP ↔ E + P
Mechanisms:
Fischer suggested the lock and key hypothesis where the substrate fits into the
active site
...

Chemical interactions occur between the substrate and enzyme (breaking old bonds
and forming new ones)
1
...
Enzymes induce strain in the substrate: the enzyme can cause bonds in
substrate to stretch and facilitating its breakdown
3
...


2 | P a g e





Factors affecting enzyme reactions:
Rate: the amount of substrate changed and product formed
...

ü Enzyme concentration: high temperature concentration will increase the rate
of reaction
...


3 | P a g e


ü Substrate concentration: rate of reaction increases with an increase in
substrate concentration
...
Any extra substrate has to ‘wait’ until the enzyme-substrate
complex has released the products
...
Changes in pH alter the concentration of H+ ions in the medium
...
The enzyme may become denatured
...
Chances of substrate and enzyme molecules colliding are
increased (optimum temperature)
...

4 | P a g e


Reactions are slowed by lower temperatures, enzymes are inactivated
...

𝑄"# =

𝑟𝑎𝑡𝑒 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑎𝑡 𝑋 + 10°𝐶
𝑟𝑎𝑡𝑒 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑎𝑡 𝑋°𝐶

Inhibitors
These reduce the rate of an enzyme – controlled reaction
ü Competitive inhibition: same shape as the substrate and fits into the active
site, preventing the true substrate from entering the active site
...
It causes a conformational change in the shape
of the enzyme
...


Allosteric Enzymes
These change shape and regulate metabolic pathways
...

They are regulated by effectors which bind to the allosteric sites
...
Allosteric enzymes have subunits; active site is on
the catalytic subunit
...


5 | P a g e


The substance binds to the allosteric site, so they are used as activators
...

Reactions are interconnected
ü Negative feedback inhibition: accumulation of the end product might inhibit
the enzyme (end-product inhibition – negative feedback mechanism)

6 | P a g e


ü Enzyme cofactors: enzymes require non-protein components for efficient
activity
...

§

Inorganic ions: mould the enzyme/substrate into a shape that allows a
complex to be formed e
...
Cl- increase activity of salivary amylase
...
E
...
FAD is involved
in respiration – acts as an H2 carrier
...
g
...


§

Coenzymes: derived from vitamins
...
They change chemically in the process
...
g
...

AH2 + NAD -> A + NADH2
NADH2+ B -> BH2 + NAD
E
...
coenzyme A: synthesis and oxidation of fatty acids and oxidation of
pyruvate during respiration carry acyl (RCO) groups (acetyl-CoA)

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