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Cell processes
Enzyme activity
Key terms
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Amino acids
Protein
Enzyme
Catalyst
Metabolism
Anabolism
Catabolism
Active site
Substrate
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Lock-and-key model
Induced fit model
Denature
pH scale
saturation point
Co-factors
Co-enzymes
Inhibitor
Optimum temperature
Proteins
• Proteins are one of the major food groups
in the diet of animals.
• They are made of chains of amino acids.
• There are only 20 different types of amino
acids.
• The varying combinations of amino acids
result in the huge diversity of proteins,
each having its own function.
Two main types of proteins
• Fibrous proteins
- long and stringy
- form structures such as collagen in muscle,
elastin in skin, keratin in hair, nails and horns
• Globular proteins
- folded into a 3-D shape
- perform regulatory functions such as
hormones, transporting other molecules,
antibodies for fighting off infections and
enzymes
Protein synthesis from DNA
Enzymes
• Enzymes are proteins that act as
biological catalysts i.e. they increase the
rate of chemical reactions in the body.
• Without enzymes, metabolism would occur
too slowly for life to exist.
• Remember what metabolic reactions are!
Two main types of
metabolic reactions
• Synthesis of large molecules from smaller
molecules – anabolic reactions
e.g. glucose molecules into starch
• Breakdown from larger molecules into
smaller molecules – catabolic reactions
e.g. food protein into amino acids for
making other proteins
Each enzyme has a specific role
• One enzyme catalyzes only one type of reaction.
• Often named after the main substance in the
reaction it catabolises.
Suffix ‘-ase’ is added.
e.g. Lipase catalyzes breakdown of lipids (fats)
Lactase facilitates catabolism of lactose from milk
Protease helps break down proteins from food
Why are enzymes specific?
• This property of enzymes relates to their
shape.
• Each enzyme has a specific shape,
depending on the sequence of amino
acids it is made of.
• Shape of an area on the enzyme known
as its active site is where the substrate
fits.
Two slightly varying models of
enzyme action
• Lock - and - key model
The shape of the substrate corresponds exactly
to the shape of the active site.
This model, although useful to gain basic
understanding, is now considered too simple to
explain most enzyme action.
Two slightly varying models of
enzyme action
• Induced fit model
Assumes that the enzyme is partially flexible, and that
the substrate plays a role in determining the final shape
of the active site.
• Enzymes get reused several times before
they get worn out.
e.g. Peroxidase catalyzes breakdown of
several million hydrogen peroxide
molecules (dangerous to body tissues)
into water and oxygen per minute.
Factors that affect enzyme activity
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Temperature
pH
Substrate concentration
Co-factors
Inhibitors
Temperature
• Up to 40 – 45 °C, temperature speeds up
enzyme activity, as molecules move faster
at higher temperatures and collide more
often.
• If temperature is too high, proteins/enzymes
get denatured.
e.g. What happens when you cook an egg?
Rate of reaction
• Temperature at which the reaction is fastest is
called optimum temperature.
Temperature (in °C)
• Optimum temperature for enzymes in different
organisms varies!
e.g. Antarctic fish, bacteria living in sulfur
springs, etc.
pH
• pH scale measures acidity; ranges from 1 to 14. The
closer pH is to 1, the more acidic a substance or
environment is.
• Most enzymes work within cells where the pH is neutral.
So, their optimum pH will be approx. 7.
• When pH is outside range for an enzyme (too low or too
high), enzyme denatures.
• Examples of exceptions
- Pepsin (works in stomach, where it is acidic, optimum
pH is low)
- Pancreatic lipase (works in small intestine, where it is
basic, optimum pH is high)
Effect of pH on enzyme acitivity
Rate of reaction
pH < 7 is acidic
pH = 7 is neutral
pH > 7 is basic
pH scale
Substrate concentration
• Rate of enzyme activity increases as the
concentration of the substrate increases.
• This happens up until saturation point i.e. there
are no more free enzymes/active sites left.
Co-factors & Co-enzymes
• Enzymes often need “helpers”.
• Sometimes ions or metal atoms are used. These
helpers are called cofactors (e.g. iron in
haemoglobin, calcium in nerve signalling, nickel
in urease etc.)
• Small molecule helpers are called coenzymes.
• Coenzymes that we can't build ourselves, that
we need to get from our food in their working
form, are called vitamins. (e.g. vitamin B in
respiration, vitamin C for turning genes “on”)
Co-factors & Co-enzymes
Inhibitors
• Inhibitors are substances that prevent enzymes from
catalysing reactions.
• Many poisons work as enzyme inhibitors.
• Also, unwanted enzyme activity may be controlled by
inhibitors.
• Sometimes reversible, sometimes not.
Heavy metals (lead, mercury) prevent enzymes in cells of the
nervous system from functioning.
Cyanide prevents the action on an enzyme in the electron transfer
chain of respiration
Not always poison – look up what ACE-inhibitors are used for!
Competitive inhibitors
• Structure closely
resembles the structure
of the enzyme’s normal
substrate.
• Takes over the enzymes
active site.
e.g. The antibiotic
penicillin inhibits an
enzyme that bacteria use
to make cell walls.
Non-competitive inhibitors
• Bond to another part
of the enzyme
molecule, but this
alters the shape of
the active site.
• Hence, substrate can
no longer bind to the
active site.
• Often a way in which
unwanted enzyme
action is controlled.