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Enzymes
What are enzymes?
Special proteins that coordinate the
cell’s activities
Important:
1) All enzymes are proteins
2) All proteins are NOT enzymes
Enzymes are Catalysts
Catalysts: speed up chemical reactions but
do not change as a result of the reaction
***Enzymes do not change the amount of
product that is formed in a reaction
What are catalysts used for?
1)Used in industrial processes to reduce heat or pressure
requirements
2)Used in living organisms
– Reactions must occur at body temp and atmospheric
pressure (LOW)
3)Without catalysts reactions would be too slow
4)Needed to sustain life
How do enzymes work?
Each chemical reaction requires a specific
enzyme
• Enzymes must be made of something that
can take many different shapes
• Proteins and their 4 levels of structure
work well
How do enzymes work?
1) Enzymes act upon a substance called a
substrate
2) The enzyme has an indent in it called the
active site where the substrate can fit
into, kind of like a lock and a key
How do enzymes work?
• Substrate fits into active site and interacts
with exposed amino acids by ionic or
hydrogen bonding
Forms enzyme-substrate complex
The Lock and Key Model
Induced Fit Model
Summary of How enzymes work
1) Substrate ( reactant) fits into the active
site of the enzyme
2) The enzyme breaks/forms bonds and
releases the products
3) The enzyme can then be used again with
another substrate
How do enzymes speed up a
chemical reaction?
Enzymes decrease the amount of
energy required to start a reaction.
The energy required to start a reaction
is the activation energy
How enzymes speed up chemical
reactions
• Enzymes work by lowering the
activation energy
– This is done by the enzyme-substrate
complex
– This complex has lower activation
energy
– Makes the reaction proceed quicker and
easier
Factors that affect the activity of an
enzyme
1)
2)
3)
4)
Temperature
pH
Amount of an enzyme
Amount of substrate ( like the reactant
part of the chemical equation)
5) Inhibitors
Temperature
– Increase temperature --> Increase movement of
molecules
– More collisions between substrate and enzyme
– More successful conversions
– Moving higher than optimum temp reduces activity
• Enzyme is made of protein
• High temperatures can denature the enzyme
– Tertiary structure is changed
– 3D structure of enzyme changes too much for substrate to fit
– Most enzymes in the human body have an optimum
temperature around 40 C
pH
– A change of pH means a change in H+ ions in
surrounding
– Affects the bonding of R-groups
– Affects the shape of the enzyme
– Most human enzymes have optimum at 7
(neutral)
– Stomach enzymes (pepsin) have optimum at
2
– Liver enzymes arginase has optimum at 10
Enzyme concentration
– Enzymes are not used up during reactions
– Can work effectively at low concentrations
– Usually substrate molecules are in excess
– Reaction rate is limited by enzyme
concentration
– Increased concentration --> increased
reaction rate
Substrate concentration
– If substrate concentration is low some
enzyme sites empty
– Rate of reaction will be lower
– If substrate conc. is then increased --> rate
increases
– This increase continues until substrate is in
excess
• Enzyme active sites are saturated with substrate
Inhibitors
Inhibitors are chemicals which reduce the rate of enzyme catalyzed reactions
– Alter the shape of the active site directly or indirectly
1)Non-reversible bind permanently disabling enzyme
– Permanent change of tertiary structure (break disulfide bond)
– Always non-competitive
2)Reversible inhibitors bind temporarily to enzyme
– Competitive inhibitors
• Similar shape to substrate and fit into active site
• Prevents substrate from entering enzyme
– Non-competitive inhibitors
• Bind to other parts of the enzyme altering shape of enzyme
• Active site no longer fits substrate
Inhibitors