Enzyme Activity
The properties of enzymes related to their tertiary structure.The effects of change in temperature,pH,substrate concentration,and competitive and non-competitive inhibition on
HOW ENZYMES WORK
• Enzymes are ORGANIC
CATALYSTS. A CATALYST is anything that speeds up a
chemical reaction that is
occurring slowly. How does a catalyst work?
• The explanation of what
happens lies in the fact that most chemical reactions that RELEASE ENERGY
Enzymes
• An enzyme is a biological catalyst
• The pockets formed by tertiary and quaternary structure can hold specific substances
(SUBSTRATES).
• These pockets are called ACTIVE SITES.
• When all the proper substrates are nestled in a particular
enzyme's active sites, the
enzyme can cause them to react quickly
Properties of Enzymes relating to their tertiary structure.
• The activity of enzymes is strongly affected by changes in pH and
temperature. Each enzyme works best at a certain pH and temperature,its activity decreasing at values above and below that point. This is because of the importance of tertiary
structure (i.e. shape) in enzyme function and forces, e.g., ionic
The effects of change in temperature
.
Temperature: enzymes work best at an optimum temperature.
Below this, an increase in temperature provides more
kinetic energy to the molecules involved. The numbers of collisions between enzyme and substrate will increase so the rate will too.
The effect of change in pH.
• pH: as with temperature, enzymes have an
optimum pH. If the pH changes much from the optimum, the chemical nature of the amino acids can change.
This may result in a change in the bonds and so
the tertiary structure may break down. The active site will be disrupted and the enzyme will be
The effect of change in concentration
• Enzyme concentration: at low enzyme concentration there is great competition for the active sites and the rate of reaction is low. As the enzyme concentration increases, there are more active sites and the reaction can proceed at a faster rate.
Eventually, increasing the enzyme concentration beyond a certain
point has no effect because the substrate concentration becomes the limiting factor.
• Substrate concentration: at a low substrate concentration there are many active sites that are not occupied. This means that the reaction rate is low.
When more substrate molecules are added, more enzyme-substrate complexes can be formed. As there are more active sites, and the rate of reaction increases.
Eventually, increasing the substrate concentration yet further will have no effect. The active sites will be saturated so no more
Competitive and non-competitive inhibition
• Inhibitors slow down the rate of a reaction. Sometimes this is a necessary way of making sure that the reaction does not proceed too fast, at other times, it is undesirable
Reversible inhibitors:
• Inhibitors
• Non-competitive reversible inhibitors: these molecules are not necessarily
Irreversible inhibitors:
These molecules bind permanently with the enzyme molecule and so effectively reduce the enzyme concentration, thus limiting the rate of reaction, for
example, cyanide irreversibly inhibits the enzyme cytochrome oxidase found in the electron transport chain used in respiration. If this cannot be used,
Applications of inhibitors
• Negative feedback: end point or end product inhibition
• Poisons snake bite, plant alkaloids and nerve gases
• Medicine antibiotics, sulphonamides, sedatives and stimulants
Cell processes (e.g. respiration or photosynthesis) consist of series of pathways controlled by enzymes
A B C D E F
Enzyme pathways
eF eD
eC eA eB
Each step is controlled by a different enzyme (eA, eB, eC etc)
This is possible because of enzyme specificity
End point inhibition
• The first step (controlled by eA) is often controlled by the end product (F)
• Therefore negative feedback is possible
A B C D E F
• The end products are controlling their own rate of production
• There is no build up of intermediates (B, C, D and E)
eF eD
eC
eA eB
Inhibition
Example:
Phosphofructokinase and ATP
Substrate: Fructose-6-phosphate
Reaction
fructose-6-phosphate + ATP fructose-1,6-bisphosphate + ADP
phosphofructokinase
ATP is the end point
• This reaction lies near the beginning of the respiration pathway in cells
• The end product of respiration is ATP
• If there is a lot of ATP in the cell this enzyme is inhibited
• Respiration slows down and less ATP is produced
• As ATP is used up the inhibition stops and the reaction speeds up again
The switch: Allosteric
inhibition
Allosteric means “other site”
E
Active site
Allosteric site
Switching off
• These enzymes
have two receptor sites
• One site fits the
substrate like other enzymes
• The other site fits an inhibitor
molecule
Inhibitor fits into
allosteric site
Substrate
cannot fit into the active site
Inhibitor molecule
The allosteric site the enzyme
“on-off” switch
E Active site Allosteric site empty Substratefits into the active site The inhibitor molecule is absent Conformational change Inhibitor fits into allosteric
site Substrate cannot fit into the active site Inhibitor molecule is present
E
A change in shape
• When the inhibitor is present it fits into its site and there is a conformational change
in the enzyme molecule
• The enzyme’s molecular shape changes • The active site of the substrate changes
• The substrate cannot bind with the substrate
Negative feedback is achieved
• The reaction slows down
• This is not competitive inhibition but it is reversible
• When the inhibitor concentration
diminishes the enzyme’s conformation changes back to its active form
Phosphofructokinase
• This enzyme an active site for fructose-6-phosphate molecules to bind with another phosphate group
• It has an allosteric site for ATP molecules, the inhibitor
• When the cell consumes a lot of ATP the level of ATP in the cell falls
• No ATP binds to the allosteric site of phosphofructokinase
• The enzyme’s conformation (shape) changes and the
active site accepts substrate molecules
Phosphofructokinase
• The respiration pathway accelerates and ATP (the final product) builds up in the cell
• As the ATP increases, more and more ATP fits into the allosteric site of the phosphofructokinase
molecules
• The enzyme’s conformation changes again and stops accepting substrate molecules in the active site
• Respiration slows down
