Enzymes Flashcards
Define catalyst
- Substance which speeds up rate of chemical reactions without being used up
- Remains chemically unchanged
Define substrate
Reactant in enzyme-controlled reactions
Define metabolism
All the reactions that occur within an organism in order to maintain life
Describe the structure of an enzyme
- Globular protein
- Active site - specific place where a substrate binds
Outline the action of enzymes
- Catalyse biological reactions
- Substrate-specific
- Lower activation energy of a chemical reaction
- Substrate binds to active site
- Enzyme–substrate complex formed
Define activation energy
Minimum amount of energy required for a reaction to occur
Outline the importance of enzymes to metabolic processes
- Increase rate of reaction
- Lower activation energy
- A specific enzyme is required for each substrate
- Metabolic pathway blocked if an enzyme is inhibited or absent
- End-product inhibition can control metabolic pathways
- Differences in metabolism as cells produce different enzymes during differentiation
- Can affect metabolism at both cellular and whole organism level
Which protein structure is responsible for the specificity of the active site?
- Tertiary (3D) structure
- Held in place by hydrogen bonds, ionic bonds, disulfide bridges and hydrophilic &
hydrophobic interactions
What is an intracellular enzyme?
- Enzyme that works inside cells
- e.g. catalase
- Found inside liver cells
- Breaks down hydrogen peroxide into water and oxygen
What is an extracellular enzyme?
- Enzyme that works outside cells
- e.g. amylase
- Found in saliva and small intestine
- Breaks down amylose into maltose
- e.g. trypsin
- Protease found in small intestine
- Hydrolyses proteins into amino acids
Explain the old lock and key model of enzyme activity
- Enzymes have specific active sites to which substrate(s) are complementary
- Enzyme-substrate complex forms when substrate binds to active site
- This reduces activation energy
- Enzyme-product complex forms
- Once reaction is complete, products leave and enzyme can work again
Explain the current induced fit model of enzyme activity
- Enzymes have specific active sites to which substrate(s) bind
- Enzyme active site and substrate not perfectly complementary
- Enzyme-substrate complex forms
- Enzyme changes shape once substrate is bound
- Change in shape causes straining of bonds, weakening them
- This reduces activation energy
- Enzyme-product complex forms
- Once reaction is complete, products leave and enzyme can work again
How is the rate of a reaction calculated?
- Rate of reaction = volume of product produced / time
- Rate of reaction = volume of substrate used up / time
What conditions affect the rate of an enzyme-catalysed reaction?
- Temperature
- pH
- Substrate concentration
What is the temperature coefficient, Q10?
How much the rate of a reaction increases with a 10°C rise in temperature
What does a Q10 value of 2 signify?
- Rate of reaction doubles with each 10°C temperature increase
- Value usually shown by enzyme-controlled reactions
Describe and explain how temperature affects the rate of an enzyme catalysed reaction
Description:
- As temperature increases, enzyme activity increases
- At high temperatures, enzyme activity decreases
Explanation:
- Enzymes and substrates have more kinetic energy and collide more frequently as
temperature increases, forming more enzyme-substrate complexes in active site
- At high temperatures, enzyme is denatured and 3D (tertiary) shape of active site is
altered. No longer able to form enzyme-substrate complexes
Describe and explain how pH affects the rate of an enzyme catalysed reaction
Description:
- Enzyme activity decreases as pH deviates from its optimum
- Optimum pH for most enzymes in a human is 7 (exceptions include protease found
in stomach with an optimum pH of 3)
Explanation:
- As pH deviates from its optimum, the increase or decrease in H+ ions causes
intermolecular bonds to break (e.g. hydrogen bonds, ionic bonds)
- Tertiary structure is altered as enzyme becomes denatured
- Enzyme-substrate complexes can no longer form
Describe and explain how substrate concentration affects the rate of an enzyme catalysed reaction
Description:
- As substrate concentration increases, enzyme activity increases to a point
- Further increases in substrate concentration see no further increase in enzyme
activity
Explanation:
- As substrate concentration increases, there are more frequent collisions between
substrate and enzyme active site so rate increases
- Eventually all the enzyme active sites are occupied, so no further rate increase
takes place
- Enzyme concentration becomes limiting factor
What is Vmax?
The maximum rate of a reaction
What happens to Vmax if the enzyme concentration is increased?
- Increases
- Concentration of substrate eventually becomes limiting factor
Describe an investigation into the effect of enzyme concentration on the rate of a reaction
- Create stock solution of enzyme
- Perform a serial dilution to produce a range of enzyme concentrations
- Stock is diluted 10 fold each time using distilled water
- Add equal volumes of a given concentration of the substrate to each dilution
- Time how long it takes the enzyme to catalyse the reaction
- e.g. by using a colorimeter
Define inhibitor
Substances that reduce enzyme activity
What are the two types of inhibitor?
- Competitive
- Non-competitive
Describe how a competitive inhibitor works
- Molecule, other than the substrate, binds to the enzyme’s active site
- Inhibitor is structurally and chemically similar to the substrate so able to bind to active site
- Competitive inhibitor blocks active site and prevents substrate binding
- As inhibitor is in competition with substrate, its effects can be reduced by increasing
substrate concentration
Give an example of competitive inhibition
- Statins are competitive inhibitors of an enzyme that synthesises cholesterol
- Statins are prescribed to treat high cholesterol levels in blood
- Aspirin inhibits the active site of COX enzymes
- Prevents the synthesis of chemicals responsible for pain
Define allosteric site
Region on the enzyme other than the active site
Describe how a non-competitive inhibitor works
- Inhibitor binds to an allosteric site on enzyme
- Binding of inhibitor to allosteric site causes conformational change to enzyme’s active site
- Active site and substrate no longer share specificity
- Substrate cannot bind
- As inhibitor is not in direct competition with substrate, increasing substrate levels cannot
mitigate the inhibitor’s effect - Maximum rate of enzyme activity reduced
Give and example of non-competitive inhibition
- Organophosphates are used as insecticides and herbicides
- Inhibit acetyl cholinesterase - necessary for nerve impulse transmission
- Can lead to paralysis and death
- Protein pump inhibitors are used to treat long-term indigestion
- Block enzyme responsible for secreting H+ into the stomach
- Reduces production of excess stomach acid
Explain the control of metabolic pathways by end-product inhibition
- Metabolic pathway is a series of enzyme-catalysed reactions
- End-product acts as inhibitor of enzyme at beginning of pathway
- Inhibitor can be competitive or non-competitive
- More inhibition if end-product concentration rises
- Prevents a build-up of intermediate products until end-product level reduces
- Example of negative feedback
What is the purpose of end-product inhibition?
- Ensure levels of an essential product are tightly regulated
- If product levels build up, the product inhibits the reaction pathway and decreases the rate
of further product formation - If product levels drop, the reaction pathway will proceed unhindered and the rate of product
formation will increase
Give an example of end-product inhibition
- ATP is produced in respiration
- PFK enzyme required for first step of glycolysis (first stage of respiration)
- ATP inhibits PFK
- When ATP levels are high, more PFK is inhibited
- Rate of respiration (and ATP production) slows
- When ATP level are low, less PFK is inhibited
- Rate of respiration (and ATP production) increases
Why are inhibitors often used in medicine?
Can target pathogenic enzymes
- e.g. antimalarial medicines target enzymes involved in parasitic development
What is a cofactor?
- Non-protein components that help enzyme function
- Binds loosely to enzyme
What is the difference between a cofactor and a coenzyme?
- Cofactors are inorganic molecules
- Coenzymes are organic molecules
How are most coenzymes derived?
- From vitamins
- e.g. vitamin B3 used to synthesise NAD
How are inorganic cofactors obtained?
- Through diet
- e.g. iron, calcium, chloride and zinc ions
Give an example of a cofactor
- Chloride ions (Cl-)
- Required for amylase to form complementary active site for starch
What is precursor activation?
- Activation of an enzyme by a cofactor
Define apoenzyme
Inactive precursor protein
Define holoenzyme
Activated enzyme
What is a prosthetic group?
- Molecules that bind and form permanent feature of the enzyme
- e.g. zinc ions (Zn2+) form part of carbonic anhydrase enzyme
- Required for metabolism of carbon dioxide
