Enzymes Flashcards
What are enzymes?
Biological catalysts that speed up metabolic reactions in living organisms but remains unchanged at the end of the reactions
What is an enzymes turnover number?
The number of reactions that an enzyme molecule can catalyse
What makes enzymes better than chemical catalysts?
Are able to function in conditions that sustain life - neural pH, lower temperature and normal pressures
More specific
Don’t produce unwanted by-products
Rarely make mistakes
What is an enzymes active site?
An indentation of the molecule consisting of a few amino acids
How can an enzyme’s ability to catalyse a reaction be altered by changes in pH or temperate?
The shape of the active site can change as the change in pH or temperate can affect the bonds that hold proteins in their tertiary structure
Why is the tertiary structure of the active site crucial?
It’s shape is complementary to the shape of the substrate molecules
Intracellular enzymes
Metabolic pathways are series of consecutive reactions with each step catalyses by specific enzymes
Reactants and intermediates = substrates for specific enzymes
Reactants, intermediates, products = metabolites
Catabolic metabolic pathways
Anabolic metabolic pathways
Catalase
Protects cells from damage by reactive oxygen by breaking down hydrogen peroxide to water and oxygen
4 polypeptide chains and a haem group
Fastest acting enzyme with highest turnover number
Found inside peroxisomes inside eukaryotes
Used by WBC to help kill pathogens
Optimum pH 7 in humans but varies in other species
What are extracellular enzymes?
Secreted from cells and act on their substrates extracellularly
Give some examples of extracellular enzymes
Bread mound fungi release hydrolytic enzymes from their hyphae and they digest carbohydrates, proteins and lipids in bread and the products of digestion are absorbed into the hyphae for use
Enzymes are secreted in our digestive system where they digest the large molecules found in food, products are then absorbed into the blood stream for use
What is the function of amylase and trypsin?
Amylase is produced in the salivary glands and digests starch to maltose in the mouth
It’s also made in the pancreas and catalyses same reaction in he small intestine
Trypsin is naked in the pancreas and digests proteins into smaller peptides by hydrolysing peptide bonds in the small intestine
What is a cofactor?
A non protein molecule that have to be present to ensure some enzymes work some are part of the enzyme structure and some for temporary associations
What is a prosthetic group?
A cofactor that is permanently bound by covalent bonds to an enzyme
Give an example of an enzyme with a prosthetic group
Carbonic anhydride constrains a zinc ion permanently bound to its active site
How do some cofactors ease the formation of some enzyme-substrate complexes?
They and the substrate together form the correct shape to bind to the active site of the enzyme (co-substrates)
They can change the charge distribution on the surface of the substrate or of the active site and make the temporary bond in the enzyme-substrate complex easier to form
What are coenzymes?
Small non protein molecules that bind temporarily to the active site of the enzyme, they are chemically changed during the reaction and need to be recycled to their original stage
What is the lock-and-key hypothesis?
The shape of the active site is complementary to that of the substrate so only a specific substrate molecule will activate the enzyme
Substrate and enzymes constantly moving A collision is successful = ES complex Substrate either broken down or built up into the product molecule = EP complex Product leaves active site Enzyme can now form another ES complex
What is the induced-fit hypothesis?
Active site changes shape to mound itself round the substrate to give a more precise confirmation that exactly fits
Enables substrate and active site to bind more effectively
ES complex formed
EP complex formed while still in the active site
Product detached from active site due to slightly different shape
Enzyme free to catalyse another reaction
Why is the induced-fit hypothesis better than the lock-and-key hypothesis?
It does not explain how the transition state/ES complex is stabilised
How is Koshland modify the lock-and-key hypothesis by suggesting?
That the active site of the enzyme is not a rigid fixed structure but that the presence of the substrate molecule in it induces a shape giving it a good fit
What is activation energy?
The energy need to activate or begin chemical reactions
How do enzymes lower the activation energy of a reaction?
Because they have an active site specific to only the substrate molecules they bring the substrate molecules close enough the reach without the need for excessive heat
Is a substance is heated then
The extra heat energy causes molecules to move faster
Increasing the rate of collision
And the force with which they collide
What will happen is the reactant mixture containing enzyme and substrate molecules is heated?
Both types of molecule will gain kinetic energy and move faster
Increasing the rate of successful collisions
Increases the rate of formation of ES complexes and rate of reactions therefore the number of EP complexes up to a point
At a particular temperature the rate of reaction is at its maximum
What effect does the vibration have on molecules?
Some bonds that hold the active site’s tertiary structure may break
The substrate will not fit into it so well and rate of reaction will decrease
As more heat is applies the shape of the active site will irreversibly change so that it’s no longer complementary to the substrate
Reaction can’t proceed
Enzyme denatured
What is optimum temperature?
The temperature at which the enzyme works best and at which it has its maximum rate of reaction
What is the temperature coefficient?
The increase in the rate of a process when the temperature is increased by 10’C
How do you work out he temperature coefficient?
Rate of reaction at (T+10)’C /
Rate of reaction at T’C
What is a buffer?
Something that resists changes in pH
What effect do excess hydrogen ions have on enzymes?
They interfere with the hydrogen bonds and ionic forces holding the tertiary structure of the active site together and so the active site will change shape
They alter the charges on the active site of enzyme molecules as more protons will cluster round negatively charged groups in the active site which interferes with the binding of a ES complex
Why do extracellular enzymes have different optimum pH values?
Because they work in different parts of the body which have values that they work best at
What is the effect of increasing substrate on rate of reaction?
More ES complexes can form
More product molecules can form
Substrate concentration becomes limiting factor because as it increases, the rate of reaction increases
What will happen if the concentration of substrate is increased further?
The reaction will reach its maximum rate
Adding more won’t increase rate of reaction as all the enzyme’s active sites are occupied with substrate molecules
If more substrate is added they can’t collide with fit into an active site successfully
What does the enzyme availability depend on in living cells?
Rate of synthesis
Rate of degradation
What is enzyme synthesis?
Switching on it or genes for synthesising particular enzymes depending on the cell’s needs
What is enzyme degradation?
When cells continuously break down old enzyme molecules to their component amino acids and synthesise new enzyme molecules from amino acids
What are advantages to enzyme degradation?
The elimination of abnormally shaped proteins that might otherwise accumulate and harm the cell
The regulation of metabolism in the cell by elimination any superfluous enzymes
What happens as enzyme concentration in an enzyme-controlled reaction increases?
More active sites in the enzyme become available
Meaning more successful collisions between the enzyme and substrate occur
More ES complexes can form per unit time = increases rate of reaction
Enzyme concentration becomes the limiting factor - as it increases, so does the rate of reaction
When does the enzyme concentration stop being the limiting factor?
When substrate concentration is fixed or limited as all substrate molecules will be occupying an active site and been released as product molecules
Why does the rate of reaction decreases over time?
When enzyme and substrate molecules are first mixed there is a great chance of successful collisions
As the reaction proceeds, substrate is used up as they are converted to product so substrate concentration drops
As a result, the frequency of successful collisions decreases
What are enzyme inhibitors?
Substances that reduce the activity of an enzyme by combining with it in a way that influences how the substrate binds to the enzyme or affects its turnover number
Some may block the active site and some change its shape
What is the difference between competitive and non-competitive inhibitors?
Competitive inhibitors are substances who molecules have a similar shape to an enzyme’s substrate molecules and so competes with the substrate for the active site
Non-competitive inhibitors are substances that attaches to a different part of the enzyme changing the shape of the active site and preventing ES complexes forming
Describe competitive inhibition
Form an enzyme-inhibitor complex that is catalytically inactive
Inhibitor is not changed once on the active site
Reduces rate of formation of ES complexes as reduces number of free active sites
Reversible - increase the substrate concentration would reduce the effect
When is a competitive inhibitor called an inactivator?
If it binds irreversibly to the enzyme’s active site
What is an allosteric site?
The region non-competitive inhibitors attach to on enzyme molecules
Describe non-competitive inhibition
Inhibitor binds to allosteric site distorting the shape of the active site so it’s not complementary to the substrate meaning ES complexes cannot form
Some bind reversibly and others bind irreversibly
What is end-product inhibition?
An example of negative feedback in which enzyme-catalyses reactions may be regulated
After the reaction is completed, product may stay bound to the enzyme so the enzyme cannot form more of the product than the cell needs
What is a metabolic pathway?
When the product of one enzyme-catalyses reaction becomes the substrate for the next
How are metabolic sequences controlled?
The product of the last enzyme-catalyses reaction in a metabolic pathway may attach to a part of the first enzyme in the pathway but not at its active site changing the shape of its active site preventing the pathway from running
When the concentration of the product falls, the molecules detach and allow the active site to resume its normal shape and the pathway can run again
How do multi-enzyme complexes increase the efficiency of metabolic reactions without increasing substrate concentration?
They keep the enzyme and substrate molecules in the same vicinity and reduce diffusion time
What are some examples of toxins that act as enzyme inhibitors?
Cyanide
- inhibits aerobic respiration
- hydrolysed when ingested to produce hydrogen cyanide (toxic gas)
- this can dissociate into H+ ions and CN- ions
- CN- ions bind irreversibly to enzyme in mitochondria and inhibit final stage of aerobic respiration
Snake venom
- contains chemical that inhibits AChE (breaks down acetylcholine as neuromuscular synapses)
- keeps muscle contracted
- causes paralysis, if the muscles involved in breathing paralyse - victims suffocate
What medicinal drugs act by enzyme inhibition?
Aspirin
- prevent formation of prostaglandins (cell signalling molecules produced when tissues are infected or damaged making nerve cells more sensitive to pain and increase swelling)
- reduce risk of blood cord
ATPase inhibitors
- extracts from foxglove leaves to treat heart failure
- inhibit the sodium potassium pump in the membranes of heart-muscle cells and allow more calcium ions to enter increasing muscle contraction strengthening heartbeat
ACE inhibitors
- inhibit ACE that increases blood pressure
- used to lower pressure in people with hypertension
- used to treat heart failure
- used to minimise risk of second heart attack or stroke
Protease inhibitors
- used to treat some viral infections
- prevent replication of virus particles within host cells by inhibiting protease enzymes
Nucleoside reverse transcriptase inhibitors
- used to treat patients who are HIV positive
- inhibit enzymes involved in making DNA using RNA as a template
Why should children under 12 not take aspirin?
As it can damage their stomach lining
