2.4 - Enzymes Flashcards
Induced fit hypothesis mechanism
- Enzymes have active sites complementary to the substrate molecule
- When the substrate molecules fit into the enzyme’s active site, the active site changes shape slightly to mould itself around the substrate
- An enzyme-substrate complex is formed, and non-covalent forces such as hydrogen bonds, ionic attractions, London forces and hydrophobic interactions, bind the substrate molecule to the enzyme’s active site.
- The bonds in the tertiary structure of the substrate destabilise
- When the substrate molecules have been converted to the product molecules and these are still in the active site, they form an enzyme-product complex.
- As the product molecules have a slightly different shape from the substrate molecule, they detach from the active site.
- The enzyme molecule is now free to catalyse another reaction with another substrate molecule of the same type.
Induced fit hypothesis short mechanism
- Enzyme active site complementary to substrate
- Substrate binds to enzyme active site
- Enzyme substrate complex forms(ES)
- Bonds in substrate destabilise
- Enzyme-product complex forms
- Products not complementary to enzymes active site
- Product released from active site
- Enzyme active site available to catalyse breakdown of another complementary substrate
Competitive inhibitors mechanism
- The inhibitor has a shape similar to the substrate
- They both compete for the active site, consequently fewer Enzyme-Substrate complexes can be formed
- so the reaction rate is lower than normal
- at higher doses of substrate the inhibition can be overcome as the substrate will greatly outnumber the inhibitor molecule and most collisions will actually form Enzyme-Substrate complexes
Non-competitive inhibitors mechanism
- This binds to the allosteric site away from the active site
- this causes the tertiary structure of the enzyme to change
- this changes the shape of the active site so it is no longer complementary to the substrate
- fewer Enzyme-Substrate complexes can form
- reaction rate is lowered
- higher substrate concentrations cannot overcome the inhibition because the limiting factor is the number of active sites
Factors affecting rate of enzyme-substrate complex formation
- pH
- Temperature
- Surface Area
- Enzyme concentration
- Substrate Concentration
Non-competitive inhibitors on graph
H
Competitive inhibitors on graph
H
Cofactor def
- A substance that has to be present to ensure that an enzyme- catalysed reaction takes place at the appropriate rate.
Coenzyme def
- Small organic, non protein molecules that are able to bind to an active site at the same time as the substrate or just before.
Active site def
-Indented area on the surface of an enzyme molecule, with a shape that is complementary to the shape of the substrate molecule
Catalyst def
- Chemical that speeds up the rate of a reaction by providing an alternative reaction pathway with lower activation energy
- Not used up or chemically changed during reaction
What are enzymes?
Biological catalysts
Features of catalysts
- Not used up in chemical reaction
- Not chemically changed during reaction
Metabolism def
The chemical reactions that take place inside living cells or organisms
Effect of temperature on enzyme activity
-As molecules are heated they gain kinetic energy and move around faster -This results in more frequent successful collisions
• This results in more ESCs forming and therefore a higher (max) rate of reaction and more product formed(more EPCs form)
Effect of pH on enzyme activity
Model answer
• Change in pH or H+ ions alters distribution of charge on the enzyme molecule
• This causes the hydrogen and ionic bonds to be disrupted
• This means the enzyme loses its tertiary structure
• This changes the shape of the active site of the enzyme
• Substrates are no longer attracted to the active site beause the H+ ions
have altered its charge
• Substrates can’t bind to the active site as it is no longer complementary
• No ESCs can form = no product = no reaction
• Enzymes are denatured at extremes of pH (for that enzyme)
Effect of temperature above optimum temperature
-Model answer
- Molecules have more kinetic energy
- Molecules/bonds vibrate too much and weaker bonds are broken (ionic and H) •The tertiary structure of the enzymes are changed
- This means the active site loses its complementary shape
- No ESCs can form as substrate doesn’t fit into active site
- The enzymes have denatured
- This is irreversible so reaction stops
What is pH a measure of?
- pH is a measure of H+ ions
- More H+ ions = more acidic conditions
Effect of optimum temps and pH on enzyme
- Different enzymes have different optimum pHs.
- At their optimum pH, the conc of H+ ions gives the tertiary structure the best shape = most complementary active site.
- At their optimum temperature, the enzymes tertiary structure is at its most stable, as ionic and hydrogen bonds aren’t vibrating too much
- So active site is at its most complementary
What is Q10?
- It is the temperature coefficient
- A measure of the rate of change of reaction when temperature is increased by 10 Degrees
- Most enzymes have a Q10 of about 2
- Means that rate of reaction doubles when temperature increases by 10 Degrees
What is enzyme specificity?
H
Vmax def
- The maximum rate of an enzyme catalysed reaction
- Specific to the type of enzyme
Effect of increasing enzyme concentration on enzyme activity
- No enzyme = no ESC formation = no enzyme catalysed reaction
- More enzyme = more active sites
- More ESCs form so more product = higher rate of reaction
- As long as substrate is in excess, rate of reaction will increase with increase in enzyme concentration
- After a point all substrate molecules are occupying active sites, and a maximum rate will be reached
- Increasing enzyme conc. will have no more effect on the rate, as substrate conc. is in the limiting factor
- When substrate conc. becomes limiting, the rate will decrease as the substrate is used up
Effect of increasing substrate conc. on enzyme activity
- No substrate = no ESC formation = no enzyme catalysed reaction
- More substrate = higher rate of enzyme activity
- More ESCs form so more product forms = higher rate of reaction
- As long as enzyme conc. is in excess, rate of reaction will increase with increase in substrate concentration
- After a point all enzyme molecules are occupied active sites, and a maximum rate will be reached
- Increasing substrate conc. will have no more effect on the rate, as enzyme conc. is in the limiting factor
- When enzyme conc. becomes limiting, the rate will plateau, as all active sites are full, so no more ESCs can form
End-product inhibition def
Enzyme inhibition that occurs when the product of a reaction acts as an inhibitor to the enzyme that produces it
Explain how cofactors increase the activity of enzymes
- Cofactors bind to the enzymes active site
- The cofactors bind to the enzyme temporarily
- They change the shape of the tertiary structure of the active site
- They affect the charges on the active site
- Cofactors bind to/interact with the substrate molecule
- This increases the likelihood of ESC formation on enzymes active site
- The cofactors can carry a (named) chemical to a (named) enzyme
Why are the lock and key and induced fit hypothesis seen as models?
They are simple visual representations of how the process/mechanism of enzyme action works
Suggest why most scientists now accept the induced-fit model rather than the lock-and-key model.
There is more research and evidence to support it
Induced fit hypothesis fits evidence more than lock-and-key mechanism
Suggest how the structure of the same enzyme may differ in organisms that live in different climates
Different no. or type of amino acids in primary structure of enzyme
Named difference in secondary structure - alpha-helixes or beta-pleated sheets
Difference in no. of bond types in tertiary and quaternary
Examples of cofactors and what enzyme they help function
Need to do…
Examples of coenzymes and what they help function
Hbebsubejdd
Some enzymes work better in the presence of other molecules or ions. Explain how these molecules or ions increase the activity of enzymes.
(5 Marks)
- Cofactors and coenzymes can bind to an enzyme’s allosteric site.
- This causes interactions between hydrogen bonds in the tertiary structure of the protein to change
- This alters the enzyme structure
- The change of structure therefore induces a change in the shape of the active site,
- Makes active site a more complementary shape to its substrate.
- This allows for the enzyme-substrate complex to be formed more easily
- This increases the activity of the enzyme
Humans and grasshoppers have very similar percentages of each base in their DNA but they are very different organisms.
Use your knowledge of DNA structure and fucntion to explain why this is possible.
(2 Marks)
- Both orgnanisms have different genes
- Codons/bases may be arranged in a different sequence
- So code for different amino acids/sequences
- different primary structure of proteins
- different protien tertiary structure
- different protein function
Explain how the structure of DNA is related to its functions.
(6 Marks)
- Sugar-phosphate (backbone) / double stranded / helix so provides strength / stability / protects bases / protects hydrogen bonds;
Must be a direct link / obvious to get the mark
Neutral: reference to histones - Long / large molecule so can store lots of information;
- Helix / coiled so compact;
Accept: can store in a small amount of space for ‘compact’ - Base sequence allows information to be stored / base sequence codes for amino acids / protein;
Accept: base sequence allows transcription - Double stranded so replication can occur semi-conservatively / strands can act as templates / complementary base pairing / A-T and G-C so accurate replication / identical copies can be made;
- (Weak) hydrogen bonds for replication / unzipping / strand separation / many hydrogen bonds so stable / strong;
Accept: ‘H-bonds’ for ‘hydrogen bonds’
How to work out Q10
Rate of reaction at (T+10)C / rate of reaction at TC
- C = degrees celsius
Cofactor def
A substance that has to be present to ensure that an enzyme-catalysed reaction takes place at the appropriate rate
- some are part of the structure - prosthetic groups
- so form temporary associations with the enzyme (organic coenzymes, mineral ion cofactors)
Extra cellular enzymes examples
Amylase - in salivary glands
Trypsin - in pancreas
Intracellular enzymes examples
Catalase - found in most cells - breaks down H2O2 - hydrogen peroxide - a toxic substance
- breaks down into water and O2
Coenzymes need to know
Cl- = cofactor for amylase Zn2+ = prosthetic group for carbonic anhydride Fe2+ = prosthetic group for haem group in haemoglobin molecule
What is a good source of coenzymes in our diet
Vitamins are a good source of minerals in diet
- they are used to make coenzymes
- e.g.
Examples of vitamins and the coenzymes they are used to produce
Different between coenzymes and cofactors and prosthetic groups
Coenzymes - organic molecules
- bind loosely/temporarily to active site of enzymes
- aid in binding to enzyme binding to substrate
Cofactors - inorganic molecules
- do not bind to enzymes
- ensure enzyme-catalysed reactions take place at appropriate/fast enough rate
Prosthetic group: usually metallic molecules
Bind to enzyme permanently - part of structure
