Enzymes Flashcards
Enzyme
biological catalysts which interact with substrate molecules to facilitate chemical reactions. Usually globular proteins
Substrate
the substance which is used up in a chemical reaction, leading to the formation of a product. It fits into the active site of an enzyme.
why are enzymes necessary to life?
remove the need for extreme conditions to make reactions happen quickly, thus enabling many chemical processes to happen by biologically speeding up reactions.
Anabolic reactions
- reactions that construct molecules from smaller units.
- require energy from the hydrolysis of ATP
- required for growth
Catabolic reactions
- reactions that break down molecules into smaller units.
- release energy.
Metabolism
- the chemical reactions which take place in the cell of an organism (anabolic and catabolic)
- multistep process, each catalysed by a diff enzyme
Vmax
maximum initial velocity or rate of an enzyme catalysed reaction
ie. when all active sites are occupied with substrate molecules and every enzyme is working as fast as it can (under a particular set of conditions)
intracellular enzyme
an enzyme which is found within a cell
extracellular enzyme
an enzyme which is found outside of a cell
activation energy
energy required for a reaction to start
the amount of energy req. for the bonds to start to break (or form)
describe the lock and key hypothesis
- substrate fits into active site due to their complementary shapes
- enzyme binds to substrate and an enzyme-substrate complex is formed.
- substrate reacts and an enzyme-product complex is formed
- products are released leaving enzyme unchanged
what’s the difference between enzymes and chemical catalysts?
- enzymes are specific, chem. catalysts are not.
- chemical catalysts are used up and give unwanted products
what does catalase break down?
H2O2 (hydrogen peroxide) into water and oxygen
describe the induced fit hypothesis
- active site changes shape as the enzyme substrate complex forms
- tertiary structure of the active site is changed by weak initial reactions between active site and substrate, which strengthens binding
- this weakens bonds in the substrate, lowering Ea for reaction
what affects the rate of an enzyme controlled reaction?
- temperature
- pH
- substrate concentration
- enzyme concentration
Suggest how the R groups of amino acids are involved in catalysing reactions
The substrate is held by the enzyme so closely that the R groups of the amino acids interact and form temporary bonds, putting strain on the substrate and helping the reaction to progress
What kind of proteins are enzymes?
globular
Why are globular proteins soluble?
hydrophobic R groups are folded into the middle of the protein and hydrophilic R groups are on the outside, so the protein dissolves in water
What happens if you heat an enzyme?
- increase in thermal energy
- increase in kinetic energy
- bonds vibrate
- bonds break: hydrogen &hydrophobic/philic interactions, ionic bonds, disulphide bridges
- change in tertiary structure of protein- active site changes shape and is no longer complementary to the substrate
- rate of reaction decreases
How does the presence of an enzyme affect the activation energy?
Enzymes lower the activation energy of a reaction by providing an alternate activation pathway with a lower activation energy.
They bind temporarily to the substrate between R groups on a.a chains.
What factors affect rate of reaction?
- substrate conc
- enzyme conc
- temperature
- pH
(-cofactors and coenzymes)
How do you control temperature during reaction?
Thermostatically controlled water bath
How do you control pH during a reaction?
pH buffer
How does temperature affect r.o.r?
- increase in thermal energy
- increase in KE
- molecules move faster and collide more often
- more enzyme-substrate complexes formed
HOWEVER…
- enzymes are proteins so when temperature exceeds optimum, it begins to denature (bonds break and active site changes shape)
What is the temperature coefficient? Q10
a measure of how much the r.o.r increases with a 10C rise in temperature.
Enzyme-catalysed reactions have a q10 of 2 ie. r.o.r doubles for every increase in 10C
How does pH affect r.o.r
- as the pH of a solution changes, the charges on the R groups of amino acids change so the bonding between amino acids changes (hydrogen and ionic)
- this changes the tertiary structure of a protein, so the active site changes shape and is no longer complementary to it’s substrate and no more enzyme-substrate complexes form
- most enzymes work in a narrow pH range
What is pH a measure of?
The concentration of H+ ions in solution
How does substrate concentration affect r.o.r?
- More substrate molecules
- more successful collisions with active sites of enzymes
- more enzyme-substrate complexes
- more product formed per time
- until Vmax, substrate conc is a limiting factor as it is in short supply
HOWEVER…
- Vmax is reached, where all active sites are engaged and enzymes are working at their maximum rate, are said to be ‘saturated’
- r.o.r plateaus
How does enzyme concentration affect r.o.r?
SIMPLY,
- while substrate is in excess, the more enzymes, the more active sites available, the higher the r.o.r as enzymes are the limiting factor.
Cofactor
- inorganic (usually mineral ions)
- enzymes cannot function w/out
- loosely binds to enzyme to activate it
- transfers the products of a reaction in a metabolic pathway
eg. DNA polymerase uses Mg2+ to stabilise -ve charge of phosphate groups on DNA
Coenzyme
- organic (carbon containing) cofactors
- used up by enzyme reaction
- bind temporarily to active site
- carry chemical groups between enzymes
eg. NAD+ carries electrons for an alcohol dehydrogenase
eg. coenzyme A carries acyl group
Prosthetic groups
- permanently bound to enzymes
Apoenzyme
- precursor enzyme before cofactor is added
inactive enzyme which needs cofactor to activate it
Holoenzyme
- activated enzyme formed when a cofactor is added to an enzyme
Explain enzyme specificity (3)
- shape of active site;
- is complementary;
- correct shape / correct molecule / correct substrate / C, will, fit /form Enzyme Substrate Complex;
- any other shape / any other molecule / any other substrate will not fit
For the induced fit hypothesis, both the active site and the substrate change shape slightly. Suggest how the substrate changing shape will assist enzyme action.(1)
- puts strain on bonds in substrate so they will break more easily;
- lowers Ea
Explain how DNA structure determines the specific shape of enzymes
- DNA codes for, protein / polypeptide;
- transcription and translation (or described);
- enzyme is globular (protein);
- sequence of bases / triplets, determines, sequence of amino acids /
- primary structure;
- coiling / a helix / b-pleated sheet / particular secondary structure;
- determines projecting side groups;
- folding / bonding, for tertiary structure;
- 3-D structure is tertiary structure;
Why does increasing pH mean an enzyme becomes inactive?
- (change in pH/[H+]) alters charge distribution on (enzyme) molecule;
- hydrogen / ionic, bonds affected;
- loss of tertiary structure / loss of 3D structure / (enzyme) denatured;
- changes (shape of) active site;
- enzyme substrate complex cannot be formed /
substrate not attracted to active site /
substrate cannot bind to active site / AW;
Why can this … act as a competitive inhibitor?
1 substrate and, inhibitor similar shape;
2 able to, bind / fit into / block, active site;
3 (shape) complimentary to active site;
How does a non-competitive inhibitor affect the rate of an enzyme-catalysed reaction?
- fits into, allosteric site / site other than active site;
- alters, shape / charge, of active site;
- so substrate cannot fit/ bind to active site / form ESC;
- will not reach Vmax;
- reduces rate; A stops R inhibits
increasing substrate concentration has no effect (on the rate);
What are inhibitors?
Molecules that prevent enzymes from carrying out their function
How does competitive inhibition work?
Molecule with similar shape to an enzyme’s substrate fits into its active site, blocking the substrate from entering. The enzyme cannot carry out its function and is said to be inhibited.
- substrate and inhibitor molecules compete for the active site
- most only bind temporarily to the active site of the enzyme so the effect is reversible
What effect does a competitive inhibitor have on the rate of reaction?
- reduces r.o.r
- does not change Vmax of the enzyme it inhibits as if the substrate concentration is increased enough Vmax can still be reached
Name two examples of competitive inhibition
Statins are competitive inhibitors on an enzyme used in the synthesis of cholesterol. Prescribed to help people reduce their blood cholesterol concentration.
Aspirin irreversibly inhibits the active site of COX enzymes. This prevents the synthesis of prostaglandins and thromboxane, chemicals which produce pain and fever
How to non-competitive inhibitors work?
Inhibitor binds to enzyme at allosteric site. Binding changes tertiary structure of enzyme, changing shape of active site. This means active site is no longer complementary to substrate molecule so can no longer carry out its function.
Effect of non competitive inhibitor on rate of reaction
Increasing substrate or enzyme conc will not overcome the effect of the non competitive inhibitor.
The more you increase the conc of the inhibitor, the more the rate of reaction decreases as more active sites become unavailable.
End product inhibition (non-competitive)
Enzyme inhibition when the product of a reaction acts as an inhibitor to the enzyme that produces it. This serves as negative feedback as excess products are not made and resources are not wasted.
