Enzymes Flashcards
What is an enzyme?
- A biological catalyst/catalyst in living cells
- Speeds up chemical reactions
What effect do enzymes have on the activation energy required for the reaction to take place?
Enzymes lower the activation energy required for the reaction to take place
What effect do enzymes have on the equilibrium of the reaction?
Enzymes do not alter the equilibrium of the reaction; they accelerate the TIME taken to reach equilibrium
Why are enzymes important?
- Vital for life: catalyse chemical reactions (our metabolism) in cells that keep us alive
- 1000s of roles in living organisms including aiding respiration, digestion, muscle and nerve function, etc
What does amylase do?
Converts starch into sugars
Where is amylase found?
In our saliva
What is lysozyme?
An antimicrobial, breaks down peptidoglycan in cell wall of bacteria, found in secretions e.g. tears, human milk, mucous
What is maltase?
An enzyme in saliva that breaks down maltose into glucose
What is the ‘lock and key’ model?
- The enzyme active site is complementary in shape to that of the substrate
- Active site is precisely shaped to hold specific substrates
What is the ‘induced fit model’?
- Active site of enzyme and substrate do not fit together exactly
- Enzyme changes shape when substrate binds
- Active site of enzyme has a shape complementary to the substrate only AFTER the substrate has bound
What are the only known enzymes that are not proteins?
Ribozymes - RNA molecules with enzymatic activity
What does sucrase break sucrose down into?
Glucose and fructose
Which 2 factors can affect enzyme activity?
pH and temperature
What can happen to an enzyme at extremes of pH or temperature?
The enzyme may become denatured (lose its 3D shape) and become totally inactive
What is the standard free energy change (△G)?
The difference between energies of the reactants and the products
What is the activation energy (EA)?
The energy input required to initiate the reaction
What does the reaction rate depend on?
- The activation energy
- Temperature
- pH
- Enzyme concentration
- Substrate concentration
- Inhibitors and activators
- Covalent modification
What effect does an increase in enzyme concentration have on the reaction rate?
Increases the reaction rate - the equilibrium is reached more rapidly
What is Vmax?
The maximum rate of reaction (enzyme is saturated with substrate)
What does the relationship between the reaction rate (V) and substrate concentration ([S]) depend on?
Affinity of the enzyme for its substrate
What is the Michaelis constant?
Km - an inverse measurement of affinity
What does Km tell us?
Km is the Michaelis constant - an inverse measurement of affinity
Km is the concentration of substrate at which the rate of reaction is half its maximum value
What does a low Km mean?
The enzyme is normally saturated with substrate and acts at a fairly constant rate regardless of variations in substrate concentration.
What does a high Km mean?
The enzyme is not normally saturated with substate, its activity will vary with as substrate concentration varies. Rate of product formation depends on substrate availability.
Why is measuring the initial rate (Vi) of an enzyme-catalysed reaction important?
Because substrate concentration changes with time
What is the optimal temperature for a typical human body enzyme?
37.5 degrees C
What is the Michaelis-Menten equation?
Represents a graph of V against [S]
V = Vmax[S]/(Km + [S])
What is Km and how is it calculated?
Km is the substrate concentration at which V = 0.5 x Vmax
Km is a measure of affinity of the enzyme for its substate
What does a low Km mean?
Enzyme has a high affinity for its substate
What is dextran?
A polymer of glucose - one of the main components of dental plaque
Which enzyme makes dextran and what is the substrate?
Dextran sucrase
Substrate = sucrose
What is the Lineweaver-Burk plot?
- A graph plotting 1/v against 1/[S]
- Give a straight line for most enzymes
What do the intercepts at the x-axis and y-axis on a Lineweaver-Burk plot tell us, respectively?
Intercept on x-axis: -1/Km
Intercept on y-axis: 1/Vmax
What effect do reversible inhibitors have on enzymes?
- Full enzyme activity is regained when the inhibitor is removed
- Most natural enzyme inhibitors fall into this category
What effect do irreversible inhibitors have on enzymes?
- Damage the enzyme beyond repair
- Generally cause covalent modification of the enzyme
Are most enzyme inhibitors reversible of irreversible?
Reversible
What is feedback inhibition?
- When the product of a metabolic pathway builds up in the cell, inhibition of the pathway often occurs
- Product of the pathway acts as an inhibitor of an earlier step in the pathway
Describe feedback inhibition of threonine deaminase by isoleucine
- If isoleucine is not metabolised by the cell, it accumulates and switches off further synthesis.
- Isoleucine acts as an allosteric inhibitor of threonine deaminase (first enzyme in the pathway)
On a Lineweaver-Burk plot, what does the steepness of the slope tell us about the speed of the reaction?
The less steep the slope, the faster the reaction is overall
What is a competitive inhibitor?
- Substrate and inhibitor compete for the same binding site on the enzyme. They are structural analogues
- Competitive inhibitors prevent the substrate from binding
- Inhibition can be overcome by adding large amounts of substrate - substrate will ultimately occupy all binding sites
- This is an example of reversible inhibition
What is a non-competitive inhibitor?
- Structurally unrelated to the substrate
Binds to an allosteric site on the enzyme i.e. one other than the active site - Alters the conformation of the enzyme so that the active site is no longer fully functional
- Inhibition cannot be reversed by adding large quantities of substrate
Describe the effects on Vmax in the presence of competitive inhibitors
- Vmax is unaltered because a large amount of the substrate can outcompete the inhibitor
Describe the effects on Km when competitive inhibitors are added
- Km is increased - as the inhibitor competes with the substrate, the affinity for the the substrate will appear to decrease
- (Km is an inverse measurement of affinity of enzyme for substrate)
Describe the effects on Vmax when non-competitive inhibitors are added
- Vmax is decreased: a proportion of enyzmes molecules will be inactive at any time due to binding by the inhibitor
Describe the effect on Km when non-competitive inhibitors are added
- Km is unchanged: binding of substrate can still occur
What is an allosteric enzyme?
- Allosteric enzymes possess multiple subunits
- In addition to active sites, they also possess regulatory (allosteric) sites to which non-substrate modulators bind, creating conformational change in subunits
What effect does a positive modulator have on an allosteric enzyme?
Increases affinity for substrate at active site
What effect does a negative modulator have on an allosteric enzyme?
Decreases affinity for substrate at active site
Which type of enzymes do NOT conform to Michaelis-Menten kinetics?
- Allosteric enzymes
- Graph of V vs [S] gives a sigmoidal rather than hyperbolic curve; Lineweaver-Burk plot cannot be used
Which amino acids is a phosphate group often adde to in covalent modification of enzymes by another enzyme?
A common modification is the addition of a phosphate group to serine, threonine or tyrosine hydroxyl group
What does glycogen phosphorylase do?
An enzyme in liver and muscle which breaks down glycogen to provide glucose; it is activated by phosphorylation
Which enzyme phosphorylates glycogen phosphorylase?
Phosphorylase kinase
Which enzyme removes the phosphate group on glycogen phosphorylase?
Phosphorylase phosphatase
