Enzymes Flashcards
What is the transition state in a reaction?
The high energy intermediate that lies between substrate and product in a chemical reaction
What is the activation energy for a reaction? What does it determine?
The minimum energy the substrate must have to allow the reaction to occur
How likely a reaction is to occur
In which two ways can increase the rate of reaction?
Changing the;
Temperature
Concentration
How does increasing the temperature affect the rate of reaction?
Increases the rate —-> increased number of molecules with activation energy
How does an increased concentration of substrate increase the rate of reaction?
Increased chance of molecular collisions
What is an enzyme?
Biological catalysts that increase the rate of reaction by lowering the activation energy
Enzymes promote the formation of ……
The transition state
Do enzymes affect the reaction equilibrium?
No
Are enzymes changed after a reaction?
No they remain unchanged
What is the active site of an enzyme? What shape do they usually take?
Where the substrate binds and where the chemical reaction occurs
Clefts or crevices
Which two models exist to describe how substrates bind to an active site? Which one is more valid?
Lock and key model
Induced fit model
Induced fit hypothesis is considered to be more correct
Describe the induced fit hypothesis of substrate binding to active site.
The binding of a substrate induces changes in the conformation of an enzyme —> still has a similar complementary shape to the active site
What is V0?
The initial rate of reaction (at t=0)
A tangent to the start of an enzyme reaction curve will show what?
The fastest rate of reaction
What shape curve do enzyme rate of reaction curves usually show?
Rectangular hyperbola
What is the relationship between substrate concentration and rate of reaction?
Increasing the [substrate] will increase the rate of reaction up until a certain point (maximum velocity)
What two outcomes are there for an enzyme-substrate complex?
Can either form product and enzyme or dissociate back to form enzyme and substrate
What does the Michalis-menten equation predict?
That a plot of V0 against [S] will be a rectangular hyperbola
What is Vmax?
The maximal rate of reaction when all enzyme active sites are saturated with the substrate
What is Km?
The [substrate] that gives 1/2 of Vmax
What is the relationship between the Km of an enzyme and its affinity for its substrate?
Low Km = high affinity
High Km = low affinity
What is the difference in affinity and activity of hexokinase vs glucokinase?
Hexokinase is always active - has a low Km and high affinity
Glucokinase is only active when glucose levels peak after feeding - has a higher Km and lower affinity
Vmax/V0 values are rates, how are they measured?
In amounts per unit time
What is 1 unit?
The amount of enzyme that converts 1umol of substrate per minute (under standard conditions)
What is the relationship between the rate of an enzyme catalysed reaction and the [enzyme]?
Rate is proportional to the [enzyme]
What can be plotted to allow easier estimation of Vmax and Km? What shape does it take?
A lineweaver-Burk plot
Straight line, linear graph
How is a lineweaver-Burk graph plotted?
Take reciprocals of V and [S]
On a lineweaver-Burk plot, the slope (the gradient) is equal to…
Km/Vmax
On a lineweaver-Burk plot the y intercept will be equal to…
1/Vmax
On a line weaver-burk plot the x intercept will be equal to…
-1/Km
What are three types of enzyme inhibition?
1) irreversible
2) reversible, competitive
3) reversible, non-competitive
What happens during irreversible enzyme inhibition? Give an example of an irreversible enzyme inhibitor.
The molecule/drug binds very tightly, usually forming covalent bonds
Nerve gases e.g. Sarin
What is common with all reversible enzyme inhibitors?
They don’t form covalent bonds and can freely dissociate
What happens in reversible, competitive enzyme inhibition? What is the effect on Km and Vmax?
Molecule binds to active site.
Affects Km (increases) and not Vmax
What happens in reversible, non-competitive enzyme inhibition? What is the effect on Km and Vmax?
Binds to another site on the enzyme
Affects Vmax (lowers) and not Km
Why does competitive inhibition have no effect on Vmax?
Increasing the substrate concentration enough will always overcome the effect of the inhibitor - Vmax can still be reached
How does a non-competitive enzyme inhibitor lower the Vmax?
Decreases the turnover number of the enzyme
What reaction does creatine kinase catalyse?
Creatine + ATP —> Phosphocreatine + ADP
Serum levels of creatine kinase are indicative of what?
Myocardial infarction if levels are high as it is released by damaged myocardial cells
