Flashcards in MCGB - Enzyme Activity Deck (32):
What is the name given to the high energy intermediate that lies between S and P in an enzyme reaction?
What is the activation energy?
The minimum energy that substrates must have to start a reaction.
Why does increasing the temperature increase the rate of reaction?
Increases number of molecules with activation energy.
Why does increasing the concentration increase the rate of reaction?
Increases chance of molecular collisions.
What are enzymes?
Biological catalysts that increase the rate of reaction by lowering the activation energy required. This facilitates formation of the transition state.
Give an exception to the statement that all enzymes are proteins
Some RNA molecules act as enzymes.
Do enzymes affect the reaction equilibrium?
If the active site of an enzyme is only made of a few amino acids out of hundreds, what is the role of the rest of the enzyme?
A scaffold to create the active site
True or false - the active site is formed by amino acids from different parts of the primary sequence?
What sort of reaction takes place when the substrate molecule binds to the active site?
What is the difference between the lock-and-key and the induced fit model of enzyme action?
Lock and key states that the active site of the enzyme is already complementary to the shape of the substrate, while induced fit states that the active site only forms a complementary shape AFTER the binding of the enzyme.
True or false - substrates bind to enzymes by covalent bonds?
False - covalent is too tight. They must be multiple weak bonds.
What does V0 mean?
Initial rate of reaction.
What shape would a graph of reaction rate against substrate concentration be?
A rectangular hyperbola.
What does the Michaelis-Menten model suggest about enzyme catalysis?
A specific complex is a necessary intermediate, and that a plot of V0 vs [S] will be a rectangular hyperbola.
What does Vmax refer to?
Maximal rate when all enzyme active sites are saturated with substrate (mol/min)
What does Km refer to?
Substrate concentration that gives half maximal velocity
What can Km values be used to indicate?
They give a measure of the affinity of an enzyme for its substrate.
What does a low Km indicate?
Enzyme has high affinity for substrate.
What does a high Km indicate?
Enzyme has low affinity for substrate.
Hexokinase has a Km of 0.1mM and is always active. Glucokinase has a Km of 5mM and is only active after eating food. What is the reason for these different Km values?
HK has low Km and therefore high affinity. GK has high Km and low affinity. As GK is active after eating, it has to digest large amounts of glucose, so it has low affinity because so much glucose is available to it. HK serves to "clean up" the left over glucose and must have high affinity to be able to do this.
What is one "unit" in terms of enzymes?
The amount of enzyme that converts 1 micromole of product per minute under standard conditions.
Often expressed per litre of serum or per gram of tissue
Give two ways Km can be estimated from a Lineweaver-Burk plot.
Slope = Km/Vmax
X Intercept = -1/Km
How can Vmax be estimated from a Lineweaver-Burk plot?
Y Intercept = 1/Vmax
What sort of bond does an irreversible enzyme inhibitor generally form?
Covalent bonds, which are very tight.
Give an example of an irreversible enzyme inhibitor.
Nerve gases such as sarin.
What are the two types of reversible enzyme inhibitor? Do they affect Km or Vmax?
Competitive (binds at active site) - affects Km not Vmax
Non-competitive (binds at another site on enzyme) - affects Vmax not Km.
Why doesn't competitive inhibition affect the Vmax?
Adding enough substrate will always overcome the effect of the inhibitor, so Vmax unaffected. Inhibitor competes for active site, so Km increases.
True or false - compounds binding outside of the active site can activate rather than inhibit enzymes?
Why do non-competitive inhibitors affect Vmax?
They bind at an alternative site, decreasing turnover number of enzyme which lowers Vmax.
How could Km be calculated from substrate concentration at Vmax?