Enzymes 1 Flashcards
What are virtually all reactions in the body mediated by?
Enzymes
What is an enzyme?
A biological catalyst that increases the rate of a reaction without being changed in the overall process
How are enzymes efficient?
They catalyse at a very high reaction rate
What is meant by the specificity of enzymes?
They are very specific, with only certain substrates binding to them
Are can enzyme reactions be controlled?
They can be regulated
What pH and temperature conditions do enzymes work in?
Mild
How do the set of enzymes in a cell determine which metabolic pathways take place?
They are so specific
What are ribozymes?
Catalytic RNA molecules with no protein content
What are enzymes named and classified according to?
The reactions that they catalyse
What is a cofactor?
Non protein component needed for activity
What is usually the cofactor?
An ion such as Fe2+, Fe3+, K+ or Mg2+
What is a coenzyme?
Complex organic molecule that is usually produced from vitamins
Give some examples of coenzymes?
FAD (comes from riboflavin)
NAD (comes from niacin)
Coenzyme A (comes from pantothenate)
What is a prosthetic group?
Cofactor covalently bound to an enzyme or very tight associated with the enzyme
What is an example of a prosthetic group?
The haem in haemoglobin
What is an apoenzyme?`
Protein component of an enzyme that contains a cofactor
What is a haloenzyme?
‘Whole enzyme’, the apoenzyme plus the cofactor
What is a substrate?
Molecules acted on by an enzyme
What is the active site?
Part of an enzyme in which the substrate bind and is acted upon
What does the name of an enzyme normally end in?
-ase, the name also normally relates to the function
What are the 6 classes of enzymes?
Oxidoreductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases
What do oxidoreductases do?
Transfer electrons
What do transferases do?
Transfer groups
What do hydrolases do?
Hydrolyse (transfer chemical groups to water)
What do lyases do?
Form or add groups to double bonds
What do isomerases do?
Transfer groups within molecules (form isomers)
What do ligases do?
Formation of C-C, C-S, C-O and C-N (coupled to ATP cleavage)
What are two things that enzymes do not do to a reaction?
Move the reaction equilbrium
Make a non spontaneous reaction spontaneous
What are 3 things that enzymes do?
Increase the rate of a spontaneous reaction
Lower the activation energy of biochemical reactions
Accerlerate movement towards equilbrium
What delta G value must spontaneous reactions have?
Negative so they will increase entropy
When does the transition state occur?
When the stable molecules become unstable and may carry on to the product or revert back to the substrate
Why are spontaneous reacitons not instantaneous?
Because of the energy barrier
What is the energy barrier?
Energy required to position chemical groups correctly, bond rearrangments, e- rearrangement etc
What does an enzyme allow a reaction to occur through?
A different route
E+S ⇔ ES ⇔ EP ⇔ E + P
What does induced fit mean?
Conformation changes in protein structure when the substrate binds
What are 3 techniques used to analyse an enzymes?
Mutagenesis
3D structure
Enzyme kinetics
What would happen to the initial rate of a reaction if we changed the substrate concentration?
The initial rate of the ration would also change
Why does the rate of the reaction decrease as time proceeds?
The substrate is used up
How can the initial velocity (vo) be studied?
By knowing the precise [S], and having lots of S
What does a graph of substrate concentration against vo look like?
What happens to vo as initial [S] increases?
It increases until it levels out due to all of the enzymes active sites being used
Why at higher [S] does vo change very little?
Due to the finite number of enzymes
What is vmax?
Occurs when [S] becomes so large that vo changes a vanishably small amount (all enzyme active sites are saturated with substrate)
When does vmax occur?
When all enzyme sites are saturated with substrate
What is the model that Machaelis and Menten proposed to acount for the curve of [S] against vo?
- First part of the reaction occurs reversible
- Second part occurs more slowly
What limits the rate of the entire reaction?
The second step (ES to E + P) is the slowest step and so is the rate determining step
What is the rate of the reaction proportional to?
Amount of ES
What is the M-M equation?
What is KM (Michaelis constant)?
Substrate concentration at half of the vmax
Whats happens to the M-M equation when [S] is equal to KM?
It looks like Vo = Vmax/2
What is the steady state assumption?
[ES] does not change with time due to the rate of formation being equal to the rate of breakdown
Why is it hard to determin vmax using the graph of [S] against vo?
The graph curves and basically goes to infiity
What is the most accurate way to determine vmax experimentally?
Drawing a Lineumer-Burk plot (double reciprical plot)
That does the intersection of the y-axis on a Lineurner-Burk plot represent?
1/vmax
What does the intersection of the x-axis on a Lineumer-Burke plot represent?
1/KM
What can KM also be defined as?
KM = (K-1 + K2) / K1
Because K2 is the rate limiting step (K2 < K-1):
KM = K-1 / K1
What can KM also be termed as?
The dissociation constant of the ES complex
What can KM be described as it terms of rate?
Ratio of rate constant for break of ES to E+S compared to the ratio contant for formation of ES from E+S
What does KM indicate about an enzyme and a substrate?
The affinity of the enzyme with that substrate
What does a large KM mean?
Less stable ES complex (low affinity)
What does a low KM mean?
More stable ES complex (high affinity)
What does vmax tell you?
How fast the reaction is proceeding when the enzyme is saturated with substrate
What do KM and vmax change in response to?
The cells condition, the same enzyme may function differently in different cells
