L10: Enzymes I Flashcards
Define enzyme structure
Proteins composed of 1 or more polypeptide chains folded into a 3D complex shape
How are enzyme structure stabilised?
By weak bonds
H-bonds
Electrostatic salt bridges
Van der Waals
Hydrophobic interactions
Define enzymes
Biological catalysts that speed up the rate of reaction, without altering the final equilibrium between reactants & products
Extremely efficient
2 theories for enzyme binding
1) Lock & Key
2) Induced Fit
lock & key
What is the lock & key theory?
Enzymes are complementary to their substrate
To explain the high specificity of enzymes
induced fit
What is the induced fit theory?
Enzymes are initially not complementary, but will undergo change upon substrate binding
Induced by weak interactions with the substrate itself
Define transition state
Unstable, high-energy intermediate in a chemical reaction, when the molecule is neither substrate/product
Stabilising the transition state is one way that enzymes can speed up a reaction
Why is the transition state high energy?
Has 5 carbon bonds
Why complementarity of enzymes to transition state necessary?
Lowering the Ea, binding tightly so reaction can’t proceed
Substrate specificity
What do enzymes usually do?
1) Catalyse only 1 type of reaction
2) Only act on a few related molecules
Substrate specificity
True or false: Enzyme will usually act only on one isomer if a compound exists in 2 steroisomer forms
TRUEEEE
What is specificity determined by?
Groove or cleft of defined shape - the ‘active site’
Which only the substrate of the correct shape & charge can fit
Enzyme specificity - classification
How many classes are enzymes divided in & according to what?
6 classes & according to the type of reaction they catalyse
enzyme specificity-classification
Are the six classes of enzymes further divided into subgroups? If yes, what is according to?
Yes, according to their substrate or source
Substrate: What they work on
Source: Where they come from
6 classification of enzymes
1) Oxidoreductases
2) Transferases
3) Hydrolases
4) Lyases
5) Isomerases
6) Ligases
classification of enzymes
What type of reaction is OXIDOREDUCTASES
Catalyse oxidation/reduction reactions
Transfer of H & O atoms/electrons from 1 substance to another
classification of enzymes
What type of reaction is TRANSFERASES
Catalyse the transfer of functional groups from 1 substance to another
classification of enzymes
What type of reaction is HYDROLASES
Catalyse the formation of 2 products from a substrate by hyrolysis (spitting using H2O)
classification of enzymes
What is the type of reaction LYASES
Catalyse non-hydrolytic addition or removal of groups from substrates
classification of enzymes
What type of reaction is ISOMERASES
Catalyse isomerisation changes within a single molecule
classification of enzymes
What type of reaction is LIGASES
JOin together 2 molecules by synthesis of new
C-O, C-S, C-N or C-C bonds
whilst breaking down ATP
Properties of enzymes: effects of temp
Weak bonds are easily broke
- Enzyme structure is stabilised by many weak bonds
- Weak bonds are easily broken
- Makes enzymes very sensitive to changes
classification of enzymes
What type of reaction is TRANSLOCASES
Catalyse the movement of ions/molecules across membranes or their separation within membranes
Define cofactors
Nin-protein substance (small organic molecules/inorganic elemtns), that is necessary for the activity of an enzyme
Define co-enzymes
Organic cofactors that are not firmly attached to the enzyme
Non-protein substance
Define apoenzyme
Enzyme represent inative becayse its been separated from its cofactors
Enzymes catalytically inactive form
Define holoenzyme
Complete molecule in its fully active state
Enzymes catalytically active form
2 examples of co-enzymes
1) NAD
2) FAD
Another term for co-enzymes
Co-substrates, as they take part in & are changed by the reaction
Define isoenzymes
Enzymes with different protein structures that catalyses the same reaction
Coded for by different genes
Define enzyme kinetics
Study of the RATE of an enzyme catalysed reaction
- How that rate varies with different substrate concentrations
- Effects of inhibitors
How is the reaction rate at low substrate concentration?
Reaction rate is directly proportional to the substrate concentration
How is the reaction rate at high substrate concentration?
Reaction rate is independent of substrate concentration
Define reaction rate
Increase in the amount of product formed per time
How can the reaction rate be measured as?
Decrease in the amount of substrate per unit time
Formula for Michaelis-Menten reaction model
S + E ⇌ ES → P + E
What does the Michaelis-Menten reaction model explain?
Explains how the rate of an enzyme-catalysed reaction depends on the concentration of the enzyme and its substrate
Define Vmax
Maximum rate of the reaction, all enzyme’s active sites are saturated with substrate
Define Km
Substrate concentration at which reaction rate is 50% of the Vmax
- Measure of the affinity an enzyme has for its substrate
- More lower the value of Km, more efficient the enzyme is
3 assumptions of the MMKinetics
1) (S) > (E), so the amount of substrate bound by enzyme is negligible
2) (ES) doesn’t change with time
3) Concentration of product is so small, back reaction of P to S is ignored
Rate of formation = Rate of dissociation
Equation for V0
V0 = Vmax (S) / Km + (S)
What does the V0 equation describe?
How initial rate of reaction (V) is affected by the initial substrate concentration
What do these represent:
1) V0
2) Vmax
3) Km
4) S
V0: Initial reaction velocity
Vmax: Maximal velocity of an enzyme catalysed reaction
Km: Michaelis constant
S: Substrate conc
Define Kcat
No. of substrate molecules converted to product on a single enzyme molecule when it is saturated with substrate
Define catalytic efficiency (Kcat/Km)
Measure of an enzyme’s specificity for a particular substrate
