Enzyme Properties Flashcards
Enzymes
Biological catalysts that speed up the rate of reaction without altering the final equilibrium between products and reactants
How do enzymes work?
Lower the activation energy required for a reaction, so it is quicker
Lock and key theory
Emil Fisher proposed that enzymes are complementary to their substrate - like lock and key - in 1884
Induced fit theory
Daniel Koshland - 1958 - suggested that enzymes undergo changes when the substrate binds, which changes shape of the active site
Transition state
Unstable high energy intermediate in a chemical reaction - stabilising the transition state is one way that enzymes speed up rate of reaction
Alcohol dehydrogenase
Convert primary alcohols to aldehydes
Can enzymes act on stereoisomers?
No, enzymes are only complementary to one isomer
6 types of enzymes
1) Oxidoreductases
2) Transferases
3) Hydrolases
4) Lyases
5) Isomerases
6) Ligases
Oxidoreductases
Catalyse oxidation or reduction reactions
Transfer of H/O atoms from one substance to another
Transferases
Catalyse transfer of functional groups from one substance to another
Hydrolases
Catalyse formation of two products from a substrate by hydrolysis (splitting using water)
Lyases
Catalyse non hydrolytic addition or removal of groups from substrates
Isomerases
Catalyse isomerisation changes within a single molecule
Ligases
Join together two molecules by synthesis of new bonds (C-O C-S C-N C-C) along with ATP breakdown
Effect of temperature on enzymes
When proteins are heated, weak bonds are easily broken, causing tangled structure, and enzyme is denatured
Effect of pH on enzymes
Causes unfolding of enzyme causing inactivation
Reaction rate equations
Change in product/time
Change in substrate/time
What is hyperbolic kinetics enzymes
At low substrate conc, reaction rate is directly proportional to substrate concentration
At high substrate conc, reaction rate is independent of substrate concentration
Michael Menten reaction model
E + S >< ES >< E + P
Assumptions in Michael Menten reaction model
[S] > [E] so that amount of substrate bound by enzymes at any one time is small
Initial velocities used, so back reaction of products to substrate can be ignored
[ES] does not change with time
Michaelis-Menten equation
V0 = Vmax [S] / Km + [S]
V0 - initial reaction velocity
V max - maximum velocity of an enzyme catalysed reaction
Km = Michaelis constant (k-1 + k2)/k1
Special relationship between Km and [S] when V0 = 0.5Vmax
Km = [S]
Km and enzyme substrate affinity
Where k2 < k-1
Km = k-1/k1
Kcat
Number of substrate molecules converted to product in a unit of time on a single enzyme molecule when the enzyme is saturated with substrate
Best way to compare catalytic efficiency
Kcat/Km
Lineweaver-Burk plot
Turn curve into straight line using the reciprocal
1/v = (km/Vmax) (1/[S]) + 1/Vmax
Effect of competitive inhibitors
Their effect can be overcome by increasing substrate concentration
Increase Km
V max does not change
Effect of non competitive inhibitors
Decrease V max
Do not change Km
Allosteric enzyme
Cooperative substrate binding
When the binding of one substrate affects the binding of another substrate
Regulation of enzyme activity
Substrate availability - immediate
Product inhibition - immediate
Allosteric control - immediate
Covalent modification - immediate to minutes
Synthesis/degradation - hours to days
Covalent modification
Reversible addition of Ser, Thr, Tyr and His residues by kinase and phosphatase regulatory enzymes
Blood glucose control
If blood glucose levels increase, insulin production increases within the pancreas
This increases rate of synthesis of key enzymes involved in glucose metabolism - glucokinase, phosphofructokinase, pyruvate kinase
