Enzymes Flashcards
Enzymes
incredibly important as biological catalysts
catalysts
do not impact the thermodynamics of a reaction; Hrxn and equilibrium position do not change but the reaction proceeds at a much faster rate
Key points about enzymes
- lower the activation energy
- increase rate of reaction
- do not alter the equilibrium constant
- are not changed or consumed in the reaction (appear in both reactants and products)
- are pH and temperature sensitive
- do not affect the overall ΔG of reaction
- are specific for a particular reaction or class of reactions
enzyme specifity
enzymes will only catalyze a single reaction/class of reactions for a particular substrate
oxidoreductase
catalyze redox reactions; transfer electrons between biological molecules; often has a cofactor that acts as an electron carrier
reductant
electron donor
oxidant
electron acceptor
transferase
catalyze the movement of a functional group from one molecule to another
kinase
catalyze the transfer of a phosphate group to another molecule
hydrolase
catalyze the breaking of a compound into two molecules by addition of water
lyase
catalyze the cleavage of one molecule into two products; do not require water and do not act as oxidoreductases
synthase
synthesis of two molecules into one molecule
isomerase
catalyze the rearrangement of bonds within a molecule; catalyze reactions between stereoisomers as well as constitutional isomers
ligases
catalyze addition or synthesis reactions, generally between large similar molecules and often require ATP; nucleic acid synthesis/repair
lyase
catalyze addition or synthesis reactions, generally between small similar molecules and often require ATP
endergonic reaction
require energy input (ΔG>0)
exergonic reaction
energy is given out (ΔG<0)
activation energy
catalysts lower the activation energy to make it easier for the substrate to reach its transition state
enzyme-substrate complex
physical interaction between substrate and enzyme
active site
the location within the enzyme where the substrate is held during chemical reaction
lock and key theory
proposes that the enzymes active site is already in the appropriate conformation for the substrate to bind
induced fit model
the substrate induces a change in the shape of the enzyme; the enzyme returns to its original state after the interaction
cofactors/coenzymes
bind to the active site of the enzyme and participate in the catalysis of a reaction
apoenzymes v. holoenzymes
apoenzymes: enzyme without its cofactors
holoenzymes: enzyme with its cofactors
prosthetic groups
tightly bound cofactors/coenzymes necessary for enzyme function
cofactors
generally inorganic molecules or metal ions; often digested as dietary minerals
coenzymes
small organic groups, the vast majority of which are vitamins or derivates of vitamins such as NAD+, FAD or coenzyme A
water-soluble vitamins
B complex and vitamin C (Ascorbic acid)
fat-soluble vitamins
A,D,E,K
saturation
at this rate, enzyme is working at maximum velocity (Vmax)
Michaelis-Menten
E+S()ES->E+P
where is k(1) and the second -> is Kcat
Michaelis-Menten
v=(Vmax[S])/(Km+[S]) or v=(Kcat[E][S])/(Km+[S])
Km=substrate concentration at which half of the enzyme’s active sites are full
Kcat
measures the number of substrate molecules turned over per enzyme molecule per second
Vmax=[E]Kcat
catalytic efficiency
Kcat/Km; a large Kcat or a small Km will result in higher catalytic efficiency
Lineweaver-Burk plots
-1/Km is the intercept of the line and the x-axis
1/Vmax is the intercept of the line with the y-axis
cooperative enzymes
- represented as a sigmoidal shape
- have multiple subunits/active sites
- attachment to one active site can lead the other sites to go from the tense (T) state to the relaxed (R) state
- detachment from one active site can lead the other states to go from the relaxed (R) state to the tense (T) state
Hill’s coefficient
portrays if there is cooperative binding or not
Hill’s coefficient > 1, positive cooperative binding
Hill’s coefficient < 1, negative cooperative binding
Hill’s coefficient = 1, no cooperative binding
temperature and enzymes
- enzyme-catalyzed reactions tend to double in velocity for every 10 degrees C until optimum temp reached
- after this optimal temp, activity falls off sharply as they begin to denature at high temps
pH and enzymes
human enzymes work best around 7.4; work less efficiently lower or higher
feedback inhibition
once there is enough of a given product, the product binds to the active site of an enzyme, making them unavailable to make more product
competitive inhibition
involves occupancy of the active site; can be overcome by adding more substrate; no effect to Vmax, but Km is increased
noncompetitive inhibition
bind to an allosteric site instead on the active site, which induces a change in enzyme conformation; cannot be overcome by adding more substrate; bind equally well to E and ES; lowers Vmax but does not effect Km
mixed inhibition
same as noncompetitive inhibitor in that in can bind to an allosteric site on both E or ES but has different affinity for each; alters Km, decreases Vmax
uncompetitive inhibition
bind only to ES complex; lowers Vmax and Km
allosteric enzymes
have multiple binding sites; alternate between active and inactive forms;
allosteric sites
sites different from the active sites that bind either activators or inhibitors
covalently modified enzymes
enzymes can be activated or deactivated by covalent modification, aka phosphorylation
glycosylation
covalent attachment of sugar to an enzyme
zymogens
enzymes are secreted as zymogens; regulatory domain must be removed in order for active site to be exposed
