Chapter 6- The Behavior of Proteins: Enzymes Flashcards
catalysis
the process of increasing the rate of chemical reactions
enzymes
- biological catalysts, usually globular proteins with self-splicing RNA as the only exception
- can increase the rate of a reaction by a factor of 10^20
standard free energy (changeG)
the difference between the energies of reactants and products under standards conditions
do enzymes alter the equilibrium constant of a reaction or free energy change?
nope
activation energy
the energy required to start a reaction
what type of reaction has a higher activation energy?
uncatalyzed or catalyzed
uncatalyzed (so the rate is slower)
sponanteous reactions have what type of delta g
negative
transition state
intermediate stage in a reaction where old bonds break and new bonds form
what happens to the rate of a chemical reaction when you increase temperature?
goes faster
isoenzymes
multiple forms of an enzyme that catalyze the same overall reaction but have subtle physical and kinetic parameters
rate constant
a proportionality constant in the equation that describes the rate of a reaction
exponents in the rate equation are determined…
experimentsally
first order
described a written whose rate depends on the first power of the concentration of a single reactant
second order
describes a reaction whose rate depends on the product of the concentrations of two reactants
zero order
- refers to a reaction that proceeds at a constant rate, independent of the concentration of reactant
- sometimes depends on catalysts
substrate
-reactant in an enzyme-catalyzed reaction
where does the substrate bind? and by what type of interactions?
enzyme; noncovalent
active site
part of enzyme where substrate binds and reactant takes place
why does substrate bind to enzyme
because of highly specific interactions between the substrate and the side chains and backbone groups of the amino acids making up the active site
lock and key model
substrate and active site exactly match each other in shape
induced fit model
binding of the substrate induces a conformational change in the enzyme that results in a complementary fit after substrate is bound
MM rate constant Eqn Terms
k1: rate constant for formation of ES
k-1: rate constant for ES–> E + S
k2: rate constant for ES—> E + P
rate depends on
substrate concentration
Vmax
maximum velocity
Km
- substrate concentration at which the reaction performs at one half its vmax
- inverse measure of the affinity of the enzyme for the substrate
- equals the concentration of substrate at which 50% of the enzyme active sites are occupied by substrate
lower Km=
higher affinity of enzyme for substrate
steady state
formation of ES complex equals the rate of its breakdown
michealis constant
Km; a numerical value for strength of binding of a substrate to an enzyme
michealis-menten eqn formula
V= Vmax [S]
________
Km + [S]
When [S]=Km, then…
V= Vmax [S]
________
[S] + [S]
and
V= Vmax
______
2
what type of curve describes the rate of a nonallosteric enzymatic reaction?
hyperbolic
Lineweaver-Burk Double Reciporcal Plot
- graphical method for analyzing the kinetics of enzyme-catalyzed reaction
- x axis: 1/[S]= (nm)-1
- y axis: 1/Vo= sec/nm
- slope: Km/Vmax
Keq=
[E][S]
_____
[ES]
larger Km
less tightly enzyme bound to substrate
turnover number (kcat)
- number of moles of substrate that react per second per mole of enzyme
- assumes enzyme is fully saturated with substrate and reaction is proceeding at max rate
chymotrypsin
- proteolytic enzyme that preferentially hydrolyzes amide bonds (peptide bonds) adjacent to aromatic amino acid residues
- can cleave peptide bonds
- catalyzes hydrolysis of ester bonds
- hyperbolic
ATCase
- allosteric enzyme that catalyzes an early reaction in pyrimidine biosynthesis
- signmodial
ATCcase and hemoglobin are
allosteric proteins (myoglobin and chymotprsin aren’t)
inhibitor
substance that decreases the rate of enzyme catalyzed reaction
reversible inhibtor
can bind enzyme and then be released, leaving enzyme in original condition
irreversible inhibitor
reacts with enzyme to produce protein that is not enzymatically active and from which the original enzyme can’t be regenerated
compettive inhibitor
- decreases enxyme activity caused by binding of substrate in active site
- inhibitor competes with substrate for active site
- SLOPE CHANGES
- KM INCREASES
how can competitive inhibition be overcame?
very high substrate concentration
noncompetitive inhibitor
- substrate binds to place other than the active site, but distorts active site so that reaction is inhibited
- SLOPE CHANGES
- VMAX DECREASES
Uncomepetitive inhibitor
- inhibitor can bind to ES, but no free E
- lines are parallel
- Vmax decreased
- apparent Km decreased
irrevisible inhibition
covalent binding of an inhibitor to an enzyme, causing permanent inactivation
suicide substrates (trojan horse)
molecules used to bind to an enzyme irreversibly and inactivate it
