Enzymes Flashcards
Enzymes are…
biological catalysts that are unchanged by the reaction they catalyze
Enzymes act by…
stabilizing the transition state
Enzymes have…
an active site (site of catalyst)
Exergonic reactions…
release energy; delta G is -
Enzymes do not…
alter the free energy (delta G) or enthalpy (delta H)
Lock and Key Theory
the enzyme and substrate are complementary
Induced Fit Model
the enzyme and substrate undergo conformational changes to interact fully
Some enzymes require…
metal cation cofactors or small organic coenzymes to be active
The large rate accelerations of enzymes…
correspond to large decreases in the free energy of activation for the reaction
All reactions…
pass through a transition state on the reaction pathway
The active sites of enzymes…
bind the transition state of the reaction more tightly than the substrate
The catalytic role of an enzyme…
is to reduce the energy barrier between substrate (S) and transition state (X)
Rate acceleration by an enzyme means…
that the energy barrier between ES and EX must be smaller than the barrier between S and X
Enzyme catalyzes reactions by…
lowering the activation energy
For a given energy of EX, raising the energy of ES will…
increase the catalyzed rate
Raising energy will be accomplished by
loss of entropy due to formation of ES
destabilization of ES
Destabilization of ES by…
strain, distortion, desolvation
How does destabilization of ES affect enzyme catalysis
catalysis does not occur if ES and X are equally stabilized; catalysis will occur if X is stabilized more than ES
Formation of the ES complex results in…
entropy loss
Substrates typically lose waters of hydration in the formation of…
the ES complex
Electrostatic destabilization of a substrate…
may arise from juxtaposition of like charges in the active site
How tightly do transition-state analogs bind to the active site?
very tight binding (10^-20 to 10^-26)
Enzymes are often targets for…
drugs and other beneficial agents
Transition state analogs often make…
ideal enzyme inhibitors
What are the mechanisms of catalysis?
covalent catalysis; general acid-base catalysis; low barrier hydrogen bonds; metal ion catalysis
Enzymes facilitate formation of…
near attack conformations
Protein motions are essential…
to enzyme catalysis
Proteins are constantly moving…
bonds vibrate, side chains bend and rotate, backbone loops wiggle and sway
Enzymes depend on protein movement to…
provoke and direct catalytic events
Active site conformation changes can…
- ) assist substrate binding
- ) bring catalytic groups into position
- ) induce formation of NACs
- ) assist in bond making/ breaking
Catalysis in enzymes active sites depends on…
motion fo active site residues
Covalent catalysis
enzymes derive much of their rate acceleration from formation of covalent bonds between E and S
The side chains of amino acids in proteins offer a…
variety of nucleophilic centers for catalysis
The covalent intermediate can…
be attacked in a 2nd step by water or a 2nd substrate
Specific acid-base catalysis
involves H+ or OH- that diffuses into the catalytic center
General acid-base catalysis
involves acids and bases other than H+ and OH-
typical H-bond strength
10-30kJ/mol
Low-barrier hydrogen bonds
pKa values of the two electronegative atoms must be similar/ energy released can assist catalysis
Residues may function in secondary roles in the active site:
- ) raising or lowering catalytic residue Pka
- ) orientation of catalytic residues
- ) charge stabilization
- ) proton transfers via hydrogen tunneling
