Chapter 2: Enzymes Flashcards
________________ catalyze redox reactions; that is, the transfer of electrons between biological molecules.
Oxidoreductases
Oxidoreductases often have a ____________ that acts as an electron carrier.
cofactor
In reactions catalyzed by oxidoreductases, the electron donor is known as the ____________, and the electron acceptor is known as the ____________.
reductant; oxidant
Enzymes with ________________ or ____________ in their names are usually oxidoreductases. Enzymes in which oxygen is the final electron acceptor often in include ____________ in their names.
dehydrogenase, reductase, oxidase
________________ catalyze the movement of a functional group from one molecule to another.
transferase
____________ are a type of transferase that catalyze the transfer of a phosphate group, generally from ATP, to another molecule.
kinase
____________ catalyze the breaking of a compound into 2 molecules using the addition of water. They are usually named for their substrate.
hydrolase
e.g. phosphatase cleaves phosphate group from another molecule
________ catalyze the cleavage of a single molecule into 2 products.
lyases
Lyases differ from hydrolases because of what?
They do not require water as a substrate
Because most enzymes can also catalyze the reverse of their specific reactions, the synthesis of 2 molecules into a single molecule may also be catalyzed by a lyase. In this case, they are commonly referred to as ____________.
synthases
____________ catalyze the rearrangement of bonds within a molecule. Can catalyze reactions between stereoisomers as well as constitutional isomers.
isomerases
____________ catalyze addition or synthesis reactions, generally between large similar molecules and often require ATP.
ligases
How do ligases differ from lyases?
ligases are for larger molecules, and lyases are for smaller molecules
What are the major enzyme classifications?
LIL’ HOT.
- Ligase
- Isomerase
- Lyase
- Hydrolase
- Oxidoreductase
- Transferase
An ____________ rxn is one that requires energy input. What is its ΔG?
endergonic; >0
An ____________ rxn is one in which energy is given off. What is its ΔG?
exergonic; <0
Enzymes do NOT alter the overall ________ ____________ change for a reaction, nor do they change the ____________.
free energy; equilibrium
What do enzymes affect about reactions?
The rate/kinetics
Catalysts exert their effect by lowering the ________________ ________ of a reaction. They make it easier for the substrate to reach the transition state.
activation energy
The ________ ________ is the location within the enzyme where the substrate is held during the chemical reaction.
active site
The ________ and ____ theory suggests that the enzyme’s active site (lock) is already in the appropriate conformation for the substrate to bind.
lock and key
The ____________ ________ model, the substrate has induced a change in the shape of the enzyme. This interaction requires energy (endergonic). Once the substrate releases the enzyme, the enzyme reverts back, which releases energy (exergonic).
induced fit
Many enzymes require nonprotein molecules called ____________ or ____________ to be effective. They tend to be small in size so they can bind to the active site of the enzyme and participate in catalysis.
cofactors or coenzymes
Enzymes without their cofactors are called ____________, whereas those containing them are ________________.
apoenzymes; holoenzymes
Tightly bound cofactors or coenzymes that are necessary for enzyme function are known as ________________ ________.
prosthetic groups
Cofactors are generally what type of molecules? In what form are they ingested?
inorganic molecules or metal ions; ingested as dietary minerals
____________ are small organic groups, mostly vitamins or derivatives of vitamins, e.g. NAD+, FAD, and coenzyme A.
Coenzymes
The fat-soluble vitamins, A, D, E, and K, are better regulated by ________________ ____________, which quanitify the ability of a molecule to dissolve in a polar vs. nonpolar environment.
partition coefficient
What affects how fast a reaction will occur?
The concentrations of substrate and enzyme
A reaction cannot go any faster once it has reached ____________.
saturation
When saturation is reached, the enzyme is working at maximum ____________. How is this denoted?
velocity; vmax
How can you increase vmax?
Increase the enzyme concentration
What does the Michaelis-Menten equation describe?
How the rate of the reaction, v, depends on the concentration of both the enzyme [E], and the substrate [S], which forms product [P]
Enzyme-substrate complexes form at a rate _____.
k1.
The ES complex can either dissociate at a rate ____ or turn into E + P at a rate ____.
k-1 or kcat
Km is what?
the Michaelis constant - the substrate concentration at which half of the enzyme’s active sites are full
What does Km a measure of?
The affinity of the enzyme for its substrates
How can we compare enzymes using Km?
The one with the higher Km has the lower affinity for its substrate because it requires a higher substrate concentration to be half-saturated
When [S] is less than Km, what effects reaction rate the most?
changes in substrate concentration
When [S] exceeds Km, the reaction rate increases much more _________ as it approaches vmax, where it becomes independent of [S].
slowly
How do we measure maximum enzyme velocity?
moles of enzyme per second
What does kcat measure?
The number of substrate molecules converted to product per enzyme molecule per second
What is catalytic efficiency?
ratio of kcat/Km
the higher, the more efficient
________________ enzymes have multiple subunits and multiple active sites.
Cooperative
What shape is the Michaelis-Menten plot of cooperative enzymes?
sigmoidal
What 2 states do subunits and enzymes exist in?
low-affinity tense state (T) or high-affinity relaxed state (R)
Binding of the substrate encourages subunits to transition from what state to the other?
T state to R state
low to high affinity
Cooperativity is quantified by ________ ________________.
Hill’s coefficient
What do different values of Hill’s coefficient mean?
> 1, positive cooperativity
<1 negative cooperativity
=1 no cooperativity
Enzyme-catalyzed reactions tend to double in velocity every ____degree C increase in temperature until optimum temperature is reached. What is optimal temp?
10; 37
Most enzymes also depend on pH in order to function properly. What is optimal?
7.4
What are the 4 types of reversible inhibition?
- competitive*
- noncompetitive*
- mixed
- uncompetitive
*most common
________________ inhibition simply involves occupancy of the active site by an inhibitor
Competitive
How can competitive inhibition be overcome?
Add more substrate so that the substrate-to-inhibition ratio is higher
What does a competitive inhibitor not alter?
It doesn’t alter vmax, because if enough substrate is added, it will reach vmax anyway.
Really know this!
What does a competitive inhibitor alter?
It alters Km, because [S] has to be higher to reach half the maximum velocity in the presence of the inhibitor
________________ inhibitors bind to an allosteric site instead of the active site, which induces a change in conformation.
noncompetitive
____________ sites are non-catalytic regions of the enzyme that bind regulators.
allosteric
Noncompetitive inhibition cannot be overcome just by ____________, unlike competitive inhibition.
adding more substrate
Noncompetitive inhibitiors bind equally well to what 2 things?
- Enzyme
- Enzyme-substrate complex
Adding a noncompetitive inhibitor ____________ the measured value of vmax because there is less enzyme available to react.
decreases
Know this well!
Adding a noncompetitive inhibitor does NOT alter the value of ____ because any copies of the enzyme that are still active maintain the same affinity for their substrate.
Km
________ inhibition results when an inhibitor can bind to either the enzyme or the enzyme-substrate complex, but has different affinity for each.
Mixed
If an inhibitor has the same affinity for both the enzyme and enzyme-substrate complex, it is a ____________________ inhibitor.
noncompetitive
Mixed inhibition alters the experimental value of ____ depending on the preference of the inhibitor for the enzyme vs. the ES complex.
Km
Mixed inhibition
If an inhibitor preferentially binds to the enzyme, it ____________ the Km value. This ____________ affinity.
increases, lowers
Mixed inhibition
If the inhibitor binds to the enzyme-substrate complex, it ________ the Km value, which ____________ affinity.
lowers; increases
mixed inhibition
Regardless if the inhibitor binds to the enzyme or enzyme-substrate complex, it ________________ vmax.
decreases
For mixed inhibition, the curves on a Lineweaver-Burk plot will intersect at a point that is where?
not on either axis
________________ inhibitors bind only to the ES complex and essentially lock the substrate in the enzyme, preventing its release.
Uncompetitive
Why do uncompetitive inhibitors lock substrate in the enzyme and prevent its release?
The affinity between enzyme and substrate is increased
Because the enzyme-substrate complex has already formed upon binding, uncompetitive inhibitors must bind at an ________________ site.
allosteric
What allows an uncompetitive inhibitor to bind to an allosteric site?
The formation of the ES complex that creates a conformational change
What effect do uncompetitive inhibitors have on Km and vmax?
lowers them!
On a Lineweaver-Burk plot, the curves for activity with and without an uncompetitive inhibitor are ____________.
parallel
In ____________ inhibition, the active site is made unavailable for a prolonged period of time, or the enzyme is permenantly altered. Not easily reversed or overcome.
Irreversible
prime drug mechanism
Enzymes that are ____________ have multiple binding sites. These other sites can regulate the activity of the active site.
allosteric
Allosteric enzymes alternate between an active and an inactive form. Molecules that bind to the allosteric site can be ?
Allosteric activators or inhibitors
Enzymes are often subject to covalent modification, such as ____________________ or ________________.
phosphorylation, glycosylation
____________ is the covalent attachment of sugar moieties.
Glycosylation
Certain enzymes can be dangerous if not tightly controlled. They may even digest the organ from which it is released, e.g. pancreas and trypsin. To avoid this danger, many enzymes are secreted as inactive ________________ like trypsinogen.
zymogens
What do zymogens contain? How do they work?
Zymogens contain a catalytic (active) domain and regulatory domain. The regulatory domain must be either removed or altered to expose the active site.
Apoptotic enzymes, ____________, exhibit similar regulation to zymogens.
caspases
What suffix do most zymogens have?
-ogen
