Chapter 2: Enzymes Flashcards by Maria Tolosa

________________ catalyze redox reactions; that is, the transfer of electrons between biological molecules.

Oxidoreductases

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Oxidoreductases often have a ____________ that acts as an electron carrier.

cofactor

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In reactions catalyzed by oxidoreductases, the electron donor is known as the ____________, and the electron acceptor is known as the ____________.

reductant; oxidant

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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

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________________ catalyze the movement of a functional group from one molecule to another.

transferase

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____________ are a type of transferase that catalyze the transfer of a phosphate group, generally from ATP, to another molecule.

kinase

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____________ 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

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________ catalyze the cleavage of a single molecule into 2 products.

lyases

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Lyases differ from hydrolases because of what?

They do not require water as a substrate

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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

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____________ catalyze the rearrangement of bonds within a molecule. Can catalyze reactions between stereoisomers as well as constitutional isomers.

isomerases

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____________ catalyze addition or synthesis reactions, generally between large similar molecules and often require ATP.

ligases

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How do ligases differ from lyases?

ligases are for larger molecules, and lyases are for smaller molecules

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What are the major enzyme classifications?

LIL’ HOT.

Ligase
Isomerase
Lyase
Hydrolase
Oxidoreductase
Transferase

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An ____________ rxn is one that requires energy input. What is its ΔG?

endergonic; >0

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An ____________ rxn is one in which energy is given off. What is its ΔG?

exergonic; <0

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Enzymes do NOT alter the overall ________ ____________ change for a reaction, nor do they change the ____________.

free energy; equilibrium

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What do enzymes affect about reactions?

The rate/kinetics

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Catalysts exert their effect by lowering the ________________ ________ of a reaction. They make it easier for the substrate to reach the transition state.

activation energy

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The ________ ________ is the location within the enzyme where the substrate is held during the chemical reaction.

active site

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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

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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

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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

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Enzymes without their cofactors are called ____________, whereas those containing them are ________________.

apoenzymes; holoenzymes

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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; v_max

How can you increase v_{max_?}

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 _____.

k_{1_.}

The ES complex can either dissociate at a rate ____ or turn into E + P at a rate ____.

k_{-1_{or k_cat}}

K_{m_{is what?}}

the Michaelis constant - the substrate concentration at which half of the enzyme's active sites are full

What does K_{m_{a measure of?}}

The affinity of the enzyme for its substrates

How can we compare enzymes using K_{m_?}

The one with the higher K_{m_{has the lower affinity for its substrate because it requires a higher substrate concentration to be half-saturated}}

When [S] is less than K_{m_{, what effects reaction rate the most?}}

changes in substrate concentration

When [S] exceeds K_{m_{, the reaction rate increases much more _________ as it approaches v_{max_{, where it becomes independent of [S].}}}}

slowly

How do we measure maximum enzyme velocity?

moles of enzyme per second

What does k_{cat_measure?}

The number of substrate molecules converted to product per enzyme molecule per second

What is catalytic efficiency?

ratio of k_{cat_{/K_{m_{| 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?

1. competitive* 2. noncompetitive* 3. mixed 4. uncompetitive ## Footnote *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 K_{m_{, 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?

1. Enzyme 2. Enzyme-substrate complex

Adding a noncompetitive inhibitor ____________ the measured value of v_{max_{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.

K_m

________ 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.

K_m

# Mixed inhibition If an inhibitor preferentially binds to the enzyme, it ____________ the K_{m_{value. This ____________ affinity.}}

increases, lowers

# Mixed inhibition If the inhibitor binds to the enzyme-substrate complex, it ________ the K_{m_{value, which ____________ affinity.}}

lowers; increases

# mixed inhibition Regardless if the inhibitor binds to the enzyme or enzyme-substrate complex, it ________________ v_{max_.}

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