Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers

Question 1

is a macromolecule that acts as a catalyst

Answer 1

enzyme

Question 2

a chemical agent that speeds up a reaction without being consumed by the reaction.

Answer 2

catalyst

Question 3

Every chemical reaction between molecules involves both bond breaking and .

Answer 3

bond forming

Question 4

To reach the contorted state where bonds can change, reactant molecules must absorb

Answer 4

energy from their surroundings.

Question 5

When the new bonds of the product molecules form, energy is released as heat, and the molecules return to stable shapes with lower

Answer 5

energy than the contorted state.

Question 6

The initial investment of energy for starting a reaction—the energy required to contort the reactant molecules so the bonds can break—is known as the

Answer 6

free energy of activation, or activation energy abbreviated Eᴀ

Question 7

We can think of activation energy as the amount of energy needed to push the reactants to the top of an energy barrier, or uphill, so that the

Answer 7

“downhill” part of the reaction can begin.

Question 8

Activation energy is often supplied by heat in the form of

Answer 8

thermal energy that the reactant molecules absorb from the surroundings

Question 9

The absorption of thermal energy accelerates the reactant molecules, so they

Answer 9

collide more often and more forcefully.

Question 10

When the molecules have absorbed enough energy for the bonds to break, the reactants are in an unstable condition known as the

Answer 10

transition state

Question 11

learn Figure 8.13 Energy profile of an exergonic reaction.

Question 12

Proteins, DNA, and other complex cellular molecules are rich in free energy and have the potential to

Answer 12

decompose spontaneously; that is, the laws of thermodynamics favor their breakdown

Question 13

high temperature denatures proteins and

Answer 13

kills cells.

Question 14

a process by which a catalyst (for example, an enzyme) selectively speeds up a reaction without itself being consumed.

Answer 14

catalysis

Question 15

an enzyme cannot change the ∆G for a reaction; it cannot make an endergonic reaction

Answer 15

exergonic.

Question 16

for any reaction to occur, even a downhill reaction, some energy must be

Answer 16

added to get the reaction going

Question 17

this energy is needed to break bonds in the

Answer 17

reactant molecules

Question 18

the energy needed to start a chemical reaction is called the

Answer 18

energy of activation (E sub A)

Question 19

this required energy input represents a barrier that prevents even energy-releasing exergonic reactions from occurring without

Answer 19

some added energy

Question 20

serves as a biological catalyst, increasing the rate of a reaction without being changed into a different molecule

Answer 20

an enzyme

Question 21

an enzyme does not add energy to a reaction: it

Answer 21

speeds up a reaction by lowering the energy barrier

Question 22

an enzyme is very selective, its three-dimensional shape allows it to act on specific molecules, referred to as the

Answer 22

enzyme’s substrates

Question 23

as the substrates bind to the enzyme’s active site they are held in a position that facilitates the

Answer 23

reaction

Question 24

this takes less activation energy than the

Answer 24

unaided reaction

Question 25

product form and are

Answer 25

released

Question 26

the enzyme emerges

Answer 26

unchanged from the reaction

Question 27

because of the specific fit between enzyme and substrate, each enzyme can catalyze

Answer 27

one kind of reaction involving specific substrates

Question 28

thousands of different enzymes may be required to carry out all of a

Answer 28

cells metabolic processes

Question 29

The reactant an enzyme acts on is referred to as the

Answer 29

enzyme’s substrate

Question 30

The enzyme binds to its substrate (or substrates, when there are two or more reactants), forming an

Answer 30

enzyme-substrate complex

Question 31

While enzyme and substrate are joined, the catalytic action of the enzyme converts the substrate to the

Answer 31

product (or products) of the reaction

Question 32

Most enzyme names end in

Answer 32

-ase.

Question 33

most enzymes are proteins, and proteins are macromolecules with unique

Answer 33

three-dimensional configurations

Question 34

The specificity of an enzyme results from its shape, which is a consequence of its

Answer 34

amino acid sequence.

Question 35

is typically a pocket or groove on the surface of the enzyme where catalysis occurs

Answer 35

active site

Question 36

Usually, the active site is formed by only a few of the enzyme’s amino acids, with the rest of the protein molecule providing a framework that determines the

Answer 36

shape of the active site.

Question 37

The specificity of an enzyme is attributed to a complementary fit between the shape of its active site and the shape of the

Answer 37

substrate.

Question 38

enzymes (and other proteins) seem to “dance” between subtly different shapes in a

Answer 38

dynamic equilibrium, with slight differences in free energy for each “pose.

Question 39

As the substrate enters the active site, the enzyme changes shape slightly due to interactions between the substrate’s chemical groups and

Answer 39

chemical groups on the side chains of the amino acids that form the active site.

Question 40

The tightening of the binding after initial contact—called

Answer 40

induced fit

Question 41

Induced fit brings chemical groups of the active site into positions that enhance their ability to catalyze the

Answer 41

chemical reaction.

Question 42

In most enzymatic reactions, the substrate is held in the active site by so-called weak interactions, such as

Answer 42

hydrogen bonds and ionic bonds.

Question 43

The R groups of a few of the amino acids that make up the active site catalyze the conversion of substrate to product, and the

Answer 43

product departs from the active site.

Question 44

The entire cycle happens so fast that a single enzyme molecule typically acts on about

Answer 44

1,000 substrate molecules per second

Question 45

although every enzyme performs a different reaction on different substrates they all go through the same general steps in accomplishing their reaction. these general steps include

Answer 45

substrate binding, formation of the enzyme-substrate complex, product formation and dissociation, and enzyme recovery

Question 46

like all catalysts, enzymes are unchanged by the

Answer 46

overall reaction

Question 47

first step in any enzymatic reaction is the

Answer 47

binding of the substrates by the enzyme

Question 48

the enzyme includes an active site that can accommodate the

Answer 48

size, shape, and electron configuration of its substrate or substrates

Question 49

according to the induced-fit model of enzyme-substrate interaction, a perfect fit does not form until the

Answer 49

enzymes and substrates bind to form an enzyme-substrate complex

Question 50

after the substrate has bound in the active site, the

Answer 50

enzyme-substrate complex is formed

Question 51

the shape of the enzyme may change to allow for an efficient chemical reaction, and amino acids may accept or donate protons or electrons to facilitate the

Answer 51

chemical reaction

Question 52

once the enzyme-substrate complex has formed and any reactive groups have moved to the right position, the reaction proceeds and products are

Answer 52

formed

Question 53

the product dissociate rapidly, leaving an empty

Answer 53

active site

Question 54

after the products leave the active site, the enzyme

Answer 54

returns to its original configuration

Question 55

any groups that donated or accepted protons or electrons are also

Answer 55

returned to their original state

Question 56

like all catalysts, the enzyme is unchanged by the

Answer 56

overall reaction

Question 57

Most metabolic reactions are reversible, and an enzyme can catalyze either the forward or the reverse reaction, depending on which direction has a negative

Answer 57

∆G

Question 58

This in turn depends mainly on the

Answer 58

relative concentrations of reactants and products

Question 59

The net effect is always in the direction of

Answer 59

equilibrium.

Question 60

Enzymes use a variety of mechanisms that

Answer 60

lower activation energy and speed up a reaction

Question 61

When there are two or more reactants, the active site provides a template on which the .

Answer 61

substrates can come together in the proper orientation for a reaction to occur between them

Question 62

The more substrate molecules that are available, the more frequently they access the active sites of the .

Answer 62

enzyme molecules

Question 63

there is a _______ to how fast the reaction can be pushed by adding more substrate to a fixed concentration of enzyme

Answer 63

limit

Question 64

As soon as the product exits an active site, another

Answer 64

substrate molecule enters

Question 65

At this substrate concentration, the enzyme is said to be ____________, and the rate of the reaction is determined by the speed at which the active site converts substrate to product.

Answer 65

saturated

Question 66

When an enzyme population is saturated, the only way to increase the rate of product formation is to

Answer 66

add more enzyme.

Question 67

Cells often increase the rate of a reaction by producing more

Answer 67

enzyme molecules.

Question 68

learn scientific skills exercise

Question 69

The activity of an enzyme—how efficiently the enzyme functions—is affected by general environmental factors, such as

Answer 69

temperature and pH.

Question 70

the rate of an enzymatic reaction increases with increasing temperature, partly because substrates collide with active sites more frequently when the

Answer 70

molecules move rapidly

Question 71

Above that temperature, however, the speed of the enzymatic reaction

Answer 71

drops sharply.

Question 72

Most human enzymes have optimal temperatures of about

Answer 72

35–40°C

Question 73

The thermophilic bacteria that live in hot springs contain enzymes with optimal temperatures of

Answer 73

70°C or higher

Question 74

The optimal pH values for most enzymes fall in the range of

Answer 74

pH 6–8, but there are exceptions.

Question 75

Many enzymes require nonprotein helpers for catalytic activity, often for chemical processes like electron transfers that cannot easily be carried out by the amino acids in proteins. These adjuncts, called _________ may be bound tightly to the enzyme as permanent residents, or they may bind loosely and reversibly along with the substrate.

Answer 75

cofactors

Question 76

The cofactors of some enzymes are inorganic, such as the metal

Answer 76

atoms zinc, iron, and copper in ionic form.

Question 77

If the cofactor is an organic molecule, it is referred to, more specifically, as a

Answer 77

coenzyme

Question 78

reduce the productivity of enzymes by blocking substrates from entering active sites.

Answer 78

competitive inhibitors

Question 79

watch and learn animation enzymes: competitive inhibition and animation enzymes: noncompetitive inhibition

Question 80

do not directly compete with the substrate to bind to the enzyme at the active site

Answer 80

noncompetitive inhibitors

Question 81

they impede enzymatic reactions by binding to another part of the

Answer 81

enzyme