6.5 Enzymes and the Rate of Chemical Reactions

Question 1

chemical reaction in a cell are accelerated by:

Answer 1

chemical catalysts

Question 2

the rate of a chemical reaction is defined as:

Answer 2

the amount of product formed (or reactant consumed) per unit of time

Question 3

catalysts are:

Answer 3

substances that increase the rate of chemical reactions without themselves being consumed

Question 4

enzyme

Answer 4

a protein that functions as a catalyst to accelerate the rate of a chemical reaction; enzymes are critical in determining which chemical reactions take place in a cell

Question 5

because enzymes are highly specific, acting only on certain reactants and catalyzing only some reactions, enzymes play:

Answer 5

a critical role in determining which chemical reactions take place from all the possible reactions that could occur in a cell

Question 6

all chemical reactions require:

Answer 6

an input of energy to proceed (ever exergonic reactions)

Question 7

if exergonic reactions release energy? how does that explain why the chemical reactions require an input of energy?

Answer 7

the energy released is more than the initial input of energy, so there is a net release of energy

Question 8

transition state

Answer 8

the brief time in a chemical reaction in which chemical bonds in the reactants are broken and new bonds in the product are formed

Question 9

the transition state is highly unstable and therefore has a large amount of …

Answer 9

free energy

Question 10

in all chemical reactions, reactants adopt at least one:

Answer 10

transition state before their conversion into products

Question 11

all chemical reactions, even spontaneous ones that release energy, require:

Answer 11

an input of energy : think of an “energy barrier”

Question 12

activation energy

Answer 12

the energy input necessary to reach the transition state

Question 13

there is an inverse correlation between the rate of a reaction and:

Answer 13

the height of the energy barrier (ex. lower energy barrier = faster reaction)

Question 14

what is used to overcome the energy barrier in labs?

Answer 14

heat as a source of energy

Question 15

in living organisms, reactions are accelerated by:

Answer 15

enzymes

Question 16

how do enzymes accelerate chemical reactions in organisms?

Answer 16

they reduce the activation energy by stabilizing the transition state and decreasing its free energy. (as the activation energy decreases, the speed of the reaction increases)

Question 17

as the activation energy decreases….

Answer 17

the speed of the reaction increases

Question 18

even with the presence of an enzyme, what doesn’t change?

Answer 18

the difference in free energy between reactants and products (delta G)

Question 19

an important characteristic of enzymes is that, as catalysts, they:

Answer 19

participate in a chemical reaction but are not consumed in the process (remain unchanged)

Question 20

how do enzymes increase the reaction rate without being consumed?

Answer 20

enzymes form a complex with the reactants and products

Question 21

substrate (S)

Answer 21

a molecule acted upon by an enzyme; AKA the reactant

Question 22

describe the process of the enzyme in a chemical reaction

Answer 22

substrate forms a complex with the enzyme (enzyme-substrate) which is converted to product (enzyme-product), while complex dissociates, enzyme and product is released

Question 23

S+E =

Answer 23

ES=EP=E+P

Question 24

proteins adopt three-dimension shapes and the shape of a protein is linked to:

Answer 24

its function

Question 25

active site

Answer 25

the portion of the enzyme that binds substrate and converts it to product

Question 26

in the active site, the enzyme and substrate form:

Answer 26

transient covalent bonds and/or weak non covalent interactions. these interactions stabilize the transition state and decrease the activation energy

Question 27

enzymes reduce the energy of activation by:

Answer 27

positioning two substrates to react

Question 28

the formation of the enzyme-substrate complex promotes:

Answer 28

the reaction between two substrates by aligning their reactive chemical groups and limiting their motion relative to each other

Question 29

the active site is very small in comparison to the entire enzyme, why is the enzyme so large then?

Answer 29

the large size of many enzymes is required to bring the catalytic amino acids into very specific positions in the active site of the folded enzyme (specific amino acids have to align correctly with the reactive group of the substrate)

Question 30

in many cases, the catalytic amino acids (which form the active site) are spaced far apart in the primary structure of the enzyme and brought close together:

Answer 30

in the formation of the active site by protein folding

Question 31

enzymes are remarkably specific for both the:

Answer 31

substrate and the reaction that is catalyzed: enzymes recognize either a unique substrate or a class of substrates that share common chemical structures; enzymes catalyze only one reaction or a very limited number of reactions

Question 32

in some cases, the enzyme does not recognize the whole substrate but a particular:

Answer 32

structural motif within it, and very small difference in the structure of this motif affect the activity of the enzyme

Question 33

the enzyme is able to discriminate between two identical bonds with:

Answer 33

different orientations within a molecule

Question 34

the specificity of enzymes can be attributed to:

Answer 34

the structure of their active sites; the enzyme active site interacts only with substrates having a precise 3D structure

Question 35

what can influence the activity of enzymes?

Answer 35

inhibitors and activators

Question 36

inhibitor

Answer 36

a synthesized compound that decreases the activity of an enzyme

Question 37

activator

Answer 37

a synthesized compound that increases the activity of an enzyme

Question 38

enzyme inhibitors are quite common because they are synthesized:

Answer 38

naturally by many plants and animals as a defence against predators

Question 39

what are some of the other uses of an enzyme inhibitor?

Answer 39

pesticides/herbicides often target enzymes to inactivate them, drugs used in medicine; infections/cancer/other diseases

Question 40

what are the two classes of inhibitors?

Answer 40

irreversible inhibitors and reversible inhibitors

Question 41

irreversible inhibitors

Answer 41

usually form covalent bonds with enzymes and irreversibly inactivate them

Question 42

reversible inhibitors

Answer 42

form weak bonds with enzymes and therefore easily dissociate from them

Question 43

the inhibitor competes with the substrate for the active site of the enzyme:

Answer 43

binding of the inhibitor prevents the binding of the substrate

Question 44

inhibitors that compete with the substrate for the active site can be overcome by:

Answer 44

increasing the concentration of the substrate

Question 45

another case of inhibitors is that they change the:

Answer 45

shape and activity of the enzyme by binding to a site (not the active site); this type of inhibitor has a structure very different from that of the substrate

Question 46

what are the two mechanisms of inhibitor function?

Answer 46

some inhibitors bind to the active site, some bind to a different site. both types reduce the activity of an enzyme and the rate of the reaction

Question 47

allosteric enzyme

Answer 47

an enzyme whose activity is affected by binding a molecule at a site other than the active site. typically, allosteric enzymes change their shape on binding an activator or inhibitor

Question 48

enzyme activators and inhibitors are sometimes important in the:

Answer 48

normal operation of a cell (ex. regulation of a metabolic pathway)

Question 49

negative feedback

Answer 49

describes the effect in which the final product of a biochemical pathway inhibits the first step; the process in which a stimulus acts on a sensor that communicates with an effector, producing a response that opposes the initial stimulus. Negative feedback is used to maintain steady conditions, or homeostasis

Question 50

cofactor

Answer 50

a substance that associates with an enzyme and plays a key role in its function (ex. metal ions in enzymes)

Question 51

what naturally occurring elements might have spurred the first reactions that led to life?

Answer 51

metal ions by themselves can catalyze chemical reactions

Question 52

what has been proposed as the evolutionary descendants of chemical reactions that took place spontaneously on the early Earth?

Answer 52

the reactions now carried out in cells by iron-sulfur proteins (similar to the reactions in ancient hydrothermal environments)