6.5 Enzymes and the Rate of Chemical Reactions

Question 1

What are catalysts?

Answer 1

substances that increase the rate of chemical reactions without themselves being consumed (ie. enzymes)

Question 2

What do enzymes play a critical role in?

Answer 2

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

Question 3

What do enzymes do?

Answer 3

reduce the activation energy of a chemical reaction by stabilizing the transition state and decreasing its free energy

Question 4

What are exergonic reactions?

Answer 4

release free energy

Question 5

What are endergonic reactions?

Answer 5

require free energy

Question 6

What do all chemical reactions require intially?

Answer 6

an initial input of energy is needed to proceed

Question 7

Why is an input of energy needed for all chemical reactions?

Answer 7

as a chemical reaction proceeds, existing chemical bonds break and new bonds form

Question 8

What is a transition state?

Answer 8

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

it is highly unstable and therefore has a large amount of free energy

all chemical reactions have at least one transition state before they convert into products

Question 9

How does a reactant reach a transition state?

Answer 9

must absorb energy from its surroundings

as a result, all chemical reactions, even spontaneous ones that release energy, require an input of energy that we can think of as an “energy barrier”

Question 10

What is activation energy?

Answer 10

energy input necessary to reach the transition state

Question 11

What happens once the transition state is reached?

Answer 11

reaction proceeds, products are formed, and energy is released into the surroundings

Question 12

What is the correlation between rate of reaction and height of energy barrier?

Answer 12

an inverse correlation between the rate of a reaction and the height of the energy barrier: the lower the energy barrier, the faster the reaction and the higher the barrier, the slower the reaction

Question 13

What does not change although an enzyme reduces the activation energy?

Answer 13

the difference in free energy between reactants and products (ΔG) does not change

an enzyme changes the path of the reaction between reactants and products, but not the starting or end point

Question 14

How do enzymes emerge from a chemical reaction unchanged?

Answer 14

they form a complex with the reactants and products

Question 15

What is a substrate?

Answer 15

reactant in a chemical reaction catalyzed by an enzyme

Question 16

Describe

S + E ⇋ ES ⇋ EP ⇋ E+P

Answer 16

in the presence of an enzyme:

• substrate forms a complex with enzyme (ES)
• still part of the complex, substrate is converted to product (EP)
• complex dissociates, releasing the enzyme and product

Question 17

What is the formation of a complex critical for?

Answer 17

accelerating the rate of a chemical reaction

Question 18

What is an active site?

Answer 18

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

Question 19

How is the active site formed?

Answer 19

enzymes are folded into three-dimensional shapes that bring particular amino acids into close proximity

Question 20

What bonds are formed in the active site? What do these interactions do?

Answer 20

enzyme and substrate form transient covalent bonds, weak noncovalent interactions, or both

together, these interactions stabilize the transition state and decrease the activation energy

Question 21

How do enzymes reduce the energy of activation?

Answer 21

by positioning two substrates to react: within the active site, their reactive chemical groups are aligned and their motion relative to each other restrained

Question 22

Why are enzymes so large? Of the many amino acids that form the active site, why are only a few actively contributing to catalysis?

Answer 22

each of these amino acids has to occupy a very specific spatial position to align with the correct reactive group on the substrate

if the few essential amino acids were part of a short peptide, the alignment of chemical groups between the peptide and the substrate would be difficult or even impossible because the length of the bonds and the bond angles in the peptide would constrain its three-dimensional structure

catalytic amino acids are spaced far apart in the primary structure of the enzyme, but brought close together by protein folding

Question 23

How big is the active small?

Answer 23

extremely small compared with the size of the enzyme

Question 24

Why is the large size of many enzymes required?

Answer 24

to bring the catalytic amino acids into very specific positions in the active site of the folded enzyme

Question 25

Why are enzymes highly specific?

Answer 25

• they recognize either a unique substrate or a class of substrates that share a common chemical structure
• catalyze only one reaction or a very limited number of reactions

Question 26

What can enzyme activity be influenced by?

Answer 26

inhibitors and activators

Question 27

What are inhibitors?

Answer 27

• synthesized compound that decreases the activity of an enzyme
• very common
• synthesized naturally by many plants and animals as a defence against predators
• many drugs to treat infections and diseases use them

Question 28

What are activators?

Answer 28

compound that increases the activity of an enzyme

Question 29

What are the 2 classes of inhibitors?

Answer 29

irreversible: usually form covalent bonds with enzymes and irreversibly inactivate them
reversible: form weak bonds with enzymes and therefore easily dissociates from them

Question 30

Describe 2 different ways inhibitors can act.

Answer 30

1. inhibitor is similar in structure to the substrate and therefore is able to bind to the active site of the enzyme
   - binding of the inhibitor prevents the binding of the substrate
   - inhibitor competes with the substrate for the active site of the enzyme
   - can often be overcome by increasing the concentration of substrate
2. usually has a structure very different from that of the substrate
   - bind to a site other than the active site of the enzyme but still inhibit the activity of the enzyme
   - in this case, binding of the inhibitor changes the shape and therefore the activity of the enzyme

Question 31

What are allosteric enzymes?

Answer 31

enzymes that are regulated by molecules that bind at sites other than their active sites

Question 32

What is the activity of allosteric enzymes influenced by?

Answer 32

both inhibitors and activators

Question 33

What do allosteric enzymes play a key role in?

Answer 33

regulation of metabolic pathways

Question 34

What is negative feedback?

Answer 34

• 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