ENZYMES (LECTURE 3)

Question 1

What are enzymes?

Answer 1

Enzymes are biological catalysts.

Question 2

In what way are enzymes important for metabolism?

Answer 2

Enzymes catalyze every step in metabolic pathways.

Question 3

What must we assume about every chemical reaction in the body?

Answer 3

Unless stated otherwise, assume every reaction in the body is catalyzed by an enzyme.

Question 4

Most enzymes are ____.

Answer 4

Most enzymes are proteins.

Question 5

How do enzymes work? (2)

Answer 5

They speed up the rate of a chemical reaction by decreasing the activation energy. In doing so, enzymes increase the number of molecules that have enough energy to react.

Question 6

Do spontaneous reactions (-ΔG) require an enzyme? Explain.

Answer 6

Yes. Even in spontaneous reactions, an initial input of energy (EA) is required to break reactant bonds. What makes these reactions spontaneous is that the product’s bonds have less potential energy than in the reactants (net release of energy).

Question 7

Describe activation energy (EA).

Answer 7

EA is the amount of energy required for reactants to reach the transition state.

Question 8

What is the transition state? (2)

Answer 8

The transition state is an in-between state when old bonds are being broken so new bonds can form (molecules are unstable in this state).

Question 9

Do enzymes change ΔG?

Answer 9

No.

Question 10

Why is it important that reactions in our body only occur with the help of an enzyme? (3)

Answer 10

Regulation; we only want reactions to occur at a specific time, in a specific place and at a specific rate. The necessity of enzymes means that enzymes can regulate these reactions. The cell can regulate the enzymes by “turning them on and off” (changing their shape).

Question 11

Enzymes catalyze the conversion of ____ to ____. Enzyme + _____ –> ____ ____ –> enzyme + ____

Answer 11

Enzymes catalyze the conversion of substrate(s) to products.
enzyme + substrate –> enzyme-substrate complex –> enzyme + products

Question 12

What happens to the enzyme after it catalyzes a reaction?

Answer 12

Nothing, enzymes are not consumed or change during a reaction.

Question 13

Enzymes are _____ specific, meaning what?

Answer 13

Enzymes are substrate specific, meaning different enzymes allow only specific reactions to occur.

Question 14

What is the active site?

Answer 14

It is a pocket or groove on the enzyme’s surface where the substrate binds.

Question 15

The substrate binds to the active site of an enzyme using what type(s) of bonds?

Answer 15

Using non-covalent interactions such as H-bonds and ionic bonds.

Question 16

Describe the model of induced fit.

Answer 16

By the model of induced fit, the binding of a substrate causes a change in the shape of the enzyme to bring amino acid side chains in position to catalyze the reaction.

Question 17

The model of induced fit is comparable to what?

Answer 17

To catching a ball (substrate) with your hands (enzyme).

Question 18

Describe the process of an enzyme catalyzing a reaction. (6)

Answer 18

(1) Substrates enter the active site which changes shape to enfold the substrates. (2) Substrates held in place in the active site by weak interactions. (3) The active site catalyzes the reaction between the substrates. (4) Substrates are converted into products. (5) Products are released. (6) Active site is now available for new substrate molecules.

Question 19

In what ways can enzymes lower activation energy? (How does it stabilize the transition state?) (4)

Answer 19

(1) Stressing bonds to make them easier to break. (2) Holding substrates together in the correct orientation. (3) Creating a more favourable microenvironment within the active site. (4) Participating directly in the reaction, although the enzyme remains unchanged as expected.

Question 20

What factors affect the reaction rate of any given reaction within the body? (4)

Answer 20

Substrate concentration, enzyme concentration, environmental conditions (temperature and pH) and cofactors and/or coenzymes.

Question 21

Describe the effect of substrate concentration on the rate of any given reaction within the body?

Answer 21

Increasing substrate concentration increases the rate of reaction until the point of saturation has been reached, at which point the rate of reaction plateaus.

Question 22

Why does the rate of reaction plateaus once substrate saturation has been reached?

Answer 22

Because all active sites are occupied, thus a further increase in substrate concentration would no longer affect the rate of reaction.

Question 23

Describe the effect of enzyme concentration on the rate of any given reaction within the body?

Answer 23

Increasing enzyme concentration increases the rate of reaction. As long as there is always an excess of of substrate (which is assumed since it is often the case), the rate of reaction will continue to linearly increase with the increase of enzyme concentration.

Question 24

Describe the effect of temperature on the rate of any given reaction within the body?

Answer 24

The effect of temperature is enzyme dependent, however the effect on reaction rate can be generally described as follows: an increase in temperature results in an increase of collisions between substrates and enzymes. This then increases the rate of reaction until the optimal temperature is reached.

Question 25

Why does enzyme activity drop off at higher temperatures?

Answer 25

After the optimal temperature is reached, any further increase in temperature will denature the protein and render it useless. This of course thus decreases the rate of reaction.

Question 26

Describe the effect of pH on the rate of any given reaction within the body? (2)

Answer 26

Enzymes are usually most active within a specific pH range (depends on the enzyme). Extreme pH affects the ionization of amino acid side chains and cause the enzyme to denature.

Question 27

What is the optimal pH range for most enzymes?

Answer 27

The optimal pH range for most enzymes is 6-8.

Question 28

Why do different enzymes have different pH optimums?

Answer 28

Because they function in environments with different pH.

Question 29

What are cofactors and coenzymes? (3)

Answer 29

Some enzymes require additional molecules to function. These are cofactors and coenzymes, which are non-protein molecules that can bind permanently or reversibly to the enzyme.

Question 30

What are cofactors?

Answer 30

These are inorganic metal ions (Zn2+, Cu2+, Fe2+) that bind either permanently or reversibly to some enzymes so that they can function.

Question 31

What are coenzymes?

Answer 31

These are organic molecules (such as vitamins) which bind weakly to some enzymes so that they can function.

Question 32

What is enzyme inhibition? (what does an enzyme inhibitor do?)

Answer 32

Enzyme inhibition is when an enzyme inhibitor (molecule) disrupts the reaction pathway between an enzyme and a substrate.

Question 33

Enzyme inhibition can be classified as ____ or ____, and as ____ or ____.

Answer 33

Enzyme inhibition can be classified as irreversible or reversible, and as competitive or non-competitive.

Question 34

What characterizes irreversible enzyme inhibition? Provide 3 examples of irreversible inhibitors.

Answer 34

The inhibitor attaches by a covalent bond, thus making it irreversible (ex. nerve gas, penicillin, some poisons).

Question 35

Why isn’t there an abundance of irreversible enzyme inhibitors in the body?

Answer 35

Because the body does not want to permanently disable enzymes - this would render them useless.

Question 36

What characterizes reversible enzyme inhibition?

Answer 36

The inhibitor attaches by weak, non-covalent bonds. This makes it reversible, reversible enzyme inhibition is an important part of regulating metabolism in our bodies.

Question 37

What characterizes a competitive inhibitor?

Answer 37

The inhibitor binds to the active site and thus competes with substrates for binding. Competitive inhibitors can thus be overcome by increasing substrate concentration.

Question 38

What characterizes a non-competitive inhibitor?

Answer 38

Non-competitive inhibitors bind somewhere else on the enzyme (other than the active site). This causes the enzyme to change shape so that it no longer binds to substrates at all.

Question 39

Would increasing substrate concentration eventually overpower non-competitive inhibitors?

Answer 39

No. Since non-competitive inhibitors render the enzyme physically incapable of binding to substrates, an increase in substrate concentration would not overcome the inhibition.

Question 40

What are allosteric enzymes involved in? What is particular about their structure?

Answer 40

These enzymes are involved in the regulation of metabolic pathways. They are comprised of more than one polypeptide (each with its own active site).

Question 41

What is the allosteric site? Where is it usually located?

Answer 41

It is the location where a regulator binds to an allosteric enzyme (regulators do not bind at the active site). Allosteric sites are usually located where the polypeptides of an allosteric enzyme join.

Question 42

How do regulators bind? Where to they bind? To what do they bind? What are the 2 types of regulators?

Answer 42

Regulators are molecules that bind weakly to an allosteric site of an allosteric enzyme. Regulators can either be the inhibitor or activator of the enzyme.

Question 43

What do allosteric activators do?

Answer 43

These regulators stabilize the active form of the enzyme.

Question 44

What do allosteric inhibitors do?

Answer 44

These regulators stabilize the inactive form of the enzyme.

Question 45

Do allosteric inhibitors and allosteric activators ever compete for the allosteric site?

Answer 45

Inhibitors and activators can often compete for the allosteric site.

Question 46

Describe negative feedback inhibition.

Answer 46

Negative feedback inhibition is when the eventual product of a metabolic pathway acts as an allosteric inhibitor to an allosteric enzyme that catalyzes an earlier reaction in the same metabolic pathway. By this method of self regulation, only a certain amount of product is made.

Question 47

Provide an example of negative feedback inhibition.

Answer 47

Enzymes in cellular respiration pathways have allosteric sites. ATP (a product of cellular respiration) acts as an allosteric inhibitor and AMP (adenosine monophosphate) acts as an allosteric activator. Recall that AMP forms when there is a shortage of ATP, at which point the body turns two ADP molecules into one ATP and one AMP molecule.