Enzymes

Question 1

What are enzymes?

Answer 1

Enzymes are globular proteins with a specific tertiary structure that catalyses metabolic reactions inside living organisms.

Question 2

What is the active site?

Answer 2

A pocket or cleft on the surface of the enzyme that is shaped specifically for a certain substrate to fit in. The active site is usually a very small part of the enzyme molecule, with the rest of the tertiary structure responsible for maintaining that specific shape of the active site.

Question 3

Why are enzymes ‘biological catalysts’?

Answer 3

They speed up reactions without being changed themselves and thus can be reused over and over again.

Question 4

How many enzymes are there?

Answer 4

Looking at all the different bonds that are formed as well as broken in the body, each bond has a specific enzyme associated with it. It’s this specificity that makes enzymes different to industrial catalysts. In total there are over 1000 different enzymes in the body.

Question 5

What is an enzyme controlled reaction?

Answer 5

When a specific enzyme acts on a specific substrate in either a catabolic reaction (breaking substrate down into products) or anabolic reaction (building substrate up into product).

Question 6

Why are enzymes a big part of digestion?

Answer 6

Usually, nutrients in food sources are present as large molecules which aren’t of much use to the consumer. They have to be broken down to smaller, usable molecules before they can be absorbed. To do this, many bonds like peptide, glycosidic and ester bonds need to be broken, which is what enzymes are for.

Question 7

Where do enzymes act?

Answer 7

Extracellular: Cells excrete the enzyme outside of the cell where it does its job and then absorb the products (usually digestive enzymes).
Intracellular: The enzymes are present in the cytoplasm or in the cell membrane. The substrates need to either be absorbed into the cell first or bind onto the membrane in order to be broken down (usually enzymes in respiration/photosynthesis).

Question 8

How do enzymes work?

Answer 8

Biological reactions require lots of energy to occur. An enzyme (as a catalyst) lowers the activation energy required for the reactions to happen. This happen because the fitting of the substrate into the active site destabalises the structure of the substrate, making it react more easily; especially in the low temperatures of organisms.

Question 9

What is the lock and key model of enzyme action?

Answer 9

Substrates have a complementary shape to the shape of the enzymes’s active site and fits in exactly. Because only one type of substrate is shaped in this way, the enzyme is specific and can only catalyse one reaction. Just like only one key can fit into the keyhole of a lock.

Question 10

What is the induced fit hypothesis of enzyme action?

Answer 10

A better model to describe enzyme action is the induced fit model. This model states that the shape of the active site isn’t exactly like the shape of the substrate, but is very similar so the substrate still has a complementary shape to the active site. However, when the substrate enters the active site, the active site moulds itself around the substrate, forming an enzyme-substrate complex. This also puts strain on the substrate and weakens some of its bonds, making it easier for a reaction to occur. Once the product(s) is formed, it has a different shape to the substrate so no longer fits the active site and thus is released.

Question 11

What is the general trend of rates of enzyme controlled reactions?

Answer 11

Rate of reaction decreases with time because concentration of substrate decreases. This means that probability of substrate-enzyme collisions decrease and thus less enzyme-substrate complexes are formed and less products are formed, so rate of reaction decreases.
Temperature could also increase.
pH can change.
Product may inhibit enzyme action.

Question 12

What major factors affect rate of enzyme controlled reactions?

Answer 12

1. Collisions: Substrate molecules need to collide with enzyme in order for enzyme-substrate complexes to form. Increasing the rate of collision will change the rate of reaction.
2. Active site shape: The shape of the active site must be complementary to the shape of the substrate otherwise the substrate molecule won’t fit and enzyme-substrate complexes can’t form so rate of reaction decreases.

Question 13

How do you describe a rate of reaction against temperature graph?

Answer 13

As temperature increases, rate of reaction also increases up to a maximum point then begins to decrease more rapidly than it increased beyond this optimum temperature.

Question 14

Why is a rate of reaction against temperature graph shaped this way?

Answer 14

An increase in heat energy increases the amount of kinetic energy enzyme and substrate molecules have, this means that more substrate-enzyme collisions occur and more successful collisions occur so more enzyme-substrate complexes form so more product is formed so rate of reaction increases. However, more kinetic energy also means that the molecules vibrate more. This vibration puts strain on the bonds in the enzyme’s tertiary structure. As temperature increases beyond optimum, the particles are vibrating so much that weaker bonds like hydrogen and ionic bonds break within the enzyme’s tertiary structure, changing it and the shape of the active site so that substrate np longer fits. Enzyme-substrate complex can’t be formed so rate of reaction decreases. The enzyme is denatured.

Question 15

How do you describe a rate of reaction against pH graph?

Answer 15

Enzyme works in a narrow range of pH. As the pH tends away from optimum pH, rate of reaction decreases.

Question 16

Why is a rate of reaction against pH graph shaped this way?

Answer 16

Acids contain lots of positively charged H+ ions. Because the tertiary structure of enzyme is held together by many charged ionic/hydrogen bonds, acidic solution affects these bonds and alters them. Tertiary structure of enzyme changes shape, active site changes shape so substrate can no longer fit so enzyme-substrate complexes can’t form and rate of reaction decreases. Enzyme is denatured.
H+ ions also disrupt charges around active site of enzyme, making it harder for substrates to bind and form enzyme-substrate complexes, reducing rate of reaction.

Question 17

How do you describe a rate of reaction against substrate concentration graph?

Answer 17

As substrate concentration increases, rate of reaction also increases in direct proportion up to a maximum point when the rate levels off.

Question 18

Why is a rate of reaction against substrate concentration graph shaped this way?

Answer 18

This is because as concentration of substrates increase, more collisions occur and more enzyme-substrate complexes form, so rate of reaction increases. This happens up to a point when all enzyme active sites are being occupied by substrate molecules so rate cannot increase further. The rate of reaction is limited by the enzyme concentration.

Question 19

How do you describe a rate of reaction against enzyme concentration graph?

Answer 19

Rate of reaction increases in direct proportion to enzyme concentration and then begins to gradually level off as enzyme concentration increases further.

Question 20

Why is a rate of reaction against enzyme concentration graph shaped this way?

Answer 20

The higher the enzyme concentration, the more active sites are available so more enzyme-substrate complexes form and rate of reaction increases. Eventually, all available substrate molecules are occupying an active site so rate of reaction can no longer increase. The substrate concentration is limiting the rate of reaction.

Question 21

What ways are there to limit rates of reactions?

Answer 21

Activating: Only produced when needed and activated by genes or needing a co-enzyme/co-factor to work.
Inhibiting: Decreasing rate of enzyme catalysed reactions using inhibitors.

Question 22

How do co-factors work?

Answer 22

They bind to the enzyme permanently and are usually prosthetic groups. They are usually inorganic compounds and contribute towards either the tertiary shape of the enzyme or the charge around the active site so substrates bind onto active site more easily.

Question 23

How do co-enzymes work?

Answer 23

Organic molecules like vitamins that bind onto the enzyme temporarily either before or as substrate binds to active site. Changed during reaction but recycled and converted back to be used again. They often act as carriers between enzymes in sequenced enzyme catalysed reactions.

Question 24

How do competitive inhibitors work?

Answer 24

They have similar shapes to the substrate so they fit into the active site but do not produce products because they are not identical. As the inhibitor occupies the active site, substrates cannot enter the active site. The inhibitors reduce the number of available active sites and less enzyme-substrate complexes form, so rate of reaction decreases.

Question 25

What does a rate of reaction against substrate concentration graph look like with a competitive inhibitor?

Answer 25

Level of inhibition decreases as substrate concentration increases. This is because the inhibitors are in competition with the substrate. If there are more substrate molecules than inhibitor molecules, the probably of a substrate molecule binding to an active site is greater than an inhibitor molecules so fewer inhibitor molecules bind to active sites and more active sites become available so rate of reaction increases.

Question 26

What does a rate of reaction against substrate concentration graph look like with a non-competitive inhibitor?

Answer 26

Level of inhibition is fixed and is not affected by substrate concentration so rate of reaction is reduced to a certain amount regardless of substrate concentration. This is because a non-competitor is not in competition with the substrate and will bind on and inhibit the enzyme molecule regardless of whether a substrate is in the active site or not.

Question 27

How do non-competitive inhibitors work?

Answer 27

They do not bind onto the active site of enzyme but binds to another part of enzyme called allosteric site. This disrupts the bonding within tertiary structure of enzyme and distorts the shape of the tertiary structure as well as the shape of the active site so substrates can no longer fit into the active site. This means less enzyme-substrate complexes are formed and rate of reaction decreases.

Question 28

What is end product inhibition?

Answer 28

When a product of a metabolic pathway acts as a non-competitive inhibitor to the enzyme at the beginning of the metabolic pathway and slows down the rate of reaction for the metabolic pathway.

Question 29

How does an inhibitor act as a poison?

Answer 29

Potassium cyanide is a non-competitive inhibitor for an enzyme in the mitochondria called cytochrome oxidase which is an essential part in aerobic respiration, when potassium cyanide inhibits the activity of cytochrome oxidase, the body respires anaerobically and lactic acid builds up in the bloodstream. This leads to unconsciousness and then a coma and death follows shortly after.

Question 30

How does an inhibitor act as a drug?

Answer 30

Chemicals used for treating HIV are usually competitive inhibitors for protease which catalyses protein synthesis in viruses. They inhibit protein production and the virus struggles to build and repair virus coats so the virus eventually dies.