Enzymes

Question 1

What is a catalyst?

Answer 1

A substance that changes the rate of reaction without changing the sibstance produced. It is unaffected at the end of the reaction and can be used again

Question 2

What are enzymes?

Answer 2

• biological catalysts which control the rate of reaction in individual cells and whole organisms
• they make life possible by speeding up the rate of reaction in cells without changing the conditions in the cytoplasm
• they are globular proteins produced during protein synthesis
• they have a specific shape as a result of their primary, secondary and quaternary structures and so each enzyme will only catalyse a specific reaction or group of reactions - they show great specificity

Question 3

Why are enzymes needed for life?

Answer 3

Because under the conditions of temperature ans pH found in living cells most of the reactions that provide cells with energy and produce new biological material would take place too slowly for life too exist

Question 4

What is specificity?

Answer 4

The characterisitic of enzymes that means that as a result of their specific shapes resulting from their primary, secondary, tertiary and quaternary structures each enzyme will only catalyse a specific reaction or group of reactions

Question 5

What is an anabolic reaction?

Answer 5

A reaction that builds up (synthesises) new molecules in a cell

Question 6

What are catabolic reactions?

Answer 6

A reaction which breaks down substances inside a cell

Question 7

What is metabolism?

Answer 7

The sum of anabolic and catabolic processes in a cell

Question 8

What is a metabolic chain (metabolic pathway)?

Answer 8

A series of linked reactions in the metabolism of a cell. Most of the reactions of metabolism occur as a sequence of events

Question 9

What are intracellular enzymes?

Answer 9

Enzymes that catalyse reactions within the cell

Question 10

What are extracellular enzymes?

Answer 10

Enzymes that catalyse reactions outside of the cell in which they were made

Question 11

What are the names that most enzymes have?

Answer 11

• a relatively short recomended name, which is often the name of the molecule that the enzyme works on (the substrate) with ‘-ase’ on the end
• a longer systematic name describing the type of reaction being catalysed e.g. ATP: creatine phosphotransferase
• a classification number

Question 12

What is the activation energy?

Answer 12

The energy needed for a reaction to get started

Question 13

How do enzymes make reactions faster?

Answer 13

They lower the activation energy needed for a reaction to take place

Question 14

How do enzymes work?

Answer 14

They form a complex with the substrate or substrates of the reaction:

Substrate + enzyme enzyme/ substrate complex enzyme + products

Question 15

What is the lock and key hypothesis?

Answer 15

• within the globular protein structure of each enzyme is an area known as the active site 5hat has a very specific shape.
• Only one substrate or type of susbstrate will fit the shape of the gap and the enzyme and the substrate slot together to form a complex
• the formation of the enzyme/substrate complex lowers the activation energy of the reaction
• the active site affects the bonds in the substrate making it easier for them to break, and the reacting substances are brought close together making it easier for bonds to form between them.
• once the reaction is complete the products are no longer the right shape to stay in the active site and the complex breaks up, releasing the products and freeing the enzyme for further catalytic action

Question 16

What evidence suggests that the active site is not a rigid shape?

Answer 16

Evidence from X-ray crystallography and chemical analysis of active sites

Question 17

What is the induced fit hypothesis?

Answer 17

Once the substrate enters the active site, the shape of the site is modified around it to form the active complex. Once the products have left the complex the enzyme reverts to its inactive, relaxed form until another substrate molecule binds

Question 18

When investigating enzymes and how they act as catalysts what do they measure?

Answer 18

• The reaction rate
• measure it with and without enzymes
• to measure different factors on the rate of enzyme-catalysed reactions biologists measure the initial rate of reaction each time the independent variable is changed
• you have to use a large excess of substrate in enzyme experiments unless the effect of substrate concentration is under investigation

Question 19

Why are only tiny amounts of most enzymes needed?

Answer 19

Because they increase reaction rates by factors of 10-8 to 10-26

Question 20

Why is the initial rate of reaction measured?

Answer 20

Because this is when the reaction proceeds at its fastest rate. This gives the maximum reaction rate for an enzyme under particular conditions

Question 21

What is the molecular activity (turnover number)?

Answer 21

The number of substrate molecules transformed per minute by a single enzyme molecule.

Question 22

Why are enzyme controlled reactions affected by the concentration of the enzyme?

Answer 22

Because if every enzyme molecule is involved in a reaction it will not go faster unless there is an increase in the enzyme concentration

Question 23

What is the evidence for an active site in an enzyme?

Answer 23

Enzymes are very specific to the reaction they catalyse which suggests there is a physical site within the enzyme with a particular shape into which a specific substrate will fit

Question 24

Why does the concentration of the substrate affect the rate of an enzyme catalysed reaction?

Answer 24

First when the substrate concentration is increased the rate of the enzyme catalysed reaction increases but eventually the enzyme becomes saturated (all of the active sites are occupied by substrate molecules) and a further increase in substrate concentration will not increase the rate of the reaction further

Question 25

How does temperature affect the rate of enzyme catalysed reactions?

Answer 25

• between about 0 and 40°c the rate of reaction doubles for every 10°c rise in temperature as it leada to more succesful collisions
• the rate of enzyme catalysed reactions in humans falls as the temperature rises and at about 60°c the reaction stops completely in most cases.
• at temperatures over 40°c most proteins including enzymes start to lose their tertiary and quarternary structures - they denture. When the enzymes start to denature the shape of the active site changes and so they lose their ability to catalyse reactions.

Question 26

What enzymes don’t denature at about 40°c?

Answer 26

The enzymes of thermophillic bacteria which live in hot springs at temperatures of up to 85°c

Question 27

How does pH affect enzyme activity?

Answer 27

It affects the shape of the protein molecules. Different enzymes work in different ranges of pH because changes in pH affect the interactiond between R groups (e.g. the hydrogen and ionic bonds that hold the 3D structure of the protein together. The optimum pH is not always the same as the pH of its normal surroundings. This is one way in which cells control the effects of their intracellular enzymes - increasing or decreasing their activity by small changes in pH

Question 28

What are enzyme inhibitors?

Answer 28

Substances that slow down enzymes or stop them from working

Question 29

What are the two main types of enzyme inhibition and what happens in each?

Answer 29

• reversible inhibition: inhibition of the action of an enzyme by an inhibitor that does not permanently affect the functioning of the enzyme and can be removed from the enzyme. It is often used to control reaction rates within the cell
• irreversible inhibition: inhibition of the action of an enzyme that is permenant and cannot be undone

Question 30

What are the two major forms of reversible inhibition?

Answer 30

competitive inhibition and non-competitive inhibition

Question 31

What is competitive inhibition?

Answer 31

The inhibitor molecule is similar in shape to the substrate molecule. It competes with the substrate for binding at the active sites of the enzymes forming an enzyme/inhibitor complex. If the amount of inhibitor is fixed the percentage of inhibition can be reduced by increasing the substrate concentration

Question 32

What is non-competitive inhibition?

Answer 32

• The inhibitor may form a complex with either the enzyme itself or with the enzyme/ substrate complex. This shows that the inhibitor is not competing for the active site. It joins to the enzyme molecule elsewhere.
• only the concentration of inhibitor affects the level of inhibition. The concentration of the substrate makes no difference
• the presence of the inhibitor on the enzyme or enzyme/ substrate complex deforms ornchanges the shape of the active site so that it can no longer catalyse the reaction

Question 33

What happens in the irreversible inhibition of enzymes?

Answer 33

The inhibitor combines with the enzyme by permanent covalent bonding to one of the groups vital for catalysis to occur. It changes the shape and structure of the molecule in such a way that it cannot be reversed. It tends to occur more slowly than reversible inhibition but the effects are much more devastating

Question 34

Why are arsenic, cyanide and mercury poisonous?

Answer 34

Because they exert irreversible inhibition on enzyme systems

Question 35

How do the nerve gases used in chemical warfare work?

Answer 35

They combine with and completely inactivate enzymes such as acetyl cholinesterase that break down chemicals used to transfer impulses from the nervous system to the muscles of the body. The normal function or acetyl cholinesterase is to destroy the neurotransmitter acetylochin at the junctions between neurones and muscle cells. It does this as soon as an impulse has been passed from a nerve to a muscle. When the enzyme is inhibited the impulse continues and the muscles go into prolonged spasm

Question 36

What factors are involved in cells controlling their reactions?

Answer 36

Membrane compartments keep reactions apart. Variations in pH can change the rate od enzyme-catalysed reactions and the amount of substrate available is another mechanism but the most important is regulatory enzymes

Question 37

What are regulatory enzymes?

Answer 37

Enzymes that have a site seperate to the active site where another molecule can bind to gave either an activating or inhibiting effect. They are widely found in complex metabolic pathways such as photosynthesis and respiration

Question 38

What is end-product inhibition and how does this work with PFK?

Answer 38

• it’s a control system in many metabolic pathways in which an enzyme at the beginning of the pathway is inhibitted by one of the end products of the reaction
• PFK is an enzyme involved in the production of ATP in glycolysis in cellular respiration. PFK is inhibitted by ATP which binds non-competitevly and changes the shape of the active site. If the ATP concentration goes up PFK is inhibitted and cellular respiration slows down. As ATP levels fall ATP molecules detach from PFK and the enzyme becomes active again.