Biological Molecules - Enzymes

Question 1

what are enzymes

Answer 1

Enzymes are biological catalysts
‘Biological’ because they function in living systems
‘Catalysts’ because they speed up the rate of chemical reactions without being used up or changed

Question 2

what are globular proteins

Answer 2

protiens that have a folded polypetide structure

Question 3

what is the critical part of an enzyme

Answer 3

Critical to the enzyme’s function is the active site where the substrate binds

Question 4

how are enzymes important to the human body

Answer 4

Metabolic pathways are controlled by enzymes in a biochemical cascade of reactions
Virtually every metabolic reaction within living organisms is catalysed by an enzyme – enzymes are therefore essential for life to exist

Question 5

what are intracellular enzymes

Answer 5

produced and function inside the cell

Question 6

what is an example of an intracellular enzyme

Answer 6

catalase - coverts hydrogen peroxide which is harmful to cells into water and oxygen to prevent damage to cells or tissue

Question 7

what are extracellular enzymes

Answer 7

enzymes are secreted by cells and catalyse reactions outside cells (eg. digestive enzymes in the gut)

Question 8

what is an example of an extracellular enzyme

Answer 8

amylase
secreted by salivary glands, and pancreas for digestion of starch into the mouth and small intestine

Question 9

what does the enzyme form when the substrate binds to it

Answer 9

enzyme-substrate complex

Question 10

what does specificity of an enzyme depend on

Answer 10

the complementary nature between the shape of the active site on the enzyme and its substrate(s)

Question 11

what is the structure of the active site

Answer 11

the complex tertiary structure of the protein that makes up the enzyme:
Proteins are formed from chains of amino acids held together by peptide bonds
The order of amino acids determines the shape of an enzyme
If the order is altered, the resulting three-dimensional shape changes
Also has ionic, hydrogen bonds and disulfuide bridges

Question 12

what are catabolic reactions

Answer 12

Catabolic reactions involve the breakdown of complex molecules into simpler products, which happens when a single substrate is drawn into the active site and broken apart into two or more distinct molecules
Examples of catabolic reactions include cellular respiration and hydrolysis reactions

Question 13

what are anabolic reactions?

Answer 13

Anabolic reactions involve the building of more complex molecules from simpler ones by drawing two or more substrates into the active site, forming bonds between them and releasing a single product
Examples of anabolic reactions include protein synthesis and photosynthesis

Question 14

what are the two types of reaction catalysed by enzymes

Answer 14

catabolic
anabolic

Question 15

what is the function of an enzyme in reactions

Answer 15

lower the activation energy

Question 16

how do enzymes lower the activation energy

Answer 16

reduces the energy required for the reactant to react together and providing an alternate pathway

Question 17

what is activation energy

Answer 17

Activation energy is the amount of energy needed by the substrate to become just unstable enough for a reaction to occur and for products to be formed
Enzymes speed up chemical reactions because they influence the stability of bonds in the reactants
The destabilisation of bonds in the substrate makes it more reactive

Question 18

what is the lock and key hypothesis of enzyme function

Answer 18

He suggested that both enzymes and substrates were rigid structures that locked into each other very precisely, much like a key going into a lock and active site and substrate were exactly complementary
This is known as the ‘lock-and-key hypothesis’

Question 19

what is the modified version of enzyme function called

Answer 19

the induced fit model

Question 20

how does the induced fit model work

Answer 20

Although it is very similar to the lock and key hypothesis, in this model the enzyme and substrate interact with each other:
The enzyme and its active site (and sometimes the substrate) can change shape slightly as the substrate molecule enters the enzyme- they are not exactly complementary
These changes in shape are known as conformational changes
This ensures an ideal binding arrangement between the enzyme and substrate is achieved
This maximises the ability of the enzyme to catalyse the reaction

Question 21

what is the evidence for the induced fit model

Answer 21

X-ray diffraction techniques allow for 3D pictures of molecules to be formed
This technique was used to produce pictures of the enzyme hexokinase before and after it bound to its substrate glucose
The images confirmed that the active site of the enzyme changed shape after the substrate bound

Question 22

what are the limiting factors of enzyme action

Answer 22

temp
ph
concentration
inhibitors

Question 23

how does temp effect the rate of enzyme action

Answer 23

Enzymes have a specific optimum temperature – the temperature at which they catalyse a reaction at the maximum rate

Question 24

what is the effect of lowering the temp

Answer 24

Lower temperatures either prevent reactions from proceeding or slow them down:
Molecules move relatively slow
Lower frequency of successful collisions between substrate molecules and active site of enzyme
Less frequent enzyme-substrate complex formation
Substrate and enzyme collide with less energy, making it less likely for bonds to be formed or broken (stopping the reaction from occurring)

Question 25

what is the effect of increasing the temp

Answer 25

Higher temperatures speed up reactions:
Molecules move more quickly
Higher frequency successful collisions between substrate molecules and active site of enzyme
More frequent enzyme-substrate complex formation
Substrate and enzyme collide with more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)

Question 26

what happens when the temp exceeds maximum

Answer 26

However, as temperatures continue to increase, the rate at which an enzyme catalyses a reaction drops sharply, as the enzyme begins to denature:
Bonds (eg. hydrogen bonds) holding the enzyme molecule in its precise shape start to break
This causes the tertiary structure of the protein (ie. the enzyme) to change
This permanently damages the active site, preventing the substrate from binding
Denaturation has occurred if the substrate can no longer bind
Very few human enzymes can function at temperatures above 50°C
This is because humans maintain a body temperature of about 37°C, therefore even temperatures exceeding 40°C will cause the denaturation of enzymes
High temperatures causes the hydrogen bonds between amino acids to break, changing the conformation of the enzyme

Question 27

at what temp does denaturing start to occur

Answer 27

40 degrees

Question 28

what is the effect of ph on enzyme action

Answer 28

All enzymes have an optimum pH or a pH at which they operate best and then denature

Question 29

what happens at extremes of ph

Answer 29

Enzymes are denatured at extremes of pH
Hydrogen and ionic bonds hold the tertiary structure of the protein (ie. the enzyme) together
Below and above the optimum pH of an enzyme, solutions with an excess of H+ ions (acidic solutions) and OH- ions (alkaline solutions) can cause these bonds to break
This alters the shape of the active site, which means enzyme-substrate complexes form less easily
Eventually, enzyme-substrate complexes can no longer form at all
At this point, complete denaturation of the enzyme has occurred

Question 30

how is the location of the enzyme related to the optimum ph of the enzyme

Answer 30

Where an enzyme functions can be an indicator of its optimal environment:
Eg. pepsin is found in the stomach, an acidic environment at pH 2 (due to the presence of hydrochloric acid in the stomach’s gastric juice)
Pepsin’s optimum pH, not surprisingly, is pH 2

Question 31

what can u use in experiments to find the effect of ph

Answer 31

use buffer solutions to measure the rate of reaction at different pH values:
Buffer solutions each have a specific pH
Buffer solutions maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change
A measured volume of the buffer solution is added to the reaction mixture
This same volume (of each buffer solution being used) should be added for each pH value that is being investigated

Question 32

what is the effect of enzyme concentration on enzyme activity

Answer 32

The higher the enzyme concentration in a reaction mixture, the greater the number of active sites available and the greater the likelihood of enzyme-substrate complex formation

Question 33

what happens to the enzyme activity as enzyme conc increases

Answer 33

if there are too many enzyme active sites to substrates then reaction slows as they cant bind quick enough

As long as there is sufficient substrate available, the initial rate of reaction increases linearly with enzyme concentration
If the amount of substrate is limited, at a certain point any further increase in enzyme concentration will not increase the reaction rate as the amount of substrate becomes a limiting factor

Question 34

what is the effect of substrate conc on enzyme action

Answer 34

The greater the substrate concentration, the higher the rate of reaction:
As the number of substrate molecules increases, the likelihood of enzyme-substrate complex formation increases- many unoccupied AS
If the enzyme concentration remains fixed but the amount of substrate is increased past a certain point, however, all available active sites eventually become saturated and any further increase in substrate concentration will not increase the reaction rate
When the active sites of the enzymes are all full, any substrate molecules that are added have nowhere to bind in order to form an enzyme-substrate complex
Enzyme conc is a limiting factor

Question 35

what does the graph for enzyme substrate reactions look like

Answer 35

there is a linear increase in reaction rate as substrate is added, which then flattens when all active sites become occupied

Question 36

what are the effects of inhibitors on enzyme action

Answer 36

An enzyme’s activity can be reduced or stopped, temporarily, by an inhibitor
Increasing the concentration of an inhibitor, therefore, reduces the rate of reaction and eventually, if inhibitor concentration continues to be increased, the reaction will stop completely

Question 37

what are the two types of enzyme inhibitor

Answer 37

Competitive inhibitors
Non-competitive inhibitors

Question 38

what are competitive inhibitors

Answer 38

similar shape,size and charge to substrate, so can fit into active site instead of sub forming enzyme-inhibitor complex
Reduce the number of ES that can form
However this is only temporary and does not damage AS

Question 39

what are non-competitive inhibitors

Answer 39

bind to the enzyme at an alternative site, which alters the shape of the active site and therefore prevents the substrate from binding to it
Permanent binding and damage to AS so destroys its ativity and no ES is made

Question 40

what is the effect of increasing the substrate conc on competitive inhibitors

Answer 40

countering the increase in inhibitor concentration by increasing the substrate concentration can increase the rate of reaction once more (more substrate molecules mean they are more likely to collide with enzymes and form enzyme-substrate complexes)

Question 41

what is the effect of increasing the substrate conc on non-competitive inhibitors

Answer 41

increasing the substrate concentration cannot increase the rate of reaction once more, as the shape of the active site of the enzyme remains changed and enzyme-substrate complexes are still unable to form

Question 42

what is the role of reversible inhibitors in metabolic pathways

Answer 42

act as regulators

Question 43

how can metabolic reactions be controlled

Answer 43

using the end product of a reaction as a non-competitive inhibitor
As the enzyme converts substrate to product, the process is itself slowed down as the end-product of the reaction chain binds to an alternative site on the original enzyme, changing the shape of the active site and preventing the formation of further enzyme-substrate complexes
The end product can then detach from the enzyme and be used elsewhere, allowing the active site to reform and the enzyme to return to an active state
This means that as product levels fall, the enzyme begins catalysing the reaction once again, in a continuous feedback loop
This process is known as end-product inhibition