chapter 4

Question 1

Why are enzymes important?

Answer 1

Most of the processes necessary to life involve chemical reactions, and these reactions need to happen very fast.
In the laboratory or in industry this would demand very high temperatures and pressures.
These extreme conditions are not possible in living cells - they would damage the cell components.
Instead, the reactions are catalysed by enzymes.

Question 2

What are enzymes:

Answer 2

Enzymes are biological catalysts.
They are globular proteins that interact with substrate molecules causing them to react at much faster rates without the need for harsh environmental conditions.
Without enzymes many of the processes necessary to life would not be possible.

Question 3

The role of enzymes in reactions:

Answer 3

Living organisms need to be built and maintained.
This involves the synthesis of large polymer-based components.
For example, cellulose forms the walls of plants cells and long protein molecules form the contractile filaments of muscles in animals.
The different cell components are synthesised and assembled into cells, which then form tissues, organs, and eventually the whole organism.
The chemical reactions required for growth are anabolic (building up) reactions and they are all catalysed by enzymes.

Question 4

Catabolic reactions:

Answer 4

Energy is released from large organic molecules, like glucose, in metabolic pathways consisting of many catabolic (breaking down) reactions.
Catabolic reactions are also catalysed by enzymes.
These large organic molecules are obtained from the digestion of food, made up of even larger organic molecules, like starch.
Digestion is also catalysed by a range of enzymes.
Reactions rarely happen in isolation but as part of multi-step pathways.

Question 5

Metabolism:

Answer 5

Metabolism is the sum of all of the different reactions and reaction pathways happening in a cell or an organism, and it can only happen as a result of the control and order imposed by enzymes.

Question 6

Speed of cellular reactions depends on:

Answer 6

dependent on environmental conditions.
The temperature, pressure, and pH may all have an effect on the rate of a chemical reaction.
Enzymes can only increase the rates of reaction up to a certain point called the Vmax (maximum initial velocity or rate of the enzyme-catalysed reaction).

Question 7

How enzymes work:

Answer 7

Molecules in a solution move and collide randomly.
For a reaction to happen, molecules need to collide in the right orientation.
When high temperatures and pressures are applied the speed of the molecules will increase, therefore so will the number of successful collisions and the overall rate of reaction.

Question 8

specificity of the enzyme:

Answer 8

Many different enzymes are produced by living organisms, as each enzyme catalyses one biochemical reaction, of which there are thousands in any given cell.
This is termed the specificity of the enzyme.

Question 9

activation energy:

Answer 9

The energy that’s needed to be supplied for most reactions to start
Sometimes, the amount of energy needed is so large it prevents the reaction from happening under normal conditions.
Enzymes help the molecules collide successfully, and therefore reduce the activation energy required.
There are two hypotheses for how enzymes do this.

Question 10

Lock and key hypothesis:

Answer 10

An area within the tertiary structure of the enzyme has a shape that is complementary to the shape of a specific substrate molecule.
This area is called the active site.
In the same way that only the right key will fit into a lock, only a specific substrate will fit the active site of an enzyme - This is the lock and key hypothesis.
When the substrate is bound to the active site an enzyme-substrate complex is formed.
The substrate or substrates then react and the product or products are formed in an enzyme-product complex.
The product or products are then released, leaving the enzyme unchanged and able to take part in subsequent reactions.

Question 11

Lock and key hypothesis chemical formation:

Answer 11

The substrate is held in such a way by the enzyme that the right atom-groups are close enough to react.
The R-groups within the active site of the enzyme will also interact with the substrate, forming temporary bonds.
These put strain on the bonds within the substrate, which also helps the reaction along.

Question 12

lock and key image

Answer 12

Question 13

Induced-fit hypothesis:

Answer 13

More recently, evidence from research into enzyme action suggests the active site of the enzyme actually changes shape slightly as the substrate enters.
This is called the induced-fit hypothesis and is a modified version of the lock and key hypothesis.

Question 14

Induced-fit hypothesis chemical formation:

Answer 14

The initial interaction between the enzyme and substrate is relatively weak, but these weak interactions rapidly induce changes in the enzyme’s tertiary structure that strengthen binding, putting strain on the substrate molecule.
This can weaken a particular bond or bonds in the substrate, therefore lowering the activation energy for the reaction.

Question 15

What is an Intracellular enzyme

Answer 15

Enzymes that act within cells

Question 16

Intracellular enzyme example:

Answer 16

Hydrogen peroxide is a toxic product of many metabolic pathways. The enzyme catalase ensures hydrogen peroxide is broken down to oxygen and water quickly, therefore preventing its accumulation.
It is found in both plant and animal tissues.

Question 17

Cellular Reactions and Nutrient Supply:

Answer 17

All of the reactions happening within cells need substrates (raw materials) to make products needed by the organism.
These raw materials need to be constantly supplied to cells to keep up with the demand.
Nutrients (components necessary for survival and growth) present in the diet or environment of the organism supply these materials.
Nutrients are often in the form of polymers such as proteins and polysaccharides.
These large molecules cannot enter cells directly through the cell-surface membrane.
They need to be broken down into smaller components first.
Enzymes are released from cells to break down these large nutrient molecules into smaller molecules in the process of digestion.
These enzymes are called extracellular enzymes.

Question 18

What are extracellular enzymes:

Answer 18

Enzymes that work outside the cell that made them.
In some organisms, for example fungi, they work outside the body.
Both single-celled and multicellular organisms rely on extracellular enzymes to make use of polymers for nutrition.
Single-celled organisms, such as bacteria and yeast, release enzymes into their immediate environment.

Question 19

extracellular enzymes examples:

Answer 19

The enzymes break down larger molecules, such as proteins, and the smaller molecules produced, such as amino acids and glucose, are then absorbed by the cells.

Many multicellular organisms eat food to gain nutrients.

Although the nutrients are taken into the digestive system the large molecules still have to be digested so smaller molecules can be absorbed into the bloodstream.

From there they are transported around the body to be used as substrates in cellular reactions.

Examples of extracellular enzymes involved in digestion in humans are amylase and trypsin.

Question 20

Digestion of starch:

Answer 20

The digestion of starch begins in the mouth and continues in the small intestine. Starch is digested in two steps, involving two different enzymes.
Different enzymes are needed because each enzyme only catalyses one specific reaction.

Starch polymers are partially broken down into maltose, which is a disaccharide. The enzyme involved in this stage is called amylase.
Amylase is produced by the salivary glands and the pancreas.
It is released in saliva into the mouth, and in pancreatic juice into the small intestine.

Maltose is then broken down into glucose, which is a monosaccharide.
The enzyme involved in this stage is called maltase. Maltase is present in the small intestine.
Glucose is small enough to be absorbed by the cells lining the digestive system and subsequently absorbed into the bloodstream.

Question 21

diagram of human digestive system

Answer 21

Question 22

Digestion of proteins:

Answer 22

Trypsin is a protease, a type of enzyme that catalyses the digestion of proteins into smaller peptides, which can then be broken down further into amino acids by other proteases.
Trypsin is produced in the pancreas and released with the pancreatic juice into the small intestine, where it acts on proteins.
The amino acids that are produced by the action of proteases are absorbed by the cells lining the digestive system and then absorbed into the bloodstream.

Question 23

Factors Influencing Interaction with Substrates and Investigative Approaches:

Answer 23

For enzymes to catalyse a reaction, they must come into contact with the substrate, and the enzyme must be the right shape (complementary) for the substrate.
Enzymes are complex proteins and their structure can be affected by factors such as temperature and pH.
These can cause changes in the shape of their active site.
Enzymes are more likely to come into contact with the substrate if temperature and substrate concentration are increased.
Factors affecting enzyme action can be investigated by measuring the rate of the reactions they catalyse.

Question 24

Temperature:

Answer 24

Increasing the temperature of a reaction environment increases the kinetic energy of the particles.
As temperature increases, the particles move faster and collide more frequently.
In an enzyme-controlled reaction an increase in temperature will result in more frequent successful collisions between substrate and enzyme.
This leads to an increase in the rate of reaction.

Question 25

The temperature coefficient:

Answer 25

The temperature coefficient, Q10 of a reaction (or process) is a measure of how much the rate of a reaction increases with a 10°C rise in temperature.
For enzyme-controlled reactions this is usually taken as two, which means that the rate of reaction doubles with a 10°C temperature increase.

Question 26

Denaturation from temperature:

Answer 26

As enzymes are proteins their structure is affected by temperature.
At higher temperatures the bonds holding the protein together vibrate more.
As the temperature increases the vibrations increase until the bonds strain and then break.
The breaking of these bonds results in a change in the precise tertiary structure of the protein.
The enzyme has changed shape and is said to have been denatured.
When an enzyme is denatured the active site changes shape and is no longer complementary to the substrate.
The substrate can no longer fit into the active sites and the enzyme will no longer function as a catalyst.

Question 27

Optimum temperature:

Answer 27

The optimum temperature is the temperature at which the enzyme has the highest rate of activity.
The optimum temperature of enzymes can vary significantly.

Question 28

Different examples of Optimum temperature:

Answer 28

Many enzymes in the human body have optimum temperatures of around 40°C, meanwhile thermophilic bacteria (found in hot springs) have enzymes with optimum temperatures of 70°C, and psychrophilic organisms (that live in areas that are cold such as the antarctic and arctic regions) have enzymes with optimum temperatures below 5°C.

Question 29

What happens once the enzymes have denatured above the optimum temperature:

Answer 29

the decrease in rate of reaction is rapid.
There only needs to be a slight change in shape of an active site for it to no longer be complementary to its substrate.
This happens to all of the enzyme molecules at about the same temperature so the loss of activity is relatively abrupt.
At this point in an enzyme-controlled reaction the temperature coefficient, Q10 does not apply any more as the enzymes have denatured.
The decrease in the rate of reaction below the optimum temperature is less rapid.
This is because the enzymes have not denatured, they are just less active.

Question 30

temperature on rate of reaction graph

Answer 30

Question 31

Temperature extremes:

Answer 31

The majority of living organisms have evolved to cope with living within a certain temperature range.
Some organisms can also cope with extremes.
Examples of extremely cold environments are deep oceans, high altitudes, and polar regions.
The enzymes controlling the metabolic activities of organisms living in these environments need to be adapted to the cold.

Question 32

Structures of enzymes adapted to the cold:

Answer 32

Enzymes adapted to the cold tend to have more flexible structures, particularly at the active site, making them less stable than enzymes that work at higher temperatures.
Smaller temperature changes will denature them.
enzymes adapted to the heat:
Thermophiles are organisms adapted to living in very hot environments such as hot springs and deep sea hydrothermal vents.
The enzymes present in these organisms are more stable than other enzymes due to the increased number of bonds, particularly hydrogen bonds and sulfur bridges, in their tertiary structures.
The shapes of these enzymes, and their active sites, are more resistant to change as the temperature rises.

Question 33

Enzymes in action - example: Siamese cats:

Answer 33

Siamese cats provide visual evidence of the effect of temperature on enzyme activity.
Tyrosinase is an enzyme responsible for catalysing the production of melanin, the pigment responsible for dark coloured fur.
Due to a mutation, Siamese cats produce a form of the enzyme tyrosinase that is denatured and therefore inactive at normal body temperature meaning that their fur is primarily white or cream coloured.
The extremities of these cats - the tails, ears, and limbs - are at a slightly lower temperature, too low to denature mutant tyrosinase.
This leads to the distinctive point coloration of these cats as melanin is produced in these areas.

Question 34

How enzymes are affected by changes in pH:

Answer 34

Hydrogen bonds and ionic bonds between amino acid R-groups hold proteins in their precise three-dimensional shape.

These bonds result from interactions between the polar and charged R-groups present on the amino acids forming the primary structure.

A change in pH refers to a change in hydrogen ion concentration.

More hydrogen ions are present in low pH (acid) environments and fewer hydrogen ions are present in high pH (alkaline) environments.

The active site will only be in the right shape at a certain hydrogen ion concentration - This is the optimum pH for any particular enzyme.

Question 35

What happens when the pH changes from the optimum:

Answer 35

When the pH changes from the optimum - becoming more acidic or alkaline - the structure of the enzyme, and therefore the active site, is altered.

Question 36

. renaturation:

Answer 36

if the pH returns to the optimum then the protein will resume its normal shape and catalyse the reaction again - This is called renaturation.

Question 37

What happens when the pH changes more significantly chemical formation:

Answer 37

Hydrogen ions interact with polar and charged R-groups.
Changing the concentration of hydrogen ions therefore changes the degree of this interaction.
The interaction of R-groups with hydrogen ions also affects the interaction of R-groups with each other.
The more hydrogen ions present (low pH), the less the R-groups are able to interact with each other.
This leads to bonds breaking and the shape of the enzyme changing.
The reverse is true when fewer hydrogen ions (high pH) are present.
This means the shape of an enzyme will change as the pH changes and therefore it will only function within a narrow pH range.

Question 38

pH on rate of reaction graph

Answer 38

Question 39

the action of different enzymes on the digestive system

Answer 39