Lesson 13/14 - enzymes

Question 1

what are enzymes?

Answer 1

• Enzymes are globular proteins that are catalysts
• A catalyst is a molecule that speeds up a chemical reaction (increases the rate) but remains unchanged/isn’t used up at the end of a reaction
• They’re called “biological catalysts” because they are made by living cells (all proteins)

Question 2

what does the term metabolism refer to?

Answer 2

all the reactions of the body

Question 3

what are the 2 types of metabolism (reaction of the body)

Answer 3

• Anabolic reactions: Building up molecules – e.g. protein synthesis (energy absorbed)
• Catabolic reactions: Breaking molecules down – e.g. digestion (energy released)

Question 4

what are anabolic and catabolic reactions controlled by?

Answer 4

enzymes

Question 5

what happens in the enzyme controlled reaction where sucrose breaks down?

Answer 5

water is absorbed, and glucose and fructose are produced

Question 6

what are the substrates and products in the enzyme controlled reaction where sucrose breaks down?

Answer 6

sucrose is the substrate, glucose and fructose are the products

Question 7

what happens in the enzyme controlled reaction of the condensation of glucose and fructose.

Answer 7

water is released, sucrose is produced

Question 8

what are the key points of an enzyme?

Answer 8

• Globular protein (made by cells)
• Speeds up a reaction
• Is not used up
• Is not changed by the reaction
• Can be used repeatedly – therefore effective in small amounts
• Have a high turn-over (They catalyse many reactions per second)

Question 9

what are the substrates and products in the enzyme controlled reaction where sucrose is produced?

Answer 9

glucose and fructose are the substrates, sucrose is the product

Question 10

what happens during an enzyme substrate complex?

Answer 10

• An enzyme acts on its substrate (with which it makes temporary bonds at the active site) forming an enzyme-substrate complex
• When the reaction is complete the products are released leaving the enzyme unchanged and the enzyme ready to receive another substrate molecule

Question 11

substrate definition

Answer 11

The specific reactant molecule

Question 12

active site definition

Answer 12

Place on enzyme where substrate binds (fits). Correct, complementary shape for substrate molecule

Question 13

catalyst definition

Answer 13

A molecule that speeds up a chemical reaction but remains unchanged at the end of a reaction

Question 14

enzyme definition

Answer 14

Globular proteins that are catalysts (made by living cells)

Question 15

enzyme substrate complex definition

Answer 15

Intermediate structure formed during an enzyme-catalysed reaction in which the substrate and enzyme bind temporarily

Question 16

explain the specificity of enzymes

Answer 16

Each enzyme will only catalyse 1 specific reaction. Because the enzyme has a specific 3D tertiary structure, resulting in the active site having a specific shape. Therefore, the active site is only complementary to and will bind to 1 substrate to form an enzyme-substrate complex.

Question 17

maltose is hydrolysed by the enzyme maltase. explain why maltase catalyses only his reaction. (3)

Answer 17

• the active site has a specific shape/tertiary structure/active site’ that is complementary to substrate/maltose
• only maltose can bind/fit
• to form an enzyme substrate complex

Question 18

where are enzymes made?

Answer 18

like other proteins, inside cells

Question 19

can enzymes act inside or outside of cells?

Answer 19

both

Question 20

what are the terms for both sites of enzyme action?

Answer 20

Intracellular (inside cells)
Extracellular (secreted from cells)

Question 21

How are enzymes secreted from cells?

Answer 21

by exocytosis

Question 22

Example of a process enzymes would be needed for?

Answer 22

Digestion – e.g. amylase, made and secreted by cells in the salivary glands

Question 23

what are the 2 models/theoris of enzyme action?

Answer 23

• The lock and key model (earlier model)
• The induced fit model (current model)

Question 24

which model for enzyme action proposes that enzymes are flexible structures?

Answer 24

the induced fit model

Question 25

Explain how enzymes use induced fit to catalyse/speed up a reaction

Answer 25

1. Before the reaction, the active site is NOT complementary to the substrate
2. As the substrate binds, the active site changes shape to better fit/become complementary to the substrate forming an enzyme-substrate complex
3. This stresses/distorts the bonds in the substrate - causing the reaction to occur and the products to be released

Question 26

Explain why the induced fit model is a better explanation of enzyme action than the lock and key model

Answer 26

• The lock and key model suggests that the active site of the enzyme is a rigid structure and that the substrate is an exact fit to the active site (1 mark)
• The induced fit model matches current observations that the active site changes shape slightly upon binding of the substrate to become a more exact fit which then allows the reaction to proceed

Question 27

what is meant by the Collision theory?

Answer 27

In order for a reaction to take place, a substrate and enzyme must collide with each other.
Both enzyme and substrate molecules are in constant motion and collide naturally.

Question 28

What important factor may affect the rate at which enzyme and substrate collide and therefore react?

Answer 28

Temperature - usually increases the rate.
In most living systems, temperatures above 40 degrees C cause irreversible damage to proteins (they denature)

Question 29

For an enzyme to catalyse a reaction, it must…

Answer 29

• Come into physical contact with a substrate(s)
• The substrate must be complementary to the active site
• They must collide with enough energy and with a suitable orientation
  = SUCCESSFUL COLLISION!

Question 30

what is the activation energy?

Answer 30

the minimum amount of energy required to activate a reaction.
The minimum energy required for molecules to react (breaking existing bonds/making new ones)

Question 31

How do enzymes actually speed up the rate of reactions?

Answer 31

They lower the activation energy of a reaction

Question 32

Explain how the active site of an enzyme causes a high rate of reaction (3)

Answer 32

lowers activation energy
induced fit causes active site (enzyme) to change shape
so enzyme -substrate complex causes bonds to form/break

Question 33

explain what determines the shape of an enzyme and how this is essential for its function

Answer 33

• Enzymes are proteins
• The primary structure of any protein is a particular sequence of amino acids
• The elements in the amino acid R groups determine the bonds the amino acids make with each other
• These are hydrogen bonds, disulphide bridges and ionic bonds and they hold the enzyme in its tertiary structure
• The shape of the enzyme (its tertiary structure) is very important as it determines the shape of the enzyme’s active site (where the substrate binds)
• Enzymes are specific for their substrates
• If the shape of the active site is not complementary the substrate and enzyme will not bind (and no reaction will take place – no product will be made)
• The protein chain folds into a spherical or globular shape with hydrophilic R groups on the outside of the molecule (making enzymes water soluble) – reactions take place in solution