Proteins and Enzymes

Question 1

Amino Acids

Answer 1

• the building blocks of proteins
• essential macromolecules involved in various functions within living organisms
• can join to from polypeptides and dipeptides

Question 2

Roles of proteins

Answer 2

Enzymes- used to break down and synthesise molecules
Antibodies- involved in immune responses
Transport- some proteins can move molecules or ions across membranes
Structural components- proteins like keratin and collagen are used to created strong fibres
Hormones- some proteins act as chemical messengers in the body
Muscle contraction- muscles are made up of proteins

Question 3

Amino acid structure

Answer 3

There are around 20 different amino acids commonly found in living organisms

All of them have the same general structure:
- a central carbon atom
- an amnio acids group (NH2)
- a carboxyl group (COOH)
- a hydrogen atom (H)
- an R group: each amino acid had a different R group which determines its properties like cysteine which has an R group containing a sulphur atom to from disulphide bonds

Question 4

Condensation reactions in proteins

Answer 4

When two amino acids join, the hydroxyl/ OH group of one of them bonds with the hydrogen/ H of the other.
This releases a water molecule/ H2O.
A peptide bond is formed between the carbon of one amino acid and the nitrogen of another.

Question 5

Hydrolysis reaction in proteins

Answer 5

When a water molecule/ H2O is added to a dipeptide, the peptide (bond between the C and N) bond is broken to release two amino acids.

Question 6

Testing for proteins

Answer 6

1. Place your food sample in a test tube.
2. Add an equal volume of Biuret solution with a pipette.
3. If proteins are present, the solution will go from blue to purple/ lilac. If no protein is present, the solution remained blue.

Question 7

The Structures

Answer 7

Primary, Secondary, Tertiary and Quaternary

Question 8

Primary structure

Answer 8

• Made up of the unique sequence of amino acids in the polypeptide chain
• Structure held together by peptide bonds
• A change to just one of the amino acids can change the structure and function

Question 9

Secondary structure

Answer 9

• Involves hydrogen bonds forming between the amino group of one amino acid and the carboxyl group of the other.
• This causes the polypeptide chain to coil into either an alpha-helix or a beta-pleated sheet structure.

Question 10

Tertiary structure

Answer 10

• Forms when the polypeptide chain folds and twists to create a complex 3D structure.

Held together by:
Hydrogen bonds- these are individually weak but provide strength in large numbers
Ionic bonds- these form between positive and negative R groups
Disulfide bridges- these form between R groups containing sulphur like cysteine
Hydrophobic and hydrophyllic interactions- these are weak interactions between polar and non-polar R groups

Question 11

Quaternary structure

Answer 11

• Involves two or more polypeptide chains held together by the same bonds found in the tertiary structure.
• Can also involve the addition of non-protein groups known as prosthetic groups
• Only some proteins contain this structure

Question 12

TYPES OF PROTEIN - Fibrous proteins

Answer 12

• e.g. collagen, keratin
• Solubility - insoluble in water
• Roles - structural roles. Not reactive.
• Common structure - little or no tertiary structure. Formed of long parallel polypeptide chains with cross-linkages. Repeated sequence of amino acids.

Question 13

TYPES OF PROTEIN - Globular protein

Answer 13

• e.g. haemoglobin, enzymes
• Solubility - soluble in water due to the many R groups containing hydrophilic side chains, which interacts its the polar water molecules. The hydrophobic side chains are often hidden in the centre of the structure.
• Roles - many are involved in metabolic reactions
• Common structure - have complex tertiary structure and sometimes even quaternary structures. These are folded into globular/spherical shapes.

Question 14

What are enzymes?

Answer 14

• proteins with a tertiary structure
• each enzyme lowers the activation of the specific reaction it catalyses

Question 15

Activation energy definition

Answer 15

The minimum amount of energy needed to activate the reaction.

Question 16

Catabolic reactions

Answer 16

Energy is released to the surroundings (exothermic).

Question 17

Anabolic reactions

Answer 17

Energy taken from the surroundings (endothermic)

Question 18

Why do we need enzymes?

Answer 18

• enzymes enable chemical reactions to happen at lower temperatures
• without enzymes, these reactions would take place too slowly to sustain life

Question 19

Explain the induced fit model

Answer 19

• the active site forms as the enzyme and substrate interact
• the proximity of the substrate leads to a change in the enzymes active site
• active site changes shape slightly to bind more tightly to the substrate

Question 20

Why does the lock and key model have limitations?

Answer 20

• scientist observed that other molecules could bind to enzymes in sites other than the active site
• this binding resulted in an altered activity of the enzyme
• this suggests that the enzyme’s shape was altered by the binding molecules
• basically, the structure was not rigid but flexible

Question 21

Active site in enzymes

Answer 21

• the properties of an enzyme are linked to the tertiary structure of its active site
• also effects its ability to combine with complementary substrates to form an enzyme-substrate complex/ESC
• the shape of the active site is complementary to one specific substance, so enzymes have specificity.

Question 22

LIMITING FACTORS - pH

Answer 22

• each enzyme had an optimum pH
• activity or the enzyme decreases as pH is increased or decreased
• this is due to the H and ionic bonds, which are at these pHs
• the tertiary structures therefore change, so the shape of the active site is lost
• this means fewer ESCs are formed
• the further away from the optimum pH, the more bonds broken and the slower the rate of reaction

Question 23

LIMITING FACTORS - temperature

Answer 23

• enzymes work best at an optimum temperature
• if the temperature is too little, then the heat provides less kinetic energy to the molecules
• the molecules move slower which decreases the frequency of collisions between enzymes and substrates
• therefore fewer ESCs are formed and rate of reaction decreases
• however when temperature is increased above the optimum, the ionic and hydrogen bonds in the tertiary structure break.
• the active site loses its shape, so fewer ESCs are formed.
• the enzyme no longer catalysed the reaction
• now, the enzymes are denatured

Question 24

LIMITING FACTORS - substrate concentration

Answer 24

• when the substrate concentration is low, few ESCs are formed, which results in a slower rate of reaction
• if the substrate concentration is increased, more ESCs are formed
• therefore there is an increased rate of reaction
• still, the rate eventually level off as the active sites are saturated.

Question 25

Explain competitive inhibitors

Answer 25

- have a similar shape to the substrate and so fit into the active site. - therefore, the inhibitors compete with the substrates for available active sites. - an increase in substrate concentration will lead to the effect of the inhibitor being reduced because when the inhibitor leaves the active site it can be immediately replaced by a substrate.

Question 26

How do non-competitive inhibitors work?

Answer 26

- they bind to the enzyme but not its active site. - they bind to an alternate site called the allosteric site - this binding changes the shape of the active site, so the rate of reaction decreases as less ESCs can be formed - because the active site shape is changed, increasing the substrate will not effect rate of reaction as no substrate can bind to the site anymore as they are no longer complementary

Question 27

What do heavy metals do?

Answer 27

- they are non competitive inhibitor - they can permanently change the active site - this is what heavy metal poisoning is

Question 28

The permanency of non-competitive inhibitors?

Answer 28

- Some are irreversible and permanent, effectively denaturing the enzymes they inhibit. - Others are reversible non-competitive inhibitors, which are vital in controlling metabolic reactions in organisms.