Proteins & Enzymes

Question 1

What are 3 types of proteins?

Answer 1

• enzymes e.g. amylase
• transport proteins e.g. channels
• signal proteins e.g. insulin
• defensive proteins e.g. antibodies

Question 2

What are the 5 components that make up amino acids?

Answer 2

Nitrogen, carbon, hydrogen, oxygen and sometimes sulphur

Question 3

What is the general structure of an amino acid?

Answer 3

A central carbon is joined to an amine group (a nitrogen bonded to two hydrogens), a carboxyl group (a carbon bonded to an hydroxyl and double bonded to an oxygen), a hydrogen and the ‘R’ group

Question 4

How do peptide bonds form between amino acids?

Answer 4

Two amino acids join together to form a dipeptide via a condensation reaction.
The hydroxyl (OH) from one amino acid reacts with a hydrogen (H) from the amine group in the other amino acid to form water and a peptide bond.

Question 5

What are the four levels of protein organisation?

Answer 5

Primary, secondary, tertiary and quaternary

Question 6

What is the primary structure?

Answer 6

The number and sequence of amino acids in the polypeptide.

Question 7

Why is the primary structure important?

Answer 7

It will determine the ultimate shape and function of a protein. Any change can alter its shape and prevent it from carrying out its function.

Question 8

What is the secondary structure?

Answer 8

The initial folding/coiling of a polypeptide into alpha helixes and beta pleated sheets. These are maintained by hydrogen bonds.

Question 9

What is the tertiary structure?

Answer 9

It is the further folding of a polypeptide into a 3D structure. It is held together by ionic bonds between R groups and may also have disulphide bonds.

Question 10

What is the quaternary structure?

Answer 10

When two or more polypeptide chains join together and non-protein groups may associate e.g. iron

Question 11

How do you preform a biuret test?

Answer 11

1. Add solution to test tube
2. Add equal amount of biuret reagent
3. If colour changes from blue to purple then peptide bonds/ proteins are present.

Question 12

How do you use chromatography to identify amino acids?

Answer 12

1. Draw a pencil line on the bottom of the chromatography paper as a start line
2. Use a pipette to drop several drops on the same spot
3. Place bottom of paper in solution making sure it doesn’t touch the pencil line
4. Wait for paper to dry and spray with ninhydrin solution to make amino acids clearer

Question 13

What is the equation of Rf value?

Answer 13

Rf = distance travelled by compound / distance travelled by solvent

Question 14

What is an enzyme?

Answer 14

They are biological catalysts that speed up the rate of reaction by lowering the activation energy by bending bonds.

Question 15

What is the activation energy?

Answer 15

The minimum energy required for a successful chemical reaction.

Question 16

What is the structure of an enzyme?

Answer 16

They have specific 3D tertiary structures with an active site which only a complimentary substrate can bind to.

Question 17

What is the induced fit theory?

Answer 17

Before a reaction, the active site is not complimentary. The active sites shape changes, as the substrate binds, to form an enzyme-substrate complex. This distorts the bonds in the substrate leading to a reaction.

Question 18

What are the four factors that affect enzyme action?

Answer 18

• Temperature
• pH
• Inhibitors
• Substrate or enzyme concentration

Question 19

What effect does temperature have on enzyme action?

Answer 19

As temperature increases, the molecules gain more kinetic energy and collide more often, forming enzyme-substrate until the optimum is reached. Beyond this, the bonds between the R groups vibrate and break, causing the substrate to no longer fit and the enzyme denatures.

Question 20

What effect does pH have on enzyme action?

Answer 20

If the pH changes from the optimum, the charge on the R groups in amino acids are altered and the ionic bonds are broken. This alters the shape of the active site so the substrate no longer fits.

Question 21

What effect do competitive inhibitors have on enzyme action?

Answer 21

They are a similar shape to the substrate so can bind the the active site, preventing the formation of ES complexes. This can be overcome by adding more substrate. Slow but maximum product is reached.

Question 22

What effect does concentration have on enzyme action?

Answer 22

As the concentration of one factor (enzyme or substrate) increases, so does the rate of reaction. However, as saturation is achieved, the concentration no longer increases the rate and it plateaus. This means the other factor is the inhibiting factor.

Question 23

What effect do non-competitive inhibitors have on enzyme action?

Answer 23

They bind to an allosteric site on the enzyme, changing the chape of the active site. This means ES complexes cannot form.
This cannot be overcome.

Question 24

What is the equation of rate?

Answer 24

Rate = measurement/time taken

Question 25

What is the equation for line gradient?

Answer 25

Gradient = change in y/change in x

Question 26

What is the equation for percentage change?

Answer 26

((final mass-initial mass)/initial mass) x 100

Question 27

What is the equation for pH?

Answer 27

pH = -log(‘concentration’)

Question 28

How many amino acids are there and how do they differ?

Answer 28

There are 20 different amino acids and they differ by their ‘R’ group

Question 29

How have models of enzyme action changed?

Answer 29

Initially, the lock and key model was accepted that claimed enzymes were always complimentary to 1 substrate. Now the induced fit model is preferred.

Question 30

Describe the induced-fit model of enzyme action and how an enzyme acts
as a catalyst. (3)

Answer 30

1. Substrate binds to the active site/enzyme OR enzyme-substrate complex forms
2. Active site changes shape (slightly) so it is complementary to
   substrate OR active site changes shape (slightly) so distorting/breaking/forming bonds in the substrate;
3. Reduces activation energy;

Question 31

Describe how the structure of a protein depends on the amino acids it
contains. (5)

Answer 31

1. Structure is determined by (relative) position of amino acid/R
   group/interactions;
2. Primary structure is sequence/order of amino acids;
3. Secondary structure formed by hydrogen bonding (between amino
   acids);
4. Tertiary structure formed by interactions (between R groups);
5. Creates active site in enzymes OR creates complementary/specific shapes in antibodies/carrier
   proteins/receptor (molecules);
6. Quaternary structure contains >1 polypeptide chain OR quaternary structure formed by interactions/bonds between
   polypeptides;

Question 32

Describe the structure of proteins. (5)

Answer 32

1. Polymer of amino acids;
2. Joined by peptide bonds;
3. Formed by condensation;
4. Primary structure is order of amino acids;
5. Secondary structure is folding of polypeptide chain due to hydrogen bonding;
6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic / disulfide bonds;
7. Quaternary structure is two or more polypeptide chains.