3.1.4 Proteins

Question 1

(Proteins) Describe the food test for proteins.

Answer 1

• Add Biuret reagent I (copper surface solution)
• Add Biuret II (dilute sodium hydroxide solution)
• Colour change from light blue to purple

Question 2

(Proteins) What are the monomers of proteins?

Answer 2

Amino acids

Question 3

(Proteins) When is a dipeptide formed?

Answer 3

When two amino acids join together by a condensation reaction.

Question 4

(Proteins) What is a polypeptide, and what relation does it have to proteins?

Answer 4

More than two amino acids join together through a condensation reaction. Proteins are made up of one or more polypeptides.

Question 5

(Proteins) What is the general structure of an amino acid?

Answer 5

A carboxylate group (COOH), an amine/amino group (-NH2) and an R group.

Question 6

(Proteins) What do R groups generally contain, and what is the exception?

Answer 6

Carbon (one expedition is glycine as it’s R group consists of one hydrogen atom).

Question 7

(Proteins) All living things share a bank of __ amino acids. And what is their only difference?

Answer 7

20
Difference: what makes up their R group.

Question 8

(Proteins) What reaction are amino acids linked together by?

Answer 8

Condensation

Question 9

(Proteins) When amino acids are linked together by condensation reactions, what is formed?

Answer 9

Dipeptides and polypeptides

Question 10

(Proteins) What are the bonds that are formed between amino acids during condensation reactions called?

Answer 10

Peptide bonds

Question 11

(Proteins) What reaction happened when dipeptides and polypeptides are broken down?

Answer 11

Hydrolysis

Question 12

(Proteins) Describe the primary structure.

Answer 12

Sequence of amino acids in the polypeptide chain.

Question 13

(Proteins) Describe the secondary structure.

Answer 13

• Polypeptide chain doesn’t remain flat&straight
• Hydrogen bonds forms between the amino acids in the chain
• Makes it coil into an alpha helix / fold into a beta pleated sheet

Question 14

(Proteins) Describe the tertiary structure.

Answer 14

• Single polypeptide chain fold up to from a precise 3D shape
• R group interaction/bonding
• 3D shape is specific and determines the biological function of the protein
• Shape maintained by: ionic bonds, disulphide bridges, hydrophobic interactions, hydrogen bonding

Question 15

(Proteins) When and how do disulphide bridges form?

Answer 15

Form when two molecules of amino acid cysteine come close together - sulphur atom in one cysteine bond to the sulphur atom in another

Question 16

(Proteins) Describe the quaternary structure.

Answer 16

More than 1 polypeptide chain can associate to form the quaternary structure of a protein.
R-group interaction.

Question 17

(Proteins) What does a proteins shape determine?

Answer 17

The protein’s function.

Question 18

(Proteins) Describe the biuret test for proteins.

Answer 18

1. Test solution needs to be alkaline, so add a few drops of sodium hydroxide solution.
2. Add some copper(II) sulphate solution .

If protein is present, solution turns purple.
No protein, solution will stay blue.

Question 19

(Many Proteins are Enzymes) How do enzymes speed up chemical reactions? (2)

Answer 19

They act as biological catalysts, by catalysing metabolic reactions.

Question 20

(Many Proteins are Enzymes) What can enzymes affect?

Answer 20

Structures in an organism.

Question 21

(Many Proteins are Enzymes) Is enzyme action intracellular or extracellular?

Answer 21

Both.

Question 22

(Many Proteins are Enzymes) What is the active site?

Answer 22

The part of the enzyme where the substrate molecules bind to.

Question 23

(Many Proteins are Enzymes) Why are enzymes highly specific?

Answer 23

A consequence of their tertiary structure.

Question 24

(Many Proteins are Enzymes) What affect do enzymes have on activation energy, and what are the consequences of it?

Answer 24

Lowers the amount of activation energy, often making reactions happen at a lower temperature than they could without the enzyme. Speeds up the rate of the reaction.

Question 25

(Many Proteins are Enzymes) Why does the formation of an enzyme-substrate complex lower the activation energy? (2)

Answer 25

• If two substrate molecules need to be joined, being attached to the enzyme hold them close together, reducing repulsion between the molecules so they can bond easier.
• If the enzyme is catalysing a breakdown, fitting into the active site puts strain on the bonds so it breaks up easier.

Question 26

(Many Proteins are Enzymes) Describe the ‘lock and key’ model. (3)

Answer 26

• Substrate is complimentary in shape to the enzyme’s active site.
• Substrate fits into the active site exactly.
• Forming an enzyme-substrate complex.

Question 27

(Many Proteins are Enzymes) Describe the ‘induced fit’ model. (3)

Answer 27

• Substrate is similar in shape to the active site.
• Active site changes shape, bonding with the substrate.
• Forming an enzyme-substrate complex.

Question 28

(Many Proteins are Enzymes) Why are enzymes very specific?

Answer 28

Only one complementary substrate will fit into the active site.

Question 29

(Many Proteins are Enzymes) What is the active site’s shape determined by?

Answer 29

The enzyme’s tertiary structure (different for each enzyme).

Question 30

(Many Proteins are Enzymes) What will happen if the tertiary structured of the protein is altered?

Answer 30

The shape of the active site will change.

Question 31

(Many Proteins are Enzymes) What can the tertiary structure of an enzyme be altered by?

Answer 31

Changes in pH or temperature.

Question 32

(Many Proteins are Enzymes) Fill in the blanks for describing a product time graph: Reaction is __1__ at the start as there is more __2__ so more __3__ can form. Reaction slows as substrate is being __4__ into __5__ so ESCs from __6__ often. Eventually all substrate has been converted into product so the reaction __7__.

Answer 32

1) fastest
2) substrate
3) enzyme-substrate complexes
4) converted
5) product
6) less
7) stopped/plateau

Question 33

(Many Proteins are Enzymes) Explain how enzymes act as biological catalysts, speeding up biochemical reactions. (10)

Answer 33

• Substrate is similar in shape to the active site (not complementary).
• Fits into the active site.
• Active site changes shape, binding to the substrate
• ESC is formed
• Induced fit model.
• Activation energy for reaction is lowered (reaction more likely to occur).
• Substrate bonds are put under strain / stress.
• Substrate changes into product.
• Product released from active site and enzyme returns to original shape.
• Enzyme is unused in the reaction

Question 34

(Many Proteins are Enzymes) How do you measure the initial rate of an enzyme catalysed reaction? (3)

Answer 34

1) Use a ruler to draw a straight line (tangent)against the steepest bit of the curve on the product-time graph.
2) Read values off the axes.
3) Divide the product by the time.

Question 35

(Many Proteins are Enzymes) How does substrate concentration affect the rate of an enzyme catalysed reaction? (3)

Answer 35

• At lower concentrations not all enzyme’s active sites are occupied.
• Increasing substrate concentration increases the frequency of “successful collisions” / ESCs form more rapidly, producing product faster.
• At high concentrations all active sites are occupied so the rate is at its maximum / Vmax

Question 36

(Many Proteins are Enzymes) How does enzyme concentration affect the rate of an enzyme catalysed reaction? (3)

Answer 36

• Increasing the enzyme concentration increases the rate of reaction.
• Proportional.
• More active sites become available so more ESCs can form at any point, forming product faster.

Question 37

(Many Proteins are Enzymes) How does temperature affect the rate of an enzyme catalysed reaction? (6)

Answer 37

• Increasing the temperature from low UP TO THE OPTIMUM increases the kinetic energy of the substrates / enzyme molecules.
• More / higher frequency of successful collision / ESCs form more rapidly.
• Product is produced faster.
• Increasing temperature ABOVE THE OPTIMUM rapidly decreases the rate.
• The heat denatures the enzymes.
• The bonds in the tertiary structure break so the active site is destroyed / cannot bind to substrate.

Question 38

(Many Proteins are Enzymes) How does pH affect the rate of an enzyme catalysed reaction? (4)

Answer 38

• Changing the pH from the optimum causes disruption of the tertiary structure of the enzyme.
• Tertiary bonds are distorted so is the shape of the active site.
• Fewer ESCs can form.
• Extreme pHs will denature the enzyme completely.

Question 39

(Many Proteins are Enzymes) Describe protein denaturation? (7)

Answer 39

• Heat / changes in pH can break the R-group interactions within the enzymes.
• The tertiary structure of the polypeptides is disrupted.
• Hydrogen bonding, hydrophobic / ionic interaction, disulphide bonds are broken.
• Polypeptide structure unfolds.
• The active site is destroyed.
• Substrate molecules can no longer bind / cannot form enzyme-substrate complexes.
• Irreversible / permanent.

Question 40

(Many Proteins are Enzymes) What does “controlling key variables” mean when conducting enzyme experiments?

Answer 40

IF you are changing a factor (independent variable) e.g. substrate concentration, then ALL OTHER factors that can affect enzyme rate must be kept “the same” i.e. temperature, enzyme concentration, pH.

Question 41

(Many Proteins are Enzymes) What is meant by “the control”?

Answer 41

A “control” is where you remove what is responsible for the change you observe.
So with enzymes you would have a separate tube where the enzyme has been boiled (denatured).
This can then be compared.

Question 42

(Many Proteins are Enzymes) Detail the effect of competitive inhibitors. (4)

Answer 42

Are similar in shape to the substrate.
They fit into the enzyme’s active site.
They block the active site and prevent the substrate binding / prevent ESC formation.

Question 43

(Many Proteins are Enzymes) How can you dilute a competitive inhibitor effect?

Answer 43

By increasing substrate concentration.

Question 44

(Many Proteins are Enzymes) Detail the effect of non-competitive inhibitors. (4)

Answer 44

Bind to a site OTHER THAN the active site.
Enzyme’s active site changes shape.
Prevents substrate binding / prevents ESC.

Question 45

(Many Proteins are Enzymes) Does increasing substrate concentration dilute a non-competitive inhibitor effect?

Answer 45

You CANNOT dilute a non-competitive inhibitor effect by increasing substrate concentration.

Question 46

(Many Proteins are Enzymes) Explain enzyme feedback inhibition with biochemical pathways. (3)

Answer 46

If a lot of product is produced at the end of an enzyme pathway it acts as an non-competitive inhibitor on the first enzyme, turning off its own production.

If the product is being used up elsewhere then there is less feedback inhibition so more product is made.

Self-regulates.