Proteins & Enzymes

Question 1

Explain the structures of proteins (6)

Answer 1

1. Primary - sequence of amino acids, determines subsequent structures
2. Secondary - hydrogen bonding between NH and C=O of different peptide bonds leads to formation of a-helices and B-pleated sheets
3. Tertiary - Forms the overall 3D shape of protein. Disulphide bridges are strong bonds.
4. Ionic bonds form between unbonded amine and carboxyl groups. They are easily broken by changes in pH
5. Hydrogen bonds are numerous but are easily broken at high temperatures
6. Quaternary - is involved when there are multiple polypeptide chains that form a protein such as haemoglobin. May have a prosthetic non-protein group associated

Question 2

Explain the test for proteins (3)

Answer 2

1. Add Biuret reagent
2. positive test = purple colour change
3. Negative test = remains blue (colour of Biuret reagent)

Question 3

Explain the two basic types of protein (2)

Answer 3

1. Globular proteins, such as enzymes and haemoglobin, carry out metabolic functions
2. Fibrous proteins, such as collagen, have structural functions

Question 4

Explain how the structure of collagen relates to its function (6)

Answer 4

1. Collagen is a fibrous protein found in tendons and so needs to be strong
2. primary structure - It has an unbranched polypeptide chain
3. Secondary structure - polypeptide is tightly wound with a-helices, amino acid glycine helps close-packing
4. Tertiary structure - chain is further twisted into a second helix
5. Quaternary structure - made up of 3 polypeptide chains that are wound together like rope held together by bonds between amino acids of adjacent chains
6. This way, the molecule is mechanically strong

Question 5

Explain the induced-fit model of enzyme action (4)

Answer 5

1. As the substrate collides with the enzyme, it causes a slight change in shape of the enzymes active site so that the substrate can fit
2. The active site is still complementary forming a enzyme-substrate complex
3. As the shape changes, this puts a strain on the substrate molecule, distorting certain bonds which decreases the activation energy needed to break the bond
4. This way, the enzyme catalyses the reaction of the substrate molecule

Question 6

Explain the effect of temperature on enzyme action (5)

Answer 6

1. An increase in temperature increases the rate in an enzyme-catalysed reaction
2. As particles have more kinetic energy, increasing the probability and frequency of substrate molecules colliding with their complementary enzymes
3. This results in more enzyme-substrate complexes formed and so the rate of reaction increases
4. However, at high temperatures hydrogen bonds and other bonds in the enzyme break that cause a change in the shape of the enzyme (denatured)
5. The shape of the active site changes and accommodates the substrate less easily decreasing the rate of reaction, until the enzyme is completely denatured and cannot accommodate the substrate at all

Question 7

Explain the effect of pH on enzyme action (3)

Answer 7

1. An increase or decrease in pH from the optimum reduces the rate of reaction, further changes can lead to the enzyme becoming denatured
2. A change in pH can alter the charges on amino acids that make up the active site, so the substrate can no longer attach
3. A further change in pH can also cause ionic bonds in the tertiary structure of the enzyme to break, causing a change in shape

Question 8

Explain the effect of enzyme concentration on enzyme action (4)

Answer 8

1. When there is excess substrate, increasing enzyme concentration proportionally increases the rate of reaction
2. As some of the excess substrate can be acted on by additional enzymes simultaneously
3. However, when substrate levels are limited, an increase in enzyme concentration has no effect
4. As there are enough enzymes to accommodate the available substrates and so the rate of reaction remains constant (graph levels off)

Question 9

Explain the effect of substrate concentration on enzyme action (3)

Answer 9

1. If enzyme concentration is fixed, increasing substrate concentration proportionally increases the rate of reaction
2. As there is an increased chance and frequency for substrate molecules to collide with enzymes
3. However, once all enzyme active sites are full at any given time, further increase in substrate concentration has no effect and so rate of reaction is constant (Vmax)

Question 10

Explain the effect of competitive inhibitors (5)

Answer 10

1. Competitive inhibitors have a shape similar to the substrate and so compete with the substrate to fit in the active site
2. The difference in concentration between the inhibitor and substrate determines the effect of inhibition
3. A higher concentration of competitive inhibitors to substrate decreases the rate of reaction more
4. And a lower concentration of competitive inhibitors to substrate decreases the reaction less (given that inhibitor is temporary)
5. Inhibitor can be temporary or permanent

Question 11

Explain the effect of non-competitive inhibitors (4)

Answer 11

1. Non-competitive inhibitors bind to a binding site separate from the active site
2. Causing a change in the shape of enzyme, including the active site, so that it can no longer form an enzyme-substrate complex, decreasing the rate of reaction
3. As they do not compete, increasing the concentration of substrate does not decrease the effect of the inhibitor
4. Can be both permanent or temporary inhibition

Question 12

Explain how enzyme can control metabolic pathways (4)

Answer 12

1. In end-product inhibition, the end product can inhibit the enzyme that catalyses the reaction that leads to the production of the end product
2. As the concentration of the end-product increases, it further inhibits the enzyme, decreasing the rate of reaction
3. If the concentration of end-product decreases, it inhibits the enzyme less, and the rate of reaction increases (from the value when it was inhibited more)
4. This way, the concentration of end-product can be kept constant (usually non-competitive inhibition)