1.3 proteins & enzyme activity

Question 1

give the basic monomer unit which all proteins are composed from.

Answer 1

amino acids are the basic monomer units, which combine to form a polymer known as a polypeptide, which can then be further combined to form a protein.

Question 2

list the four different chemical groups found within amino acids.

Answer 2

• amino group.
• carboxyl group.
• hydrogen atom (group)
• R group.

Question 3

give the type of bond that links amino acid monomers together.

Answer 3

peptide bond.

Question 4

describe the formation of a peptide bond within a protein.

Answer 4

• amino acid monomers can combine to form dipeptides through the removal of a water molecule in a condensation reaction.
• the water molecule is formed by combining an -OH from the carboxyl group of one amino acid with the -H from the amino group of another amino acid.
• the two amino acids then become linked by a new peptide bond between the carbon atom of one amino acid and the nitrogen atom of another.

Question 5

describe the primary structure of a protein.

Answer 5

a sequence of amino acids, linked by peptide bonds, found in the polypeptide chain.

Question 6

describe the secondary structure of a protein and identify the shapes which form as a result of this structure.

Answer 6

• the secondary structure of a protein refers to the shape the polypeptide chain forms as a result of hydrogen bonding.
• the secondary structure of proteins generally form two shapes - α-helix coils and 𝛽-pleated sheets.

Question 7

describe the tertiary structure of a protein, including the bonds that it contains.

Answer 7

• the tertiary structure of a protein is formed due to the bending and twisting of the polypeptide helix into a compact structure.
• as well as peptide and hydrogen bonds, the tertiary structure also contains disulfide bridges, ionic bonds and hydrophobic interactions.

Question 8

describe the quaternary structure of a protein and give an example of a protein with this structure.

Answer 8

the quaternary structure of a protein forms from the combination of a number of different polypeptide chains and non-protein prosthetic groups into a large, complex protein molecule, such as haemoglobin.

Question 9

give the test used to test for the presence of a protein.

Answer 9

Biuret’s test.

Question 10

what indicates the presence of a protein? what is detected?

Answer 10

• a purple or lilac colouration indicates the presence of a protein by detecting peptide bonds.
• if no protein is present, the solution remains blue.

Question 11

identify the two different molecular protein shapes.

Answer 11

fibrous and globular proteins.

Question 12

describe the different functions of fibrous and globular proteins, and give an example for each.

Answer 12

• fibrous proteins - such as collagen, have structural functions.
• globular proteins - such as enzymes and haemoglobin, carry out metabolic functions.

Question 13

what is an enzyme?

Answer 13

enzymes are globular proteins which catalyse metabolic reactions, by lowering the activation energy of the reaction.

Question 14

what type of structure do enzymes have?

Answer 14

enzymes have a tertiary protein structure.

Question 15

explain why are enzymes described as ‘specific’.

Answer 15

• enzymes are described as specific because they can only catalyse one type of reaction.
• this means that only one type of complementary substrate is able to bind to the enzyme’s active site.

Question 16

describe and explain two factors which affect enzyme structure.

Answer 16

changes to the pH or temperature of the conditions alter the shape of an enzyme’s active site, meaning that the enzyme-substrate complex is no longer able to form.

Question 17

describe what is meant by the ‘induced fit model’ of enzyme action, and explain how enzymes use this model to hydrolyse substrates.

Answer 17

• the induced fit model proposes that the shape of an enzyme is flexible, and the enzyme’s active site can mould itself around a substrate.
• the enzyme has a general shape, but this alters in the presence of a substrate.
• as the shape of the enzyme’s active site changes, this puts a strain on the substrate molecule, distorting the bonds in the substrate and lowering the activation energy needed to break these bonds.

Question 18

describe what is meant by the ‘activation energy’ of a reaction.

Answer 18

the activation energy refers to the minimum amount of energy needed in order for a reaction to take place.

Question 19

describe how enzymes cause a change in the activation energy required for a reaction to occur.

Answer 19

enzymes increase the rate at which a reaction occurs by lowering the activation energy and providing an alternative pathway through which the reaction can take place.

Question 20

give two changes which can be observed to measure the frequency of an enzyme-controlled reaction.

Answer 20

• the formation of the products of a reaction, such as the volume of oxygen produced.
• the reduction of the concentration of substrate.

Question 21

explain why increasing the temperature increases the rate of an enzyme-catalysed reaction.

Answer 21

• an increase in temperature results in an increase in the kinetic energy of the molecules, which causes them to collide more rapidly.
• this means that a higher number of collisions between enzymes and substrates will be successful, leading to the formation of more enzyme-substrate complexes.
• as a result, the overall rate of reaction will increase.

Question 22

describe what happens to an enzyme when the temperature of the surroundings surpasses the enzyme’s optimum temperature.

Answer 22

• when the temperature of the surroundings surpasses the enzyme’s optimum temperature, the hydrogen and peptide bonds between amino acids in the enzyme begin to break.
• this causes a change in the active site of the enzyme, meaning that an enzyme-substrate complex is unable to form.
• once the surroundings reach a certain temperature, the enzyme will become permanently denatured and eventually stop functioning altogether.

Question 23

describe how changes in pH can affect enzyme action.

Answer 23

• a change in pH alters the charges on the amino acids which determine the shape of the active site of the enzyme.
• as a result, a substrate can no longer bind with the enzyme’s active site, so the enzyme-substrate complex cannot form.

Question 24

what are enzyme inhibitors?

Answer 24

enzyme inhibitors are substances which directly or indirectly interfere with the functioning of the active site of an enzyme, reducing the enzyme’s activity as a result.

Question 25

give two types of enzyme inhibitor.

Answer 25

• competitive inhibitors - inhibit enzyme activity by binding to the active site of the enzyme.
• non-competitive inhibitors - inhibit enzyme activity by binding to the enzyme at a position other than the active site.

Question 26

describe how competitive inhibitors affect enzyme activity, and give an example of competitive inhibition.

Answer 26

• competitive inhibitors have a molecular shape similar to that of a substrate.
• as a result, they are complementary to, and able to bind with the active site of an enzyme.
• competitive inhibitors therefore decrease the rate of enzyme activity by competing with the substrate for available active sites.
• an example of competitive inhibition is the inhibition of the activity of the enzyme transpeptidase by penicillin.

Question 27

explain how non-competitive inhibitors affect enzyme activity.

Answer 27

• non-competitive inhibitors attach to the enzyme at a binding site other than the active site.
• upon attaching to the enzyme, the inhibitor alters the shape of the enzyme, and it’s active site, meaning that substrate molecules can no longer bind to the active site.
• as a result, although they do not bind directly to the active site, non-competitive inhibitors reduce the rate of enzyme activity.