Proteins 3.1.4

Question 1

What are proteins made up of?

Answer 1

Amino acids are the monomers which make up proteins

Question 2

What does the following represent?

• NH2
• COOH
• R

Answer 2

• NH2 is the amine group
• COOH is the carboxyl group
• R represents the side chain

Question 3

What bond is formed between the condensation of two amino acids?

Answer 3

peptide bond

Question 4

How are dipeptides formed?

Answer 4

From the condensation of two amino acids

Question 5

What is a proteins primary structure?

Answer 5

The sequence of amino acids in a polypeptide chain

Question 6

What is the secondary structure of proteins? Give examples

Answer 6

The positioning of hydrogen bonds in a polypeptide chain causing folds in a repeated pattern

• a-helix
• beta-pleated sheet

Question 7

What is the tertiary structure of proteins?

Answer 7

The 3D structure of a polypeptide chain due to the positioning of ionic bonds, hydrogen bonds and disulphide bridges

Question 8

What is the quaternary structure of a protein?

Answer 8

Two or more polypeptide chains held together by hydrogen, disulphide and ionic bonds

Question 9

How do the 20amino acids common in all organisms differ?

Answer 9

They differ in the side chains

Question 10

What are enzymes?

Answer 10

Enzymes are biological catalysts that speed up rate of reaction by lowering the activation energy of the reaction it catalyzes

Question 11

Describe the active site of an enzyme

Answer 11

• the active site has a tertiary structure
• making the enzyme specific to a certain substrate
• active site is complementary in structure
• it can then form enzyme-substrate complexes

Question 12

What is the lock and key model?

Answer 12

-This model suggests that the active site has a rigid shape and only a substrate with the correct complementary shape can bind to the active site

Question 13

What are the limitations of the lock and key model?

Answer 13

• doesn’t explain how activation energy is lowered
• doesn’t explain the role of competitive inhibitors
• doesn’t explain the role of non-competitive inhibitors

Question 14

What is the indued fit model?

Answer 14

This model suggests that as the enzyme binds to a substrate the active site of the enzyme is altered slightly to maximise the enzymes ability to catalyse its reaction

Question 15

What are the reasons that the induced fit model is supported?

Answer 15

• explains how the activation energy is lowered by the stretching and distorting of bonds or causing the closer orientation of reactive groups.
• explains how non-competitive inhibitors can bind to a region away from the active site and change its shape so that substrate can no longer bind to the active site.
• explains how competitive inhibitors can bind to the active site or other molecules with similar shapes to the substrate.

Question 16

How does temperature affect enzyme activity?

Answer 16

• Increase in temperature increases kinetic energy
• so more enzyme substrate complexes form
• High temperatures cause denaturation, due to the breaking of breaking of bonds holding the tertiary structure together (H bonds/disulphide bridges/ionic bonds)
• Active site altered so substrate cannot bind, no enzyme substrate complexes form

Question 17

How does pH affect enzyme activity?

Answer 17

-small deviations from optimum pH change the charge at the active site and affect the binding of the
substrate
-larger deviations can cause the hydrogen and ionic bonds holding the tertiary structure together to change and the enzyme denatures, meaning enzyme-substrate complexes can no longer form

Question 18

How does enzyme concentration affect enzyme activity?

Answer 18

• As enzyme concentration increases so will the rate: as there are more enzyme-substrate complexes forming.
• At very high concentrations of enzyme the rate remains constant as substrate becomes the limiting factor

Question 19

How does substrate concentration affect the rate of enzyme activity?

Answer 19

-as the substrate concentration increases, the rate increases because more substrate molecules can collide with enzyme molecules, so more reactions will
take place.
-at higher concentrations the enzyme molecules become saturated with substrate, so there are few free enzyme molecules, so adding more substrate doesn’t make much difference because enzymes become the limiting factor

Question 20

What are competitive inhibitors and what do they do?

Answer 20

• have a similar shape to the substrate molecules
• they compete with the substrate molecules to bind to the active site
• Instead of a reaction, they block the active site so no substrate molecules can fit
• high concentration of the inhibitor will take up nearly all of the active sites and hardly any substrate will get to the enzyme
• lower concentrations of inhibitor increase the chances of the substrate getting to an active site before the inhibitor
• so, increasing the concentration of a substrate will increase the rate of reaction - up to a point.

Question 21

What are non-competitive inhibitors and what do they do?

Answer 21

• they bind to the enzyme away from its active site.
• causes the active site to change shape so the substrate molecules cannot bind
• don’t “compete” with the substrate molecules to bind to the active site because they are different shapes.
• Increasing the concentration of substrate doesn’t affect the reaction rate - enzyme activity is still inhibited.