Enzymes

Question 1

Enzymes

Answer 1

1. Biological catalysts
2. Increase the rate of a reaction
3. Chemically unaltered at the end of the reaction
4. Effective in small amounts

Question 2

Properties of enzymes

Answer 2

Globular proteins
Exhibit specificity -> catalyse a specific chemical reaction (absolute/group)
Operate at milder reaction conditions

Question 3

Active site

Answer 3

Catalytic centre -> binds with substrate

Determined by primary structure -> determines secondary and tertiary structure -> overall specific 3D conformation

Question 4

4 types of amino acid residues

Answer 4

1. Contact/binding
   Interact reversibly with substrate via weak hydrogen and ionic bonds -> determine specificity
2. Catalytic
   Catalyse conversion of substrate to product
3. Structural
   Maintain overall 3D conformation
4. Non-essential

Question 5

Specificity of enzyme

Answer 5

Complementary conformation/shape and charge between substrate and active site

Question 6

Lock and Key hypothesis

Answer 6

Substrate is the “key” whose specific conformation and charge is complementary to the enzyme active site/”lock”

Effective collision (collide in the correct orientation) between and substrate form a temporary enzyme-substrate complex, held together by weak interactions

Catalysis occurs and products are formed

Products no longer fit into the active site and are released

Enzyme is unchanged and can be used again

Question 7

Induced fit model

Answer 7

Substrate induces a change in shape in enzyme active site so that active site is a more precise fit for substrate for effective catalysis

Question 8

Catalysis

Answer 8

Enzyme lowers the activation energy barrier by

1. Proximity effects -> aligning substrates next to each other in active site for reaction to occur
2. Strain effects -> strain on bonds to be broken / distorts the substrate and reduces activation energy to achieve transition state
3. Orientation effects -> orientates substrate such that its bonds are exposed to chemical attack
4. Microenvironment effects -> provide a favourable microenvironment
5. Acid-base catalysis -> R-groups of amino acid residues in active site participate in direct catalysis

Question 9

Enzyme cofactors

Answer 9

Non-protein substances for catalytic activity

1. Inorganic ions -> attachment changes shape of enzyme
2. Coenzyme -> organic in nature
3. Prosthetic group -> permanently bound

Question 10

Following time course

Answer 10

Formation of products
Disappearance of substrate
Calculate rate based on gradient

Highest frequency of effective collisions at the start -> rate of formation of enzyme-substrate complexes is highest -> rate of reaction maximum

Question 11

Effect of low temperature on rate

Answer 11

Increase in temp -> increase kinetic energy of enzyme substrate molecules -> increase frequency of effective collisions between substrate and enzyme active sites -> increase rate of formation of enzyme substrate complexes

Increased number of molecules having sufficient energy to overcome activation energy barrier

Rate doubles for each 10 degree C rise

Optimal temperature -> maximum rate
Higher optimal temperature -> higher proportion of disulfide bonds and numerous intramolecular interactions

Question 12

Effect of high temperature on rate

Answer 12

Increase in kinetic energy -> increase in intramolecular vibrations -> brakes weak interactions that stabilises conformation/shape -> denaturation

Substrate no longer complementary to conformation/shape of active site -> fewer ES complexes formed -> lower rate of reaction

Question 13

Effect of suboptimal pH on rate

Answer 13

Affect ionisation of R-groups
Excess H+ -> COO- groups become COOH
Excess OH- -> NH3+ groups become NH2

1. Structural -> disruption ionic and hydrogen bond which determines tertiary structure -> specific 3D conformation of active site changed -> denatured
2. Contact and catalytic at active site -> ES interaction disrupted -> catalysis may not take place
3. Protein substrate -> affect interaction with active site and/or catalysis

Reduced rate of ES complex formation

Question 14

Enzyme concentration on rate

Answer 14

Increase [enzyme] -> increase frequency of effective collisions -> increase rate of formation of ES complexes

Limiting (linear) vs not limiting (curved portion)

Question 15

Substrate concentration on rate

Answer 15

At low [S], active sites of enzymes readily available -> substrate concentration limiting

Active sites start to get saturated

Enzyme saturation -> all active sites occupied -> Vmax

Km = [S] required for reaction to attain 1/2 Vmax -> measure of affinity of enzyme for its substrate

Question 16

Competitive inhibition

Answer 16

Prevent substrate molecules from binding to enzyme active site

Similar shape/conformation to substrate -> competes

Binds reversibly via weak interactions -> reduces availability of enzyme active site for substrate binding

Can be overcome by increasing [S]

Question 17

Non-competitive inhibition

Answer 17

No structural similarity to substrate -> binds to site other than enzyme active site

Alters conformation of specific active site -> form inactive enzyme-inhibitor complex -> decrease availability of enzymes -> substrate cannot bind

Cannot be overcome by increasing [S]

Question 18

Allosteric enzyme

Answer 18

Regulated by inhibitors and activators
Usually multimeric (2 or more subunits)

1. Active site that binds substrates
2. Allosteric site that binds activators or inhibitors

Question 19

Allosteric regulation

Answer 19

Allosteric enzyme oscillates between active and inactive conformation

1. Binding of allosteric activator at allosteric site/binding of substrate at active site stabilises active conformation
2. Binding of allosteric inhibitor stabilises inactive conformation

Binding of substrates exhibits cooperativity

Question 20

Feedback/End-product inhibition

Answer 20

Metabolic pathway inhibited by the binding of the end product to an enzyme that acts early in the pathway

Prevents cell from wasting resources in producing excess product