Properties of enzymes

Question 1

What are enzymes

Answer 1

Biological catalysts that speed up the rate of a reaction without altering the final equilibrium between products and reactants.

Question 2

What are the two types of specificity that an enzyme can show?

Answer 2

Group specificity for example alcohol dehydrogenase oxidises primary alcohols to aldehydes; works on methanol, ethanol, propanol etc. because of the -OH group.

Absolute specificity, will only act on one substrate (or one stereoisomer of a substrate only)

Question 3

What are the classes of enzymes

Answer 3

Oxidoreductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases

Question 4

What is an oxidoreductase? Give an example

Answer 4

Oxidoreductases catalyse transfer of H and electrons.

For example, lactate dehydrogenase:
Pyruvate + NADH -> L-lactate + NAD

Question 5

What is a transferase? Give an example

Answer 5

Transferases swap groups between molecules.

For example, alanine aminotransferase:

Pyruvate + glutamate -> L-alanine + a-ketoglutarate

Question 6

What is a hydrolase? Give an example

Answer 6

Hydrolyses a covalent bond

An example is trypsin:

Gly-Lys-Val-Ala + water -> Gly-Lys + Val-Ala

Question 7

What is a lyase? Give an example

Answer 7

Lyases are involved in the cleavage of bonds - forms double bonds by the removal of groups

For example, ATP-citrate lyase:
citrate + ATP oxaloacetate + acetate + ADP + Pi

Question 8

What is an isomerase? Give an example

Answer 8

Isomerases swap groups within molecules (creating an isomer of the original molecule)

For example, phosphoglucose isomerase

glucose-6-phosphate fructose-6-phosphate

Question 9

What is a ligase? Give an example

Answer 9

Ligases use ATP to produce a covalent bond (kinda the opposite of a lyase)

An example is DNA ligase (sticks two complementary strands of DNA together using ATP)

Question 10

What does chymotrypsin do?

Answer 10

Chymotrypsin is a hydrolase and uses water to hydrolyse the peptide bond on the carboxyl side of tyrosine, tryptophan, phenylalanine and methionine.

Question 11

What enzyme/s use Cu2+ as a cofactor?

Answer 11

Cytochrome oxidase

Question 12

What enzyme/s use Fe2+/3+ as a cofactor?

Answer 12

Cytochrome oxidase, catalase, peroxidase

Question 13

What enzyme/s use K+ as a cofactor?

Answer 13

Pyruvate kinase

Question 14

What enzyme/s use Mg2+ as a cofactor?

Answer 14

Hexokinase, G-6-phosphatase, pyruvate kinase

Question 15

What enzyme/s use Ni2+ as a cofactor?

Answer 15

Urease

Question 16

What enzyme/s use Se as a cofactor?

Answer 16

Glutathione peroxidase

Question 17

What enzyme/s use Zn2+ as a cofactor?

Answer 17

Carbonic anhydrase, alcohol dehydrogenase

Question 18

What is an isoenzyme and give an example

Answer 18

Isoenzymes are enzymes with different protein structures which catalyse the same reaction. Hexokinase and glucokinase are isoenzymes.

Question 19

What can effect the rate of an enzyme catalysed reaction?

Answer 19

Amt of substrate/enzyme; temp; pH; presence of cofactors; amount of metal ions.

Question 20

What assumptions are made when working under the Michaelis-Menton reaction model?

Answer 20

1. [S]&raquo_space; [E]
2. [ES] does not change as it is formed at the same rates as it is used up or at the same rate at which the backwards reaction occurs.
3. Initial velocities used (t=0); [P] is small therefore the back reaction can be ignored.

Question 21

When can Km be used as a measure of the affinity of an enzyme for a substrate

Answer 21

When k2 &laquo_space;k1; the formation of the product from the ES complex can be ignored and so the Km value reduces to the dissociation constant of the initial forward reaction of the formation of the ES complex

Question 22

How can the measurement of the activity of an enzyme be used clinically?

Answer 22

Lactate dehydrogenase (LDH) and creatine kinase 2 present (CK2) in blood following a myocardial infarction as endothelial cells rupture releasing their contents into bloodstream.

Question 23

What are the types of inhibitors for an enzyme?

Answer 23

Competitive (blocks active site)

Non-competitive (Interferes in another way with catalytic mechanism)

Question 24

How does a competitive inhibitor affect the Km and Vmax of an enzyme?

Answer 24

It lowers Km, does not change Vmax

Question 25

How does a non-competitive inhibitor affect the Km and Vmax of an enzyme?

Answer 25

It lowers Vmax, does not change Km

Question 26

Describe the inhibition of angiotensin production.

Answer 26

Prevents conversion of AT1 to AT2, done either by reversible or irreversible inhibition.
Inhibition of this process prevents formation of AT2 which can lead to peripheral vasoconstriction.

Question 27

What is allosteric regulation of an enzyme?

Answer 27

A substrate binds to an enzyme and causes a conformational change in it, so makes it easier for another substrate to bind to it in the future.

Question 28

What amino acids can have phosphate groups added/removed?

Answer 28

Ser, thr, tyr, his

Question 29

How does phosphorylation be used to regulate the synthesis and breakdown of glycogen

Answer 29

Via adrenaline and glucagon.
Two enzymes involved: glycogen phosphorylase (degrades glycogen) activity increased by phosphorylation.
And glycogen synthase (synthesises glycogen) activity reduced by phosphorylation.

Question 30

How can the amount of an enzyme released be regulated?

Answer 30

An example is that high blood sugar leads to an increase in insulin production and so in turn that leads to an increase in the synthesis of other enzymes involved in glucose metabolism: glucokinase, phosphofructokinase and pyruvate kinase.

Question 31

Give an example of reversible inhibition of an enzyme

Answer 31

Inhibition of a Mg2+ requiring enzyme with EDTA/chelation

Question 32

Give an example of irreversible inhibition of an enzyme

Answer 32

Inhibition of cholinesterase by organophosphate