Lecture 7.1: Enzymes

Question 1

What is rate of reaction influenced by? (4)

Answer 1

• Concentration of reactants
• Activation energy
• Temperature
• Presence of a catalyst

Question 2

Activation Energy

Answer 2

For a reaction to occur, the reactants must acquire enough energy to enable
them to react with each other

This energy comes from e.g. reactants colliding with each other

The greater the activation energy, the slower the rate of reaction

Question 3

Catalysts

Answer 3

They reduce Ea needed for a reaction to occur

They may bring reactants closer together and in an appropriate orientation for the reaction to proceed

A compound that speeds up a chemical reaction without itself undergoing any permanent change, so is free to be used again

Question 4

What is an enzyme?

Answer 4

It is a biological catalyst – made from proteins each has a unique amino
acid composition each has a unique 3D structure

Question 5

Mechanisms of Enzyme Action? (2)

Answer 5

1) Lock and key
2) Induced fit

Question 6

Why are enzymes selective?

Answer 6

Because they have unique 3D structures

This means they display selectivity for the reactions they catalyse

The substrate(s) they work with must fit the active site (analogous to ligand bind region on receptors)

Question 7

Absolute Enzymes

Answer 7

Only catalyse one specific reaction

Question 8

Group Enzymes

Answer 8

Only act on molecules with a specific functional group (e.g. methyl groups)

Question 9

Linkage Enzymes

Answer 9

Will only act on a specific type of chemical bond (e.g. peptide bonds)

Question 10

Stereochemical Enzymes

Answer 10

Will only act on a particular optical isomer of a molecule

Question 11

Classification of Enzymes (6)

Answer 11

Oxidoreductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases

Question 12

Oxidoreductases Reaction Type

Answer 12

Oxidation-Reduction

Question 13

Transferases Reaction Type

Answer 13

Transfer of groups e.g. amino, carboxyl between molecules

Question 14

Hydrolases Reaction Type

Answer 14

Cleave bonds coupled with the insertion of water

Question 15

Lyases Reaction Type

Answer 15

Cleavage of C-C, C-S and C-N bonds

Question 16

Isomerases Reaction Type

Answer 16

Bond rearrangement

Question 17

Ligases Reaction Type

Answer 17

Bond formation between C-O, C-N and C-S

Question 18

What can rate of enzyme catalysed reactions can be influenced by? (7)

Answer 18

Substrate concentration

Enzyme concentration (in real life, [S] are generally&raquo_space; [E])

Enzyme activity (influence of other factors e.g. allosteric modulators that may alter properties of the active site e.g. increasing affinity for substrate)

pH
Temperature
Presence of ions
Presence of inhibitors

Question 19

The significance of Km values: low Km and high Km

Answer 19

Km values represent affinity of an enzyme for its substrate

Low Km = high affinity for substrate
High Km = low affinity for substrate

Question 20

What is 1 unit on a Vmax/V0 graph

Answer 20

1 unit = the amount of enzyme that converts 1micromol of product per min under standard conditions

Question 21

What is the standardised rate?

Answer 21

Per litre (L) of serum or per gram (g) of tissue

Question 22

Enzyme Inhibitor Types (2 + 2 sub)

Answer 22

Irreversible
Reversible [Competitive and Non-Competitive]

Question 23

Irreversible Enzyme Inhibitor

Answer 23

Bind very tightly, generally form covalent bond(s)

Examples: nerve gases, sarin blocks acetylcholine esterase

Question 24

Competitive Enzyme Inhibitor

Answer 24

Binds at active site

Affects Km, not Vmax

Question 25

Non-Competitive Enzyme Inhibitor

Answer 25

Binds at another site on the enzyme Affects Vmax but not Km

Question 26

What is Km?

Answer 26

The concentration of substrate which permits the enzyme to achieve half Vmax

Question 27

What is Vmax?

Answer 27

The reaction rate when the enzyme is fully saturated by substrate

Question 28

Competitive Inhibition Mechanism

Answer 28

• Resembles the substrate • Binds to the active site • Reduces the proportion of enzyme molecules bound to the substrate

Question 29

Non-Competitive Inhibition Mechanism

Answer 29

• Binds at an alternative site on enzyme (allosteric site) • Alters the shape of the molecule (enzyme) • Decreases the turnover number of the enzyme→ lowering Vmax

Question 30

What is the Turnover Number?

Answer 30

The turnover number is the number of substrate molecules transformed per minute by a single enzyme molecule

Question 31

Long Term Regulation of Enzyme Activity

Answer 31

Change in rate of protein synthesis • Enzyme induction/repression • Through a change in transcription/translation Change in rate of protein degradation • Ubiquitin-proteasome pathway

Question 32

Short Term Regulation of Enzyme Activity (seconds to hours)

Answer 32

Substrate and product concentration Change in enzyme conformation: • Allosteric regulation • Covalent modification • Proteolytic cleavage (irreversible)

Question 33

Feedback Inhibition

Answer 33

The end product of a metabolic pathway has the ability to inhibit a enzymatic step upstream

Question 34

Allosteric Enzymes

Answer 34

• Distinct regulatory sites • Regulation by small signal molecules • Multiple active sites (multi-subunit) • Binding at regulatory site induces conformational change that affects active site • Activity at one active site affects other active sites, known as co-operativity

Question 35

Allosteric Regulation: T vs R & Co-operativity

Answer 35

Tense & Relaxed: • T state – low affinity • R state – high affinity Co-operativity: • Substrate binding to one subunit favours others becoming relaxed

Question 36

Allosteric Activators

Answer 36

Increase the proportion of enzyme in the R state Shift curve to the left

Question 37

Allosteric Inhibitors

Answer 37

Increase the proportion of enzyme in the T state Shift curve to the right

Question 38

Allosteric regulation of phosphofructokinase

Answer 38

Phosphofructokinase is allosterically regulated and sets the pace of glycolysis It is the key regulator of glycolysis Allosteric activators: AMP, fructose-2,6-bisphosphate Allosteric inhibitors: ATP, citrate, H+

Question 39

Covalent Modification - Phosphorylation

Answer 39

Protein kinases transfer the terminal phosphate from ATP to the –OH hydroxyl group of Serine, Threonine, Tyrosine

Question 40

Covalent Modification - Dephosphorylation

Answer 40

Protein phosphatases reverse the effects of kinases They do this by catalysing the hydrolytic removal of phosphoryl groups from proteins

Question 41

Clinical Significance of Enzymes (2)

Answer 41

Drug Targets Biomarkers

Question 42

Clinical Significance of Enzymes: Drug Targets

Answer 42

Angiotensin converting enzyme inhibitors (e.g. Enalapril) – mainstay of the treatment of hypertension Inhibitors of dihydrofolate reductase (e.g. methotrexate) causes cells to arrest in S phase of cell cycle and die – some cancers and rheumatoid arthritis

Question 43

Clinical Significance of Enzymes: Biomarkers

Answer 43

Liver function tests measure a range of hepatic enzymes along with plasma levels of other substances Creatine kinase is a marker of muscle damage