Enzymes

Question 1

What are enzymes

Answer 1

Catalysts in living cells which accelerate specific reactions in biological systems

Question 2

What are catalysts

Answer 2

Speed up chemical reactions (lower activationenergy) while remaining chemically unchanged

Question 3

Functions of enzymes

Answer 3

Vital for life: catalyse chemical reactions in cells that keep us alive

They have 1000s of roles including aiding:
Respiration
Digestion
Muscle and nerve function

Question 4

Examples of enzymes within the oral cavity

Answer 4

-Alkaline Phosphatase
Found throughout body, involved in mineralisation of tissue and bone
-Maltase
Found in saliva, breaks the sugar maltose into glucose
-Amylase
Found in saliva, converts starch to sugars
-Lysozyme
Antimicrobial, breaks down peptidoglycan layer in cell wall of bacteria, found in many secretions e.g. saliva, tears, human milk, mucous

Question 5

Examples of enzymes throughout the body

Answer 5

-Pepsin
digestive enzyme of stomach, breaks down proteins into smaller peptides
-Trypsin
found in the small intestine, breaks down proteins into amino acids
-Acetylcholinesterase
breaks down the neurotransmitter acetylcholine in nerves and muscles
-DNA polymerase
synthesises DNA from deoxyribonucleotides

Question 6

Two enzyme models

Answer 6

Lock and key (perfect fit)

Induced fit (slightly different fit so enzyme changes shape after binding)

Question 7

What enzyme is not a protein

Answer 7

Ribozymes (catalytic RNA molecules)

Question 8

factors affecting function of enzymes

Answer 8

-Enzymic activity of proteins is dependent on the maintenance of their 3D structure
-Enzyme activity is affected by changes in pH and temp
-Each enzyme will usually have pH and temp optima at which is works best
-At extremes of pH and temp, the enzyme may lose its 3D structure (denature) and become totally inactive

Question 9

Factors affecting the rate of enzyme reaction

Answer 9

Temperature
pH
Enzyme concentration
substrate concentration
Inhibitors and activators
Covalent modification

Question 10

Effect of enzyme conc on reaction rate

Answer 10

An increase in enzyme concentration will increase the reaction rate
The equilibrium will be reached more rapidly
After equilibrium is reached the concentrations of product and substrate will not change

Question 11

What is Vmax

Answer 11

The maximum rate of reaction

Question 12

When substrate conc is smaller and in excess what does the graph look like

Answer 12

When substrate conc is small the rate of reaction is linearly proportional to the conc:
Therefore the rate of product formation is limited by the amount of available substrate

When the substrate conc is high the rate is nearly independent of conc:
rate of product formation now depends on the activity of the enzyme itself further increase of substrate conc will likely have little effect on rate

Question 13

What is Km

Answer 13

The Michaelis constant, an inverse measurement of affinity
The conc of substrate at which the rate of reaction is half it’s max value V = 1/2 Vmax

Question 14

What does low Km mean

Answer 14

Very high affinity for substrate, acts at a fairly constant rate regardless of variation in substrate concentration
RATE OF PRODUCT FORMATION IS NOT AFFECTED BY SUBSTRATE AVAILABILITY (vice versa for high Km)

Question 15

What does Vi stand for

Answer 15

Initial rate

Question 16

General symbol equation for an enzyme substrate reaction

Answer 16

[E] + [S] <=> [ES] => [E] + [P]

Question 17

What is the Michaelis-Menten equation

Answer 17

V = Vmax [S] / (Km + [S])

Question 18

Name the polysaccharide which is one of the main components of dental plaque

Answer 18

Dextran

Question 19

What produces a more accurate estimate of Vmax and Km

Answer 19

Lineweaver-Burk plot
-Plot 1/V against 1/S

Question 20

What do irreversible enzyme inhibitors do

Answer 20

-Damage enzymes beyond repair
-Generally cause covalent modification of the enzyme

Question 21

What is the most common (natural) enzyme inhibitor

Answer 21

Reversible inhibitors
-Full enzyme activity is regained when the inhibitor is removed

Question 22

What is feedback inhibition

Answer 22

When the product of a metabolic pathway builds up in the cell, inhibition of further synthesis of this product often occurs
EXAMPLE: threonine deaminase is inhibited by a build up of isoleucine

Question 23

What is the function of a competitive inhibitor

Answer 23

Substrate and inhibitor compete for the same active binding site preventing substrates from binding once attached however this can be reversed with large quantities of the substrate

Question 24

Function of Non-Competitive inhibitors

Answer 24

These bind to an allosteric site (not binding site) on the enzyme and alters the conformation of the enzyme so that the active site is no longer fully functional preventing the substrate from binding

Question 25

Can inhibition be reversed

Answer 25

Competitive inhibitors are not permanent and can be overcome

Question 26

What are allosteric enzymes

Answer 26

-Possess multiple subunits
-They possess allosteric sites to which non-substrate modulators bind changing their formation
-Subunits interact such that the binding of a substrate, inhibitor, or activator (modulators) to one subunit alters all the subunits:
-Positive modulators (increase affinity for substrate at active site)

-Negative modulators (decrease affinity for substrate at active site)

Many important regulatory enzymes are allosteric

Question 27

Which enzymes do not conform to Michaelis-Menten kinetics

Answer 27

Allosteric enzymes

Question 28

Control of enzyme activity by covalent modification

Answer 28

Common modification is the addition of a phosphate group to a serine, threonine or tyrosine hydroxyl group to modify an enzymes activity