Enzymes

Question 1

Enzymes as biological catalysts

Answer 1

1. Enzymes are proteins that increase the rate of reaction by lowering
   the energy of activation
2. They catalyze nearly all the chemical reactions taking place in the
   cells of the body.
3. Not altered or consumed during reaction.
4. Reusable

Question 2

When the Substrate (S) binds in an
Enzyme active site, an…

Answer 2

Enzyme-substrate complex is formed

Question 3

Enzymes may recognise and catalyse:

Answer 3

• single substrate
• a group of similar substrates
• particular type of bond

Question 4

Enzymes increase reaction rates by…

Answer 4

Decreasing activation energy

Question 5

What happens to ΔG when an enzyme is added

Answer 5

It remains unchanged

Question 6

What affects enzyme activity

Answer 6

Three factors

1. Environmental Conditions – pH, Temperature,
   Substrate concentration
2. Cofactors and Coenzymes
3. Enzyme Inhibitors

Question 7

What is an apoenzyme and a holoenzyme

Answer 7

Apoenzyme -
An enzyme without its non-protein component (Cofactor) it is therefore inactive

Holoenzyme -
An enzyme with its non protein component (cofactor) it is therefore active

Question 8

Cofactor vs coenzyme vs prosthetic group

Answer 8

Cofactor
-Non-protein compound, inorganic/organic
-Bound covalently or loosely to enzyme
-Required for catalysis: coenzymes, prosthetic groups, metal ions (Fe2+, Mg2+, Ca2+, Zn2+),

Coenzyme
-Non-protein component, organic
-Loosely bound to apoenzyme by non-covalent bond
-NAD, FAD, ATP, vitamins (contribute to catalysis)

Prosthetic group
-Permanent part of 3D structure
-Tightly bound to apoenzyme by covalent bonds
-Heme in hemoglobin, does not dissociate from enzyme until it is degraded

Question 9

What is irreversible inhibition

Answer 9

Inhibitors covalently interact with enzyme that their inhibition is irreversible

Any compound that modifies the enzyme active site

Modified enzyme is dead and can only be overcome with new
enzymes

Neurotoxins are irreversible inhibitors as the inhibit acetylcholine esterase in the synapses of neurones

Question 10

What is non competitive inhibition

Answer 10

Noncompetitive inhibitor binds to a site on enzyme other than the active site

Reduces enzyme activity but does not affect binding of substrate

Binds to Enzyme or
Enzyme-Substrate complex

Inhibition is reversible

Extent of inhibition depends on inhibitor concentration

Cyanide is an example of a non competitive inhibitor as it binds to cytochrome c oxidase enzyme in electron transport chain so ATP can no longer be made as enzyme can not pass electrons to oxygen so only anaerobic respiration is possible, the build up of lactic acid results in death

Question 11

What is Uncompetitive inhibition

Answer 11

Inhibitor binds only to Enzyme-substrate complex

Inhibitor affects substrate binding by increasing enzymes affinity for substrate

The Enzyme-substrate-inhibitor complex is inactive and decrease rate of reaction

Question 12

What is allosteric inhibition

Answer 12

Enzymes contain both an active site and a modulator site

Inhibitor binds to enzyme at modulator site, changes conformation of enzyme

Decrease catalytic activity
of the substrate at the active site

E.g. Direct Thrombin Inhibitors act as anticoagulants of Thrombin

Question 13

What is feedback inhibition

Answer 13

Enzyme activity is inhibited by end product in a biochemical pathway.

Occurs when there is buildup of product being produced.

Negative feedback slows down the production to maintain homeostasis in a cell.

Question 14

What is an apenzyme

Answer 14

Enzyme without cofactor

Question 15

What is holoenzyme

Answer 15

Enzyme with cofactor

Question 16

What is vmax

Answer 16

Full saturation so substrate conc no longer limiting factor