Enzymes

Question 1

Definition of enzyme

Answer 1

Enzymes are biological catalysts which speed up the rate of reaction without being used up and changed in the reaction. Enzymes are globular proteins

Question 2

How do enzymes work?

Answer 2

1. Activation energy is the minimum amount of energy required for a reaction to occur, often supplied as heat energy
2. Enzymes work (speed up reaction) by taking an alternative reaction pathway
3. This requires lower activation energy
4. Therefore, less energy needed for reaction to start
5. Reaction can happen faster at a lower temp

Question 3

“Lock and key” model

Answer 3

Substrate fits into active sit region of enzyme similar to lock and key

Question 4

Induced fit model

Answer 4

More accurate than “lock and key”

1. Before reaction, shape of active site not complementary to substrate
2. Shape of active site changes as substrate binds / as enzyme substrate complex forms
3. This distorts the bonds holding substrate together, which lowers activation energy leading to reaction
4. Enzymes are very specific
5. Only one type of substrate fits into the active site region of each enzyme
6. When the substrate fits into active site region of enzyme, enzyme substrate complex is formed
7. However, as the substrate binds to active site, the enzyme changes shape slightly to complete the fit

Question 5

Enzymes are very specific

Answer 5

1. Enzymes have a specific tertiary structure
2. Tertiary structure of enzyme determines / creates shape of active site
3. Shape of active site complementary to one specific substrate
4. Only that specific substrate can bind to complementary active site forming an enzyme-substrate complex
5. Therefore, enzymes only catalyse one specific reaction. Maltase only catalyses hydrolysis of maltose into glucose
6. Enzymes are specific
7. Only one type of substrate can fit with active site region of each enzyme
8. Substrate must have a complementary shape to active site
9. Therefore, each enzyme can only catalyse one type of reaction. For example, maltase catalyses breakdown of glucose into maltose
10. Shape of enzyme’s active site is determined by enzyme’s tertiary structure, which is determined by primary structure - sequence of amino acids
11. If tertiary structure of enzyme is altered, shape of active site will also be altered, therefore, substrate will no longer be complementary to active site, and fit into active site, and form enzyme-substrate complex, therefore, enzyme cannot catalyse reaction - it becomes denatured

Question 6

What could happen if a mutation occurs to a gene which codes for an enzyme?

Answer 6

1. Primary structure of enzyme is determined by gene
2. If mutation occurs to that gene, primary structure of enzyme (sequence of amino acids) will be altered
3. Tertiary structure will be altered -
4. THEREFORE, Shape of active site will be altered
5. Substrate will no longer be complementary + fit with active site
6. No enzyme subtstrate complexes can form
7. Enzyme cannot catalyse reaction
8. Enzyme denatured

Question 7

Factors affecting enzyme activity

Answer 7

1. Temp
2. pH
3. Enzyme conc
4. Substrate conc
5. Presence of competitie/ non competitive inhibitors

Question 8

Temp

Answer 8

1. Increasing temp increases rate of enzyme controlled reaction
2. Enzyme + subtstrate molecules have more KE
3. Move faster + more often, higher collision frequenciy
4. More enzyme substrate complexes formed
5. Energy of these collisions higher, therefore more succesful collisions therefore, more enzyme subtstrate complexes

HOWEVER…

1. If the temp gets too high (beyond the optimum temp, temp where enzyme works best), the hydrogen bonds within the tertiary stucture of enzyme break
2. Alters tertiaty structure of enzyme, alters shape of active site
3. Substrate shape complementary to shape of active site
4. Therefore, subtsrate no longer fits into active site
5. No more enzyme substrate complexes formed
6. Enzyme denatured
7. Rate of enzyme controlled reaction starts to decrease

Question 9

pH

Answer 9

1. Above / below optimum pH, (pH at which enzyme works best), enzyme will be denatured
2. Above the optimum pH, the solution would be alkaline and have an excess of OH- ions, and therefore, can alter the ionic + hydrogen bonds in enzyme’s tertiary structure
3. Below optimum pH, solution would be acidic, and have excess H+ ions, this would alter the hydrogen + ionic bonds in enzymes tertiary structure
   Tertiary structure determines SHAPE of active site
4. Shape of enzyme’s active site altered
5. Substrate shape no longer complementary to active site, therefore no longer fits into active site
6. No more enzyme substrate complexes formed
7. Enzyme denatured
8. Rate of enzyme controlled reaction decreases

Question 10

Enzyme conc

Answer 10

1. Increasing enzyme conc increases rate
2. More enzyme molecules in reaction mixture
3. Therefore, more collisions between enzyme + substrate molecules = higher collision frequenct
4. More enzyme substrate complexes form

HOWEVER…

1. After a certain point, there will be too many enzyme molecules + not enough substrate molecules
2. This will lead to empty active sites
3. No substrate left to bind to active site + form enzyme substrate complexes, no more enzyme-substrate complexes formed, rate does not increase
4. Substrate conc is a limiting factor
5. To increase rate of reaction, must increase substrate conc, if not, rate of reaction will not increase or decrease

Question 11

Substrate conc

Answer 11

1. Increasing substrate conc increases rate of reaction
2. More substrate molecuels in reaction mixture
3. More collisions between subtsrate + enzyme molecules = higher collision frequency
4. More enzyme substrate complexes formed

HOWEVER…

1. After a certain point, mroe substrate molecules than enzyme molecules
2. All active sits filled
3. Not enough active sites for all substrate molecules
4. More substrate molecules cannot bind to active sites, no more enzyme substrate complexes form
5. Therefore, enzyme conc is limiting factor
6. Rate stops increasing unless enzyme conc is increased

Question 12

Enzyme Inhibitors Definition

Answer 12

Molecules which bind to enzymes and reduce or stop their activity TEMPORARILY Can be competitive + non-competitive

Question 13

Competitive Inhibitors

S C B

Answer 13

1. Competitive inhibitors have similar/ same shape to the substrate
2. They compete with the substrate to bind to active site
3. They then block active site so substrate cannot bind
4. Therefore, enzyme-substrate complexes cannot form
5. Enzyme activity will be reduced / stopped temporarily
6. Increasing conc of competitive inhibitor reduces rate of reaction and then eventually, if inhibitor concentration further increases, reaction will stop
7. This is because more competitive inhibitor molecules than substrate molecules, therefore, they have a higher chance of binding to the active sites, more active sites blocked, less enzyme substrate complexes form, this happens till all active sites are blocked

HOWEVER…

8. Inreasing substrate concentration can increase rate of reaction. More substrate that competitive inhibitor. Substrates chances of binding to active site is higher. More enzyme substrate complexes formed therefore, higher rate of reaction

Question 14

Non-competitive inhibitors

Answer 14

1. Non-competitive inhibitors bind to enzyme on a region that is not the active site
2. This causes active site to change shape
3. Substrate molecule no longer has complementary shape to bind with active site
4. Enzyme substrate complexes cannot form
5. This reduces the rate of reaction and eventually, if inhibitor concentration is continuosly increased, the reaction will stop

IN THIS CASE…

6. Increasing concentration of substrate will not increase rate of reaction
7. This is because non-competitive inhibitor binds to enzyme on region that is not active site, causing active site to change shape
8. Substrate cannot bind to active site, no longer complementary, active sites cannot form.
9. Non-competitive inhibitors do not compete with substrate molecules to bind to active site