Enzymes

Question 1

What is the role of enzymes

Answer 1

Catalyse reactions that affect/speed up metabolism at a cellular and whole organism level. They affect both structure and function and do not get used up

Question 2

Enzyme can affect structure and function examples

Answer 2

Structure - Involved in production of collagen (tissue that joins muscle and bone)
Function - involved in respiration

Question 3

If 2 substrate molecules need to be joined… (anabolic)

Answer 3

Enzyme holds them together so they can bond more easily

Question 4

If enzyme is catalysing a breakdown… (catabolic)

Answer 4

Fitting into active site puts a strain on substrate bonds so they can be broken more easily

Question 5

Intracellular reactions

Answer 5

Made and retained inside the cell

Question 6

Catalase

Answer 6

Catalase breaks down hydrogen peroxide into water and oxygen in liver
- H2O2 -> O2 + H20

Question 7

Extracellular reactions

Answer 7

Made inside, but works outside the cell
- Amylase
- Trypsin

Question 8

Trypsin

Answer 8

Trypsin is produced by pancreatic cells and released into small intestines. It catalyses the hydrolysis of peptide bonds of large polypeptides into smaller ones that eventually get broken down into amino acids by other enzymes

Question 9

Amylase

Answer 9

Amylase in saliva catalyses the hydrolysis of starch into maltose

Question 10

Structure of an enzyme

Answer 10

Globular - 3D, almost spherical, soluble in water due to position of hydrophilic/phobic R groups
Active site - specific indented area on enzyme surface, where substrate binds

Question 11

What determines the shape of an enzyme’s active site

Answer 11

Tertiary structure of the enzymes active site gives it a shape that is complementary to that of the substrate molecule. They are specific so the enzyme only works on one particular substrate.

Question 12

Lock and key hypothesis

Answer 12

1) S-molecules and E-molecules have KE and are constantly moving randomly
2) S-molecule successfully collides with an E molecule and fits into the complementary shaped active site. ES complex forms
3) S-molecules are broken down or built up into the P-molecules. EP complex forms in the active site
4) P-molecules leave active site
5) E-molecule is able to form another ES complex

Question 13

Pros and cons of lock and key hypothesis

Answer 13

• Explains enzymes specificity,
• Does not explain how to transition state (ES complex) is stabilised

Question 14

Induced-fit hypothesis

Answer 14

1) Active site is not complementary
2) When S-molecule enters active site, it changes shape to mould around the molecule
3) This allows substrate to fit/bind more effectively to active site.
4) Forms ES complex
5) S-molecule converted into P-molecules, forming EP complex. Products have different shape to S-molecule and detach
6) Enzyme returns to original shape

Question 15

How does the enzyme change shape in induced fit

Answer 15

Conformational change
- Subtle changes to side chains (R groups) of the amino acids that make up the active site gives a more precise conformation that exactly fits the substrate molecule

Question 16

Difference between lock and key and induced fit model

Answer 16

Active site is fixed and does not change shape in lock and key. It is not complementary in induced fit.

Question 17

Enzyme substrate complex

Answer 17

Enzyme molecule with substrate molecules in active site. They are joined temporarily by non-covalent forces (e.g. hydrogen bonds, ionic attractions, Van der Waals forces, hydrophobic interactions)

Question 18

Enzyme product substrate

Answer 18

Enzyme molecule with product molecules in its active site. They are joined temporarily by non-covalent forces

Question 19

How do enzymes lower activation energy

Answer 19

Energy (usually heat) is needed to activate chemical reactions. In a living cell, temperature cannot be raised as proteins would denature and lipids would melt. Enzymes bring specific S-molecules close enough together to react, without the need for excessive heat. Therefore they lower activation energy and speed up metabolic reactions

Question 20

Difference between lock and key and induced fit model

Answer 20

Active site is fixed and does not change shape in lock and key. It is not complementary in induced fit.

Question 21

Effect of temp on enzyme activity

Answer 21

1) As the temperature ^, KE ^, number of successful collisions ^, the rate of formation of ES complex ^
2) Optimum temp - highest enzyme activity
3) Temp too high, vibration, bonds break, enzyme changes shape (irreversible) and denatures

Question 22

What happens when enzyme denatures

Answer 22

• Bonds that hold tertiary structure of the enzymes active site breaks (e.g. hydrogen and ionic bonds) so it is no longer complementary
• Peptide bonds between amino acids don’t change (primary structure unaltered)

Question 23

What is temperature coefficient (Q10)

Answer 23

• The increase in the rate of a process (ror) when the temperature is increased by 10C.
• At temps before the optimum, a Q10 value of 2 means that rate doubles every 10C ^
• At temps after optimum, Q10 drops as enzyme changes shape

Question 24

Q10 formula

Answer 24

Rate of reaction at T+10 (C) / Rate of reaction at T (C)

Question 25

What is a buffer

Answer 25

• Something that resists changes in pH.
• certain chemicals in blood can donate or except hydrogen ions.
• some proteins such as haemoglobin can also donate/accept protons and act as buffers
• buffers solutions maintain the desired pH or keep pH constant

Question 26

Effect of pH on enzyme activity

Answer 26

• H-bonds and ionic forces between amino acids hold tertiary structure of enzyme together.
• Excess H+ ions interfere with these bonds and enzyme changes shape, so substrate doesn’t fit. Increasing conc of H+ alter the charges on active site of enzyme molecules, as more protons will cluster around -vely charged groups (amino acid R-groups) in active site. This interferes with binding of the substrate molecule to the active site.

Question 27

Enzymes work within a narrow range of pH

Answer 27

• Small changes of pH on either side of optimum pH slow ror because active site shape is disrupted
• If normal optimum pH is restored, the hydrogen bonds can reform and active site is restored
• At extreme pH, enzymes active site denature (permanently changed), cannot catalyse reaction

Question 28

Not all enzymes have the same optimum pH

Question 29

Effect of substrate conc on enzyme activity

Answer 29

• No substrate -> no ES complex -> no reaction
• As substrate conc increases (is added), ror increases
• As conc of substrate is increased further, the reactions will reach max rate

Question 30

Explain why ror increases with substrate conc

Answer 30

More ES complexes can form -> more product molecules forms -> substrate concentration is the limiting factor (it increases as the rate of reaction increases)

Question 31

Explain why ror reaches a max as substrate conc increases

Answer 31

More substrate molecules does NOT increase rate of reaction (not limiting the reaction)
- all the enzymes’ active sites are occupied with substrate molecules and working at max rate
- if more substrate molecules are added, they cannot successfully collide with and fit into active site

Question 32

Availability of enzymes

Answer 32

Depends on rate of enzyme synthesis and degradation
- Genes for synthesising particular enzymes can be switched on or off
- Cells continuously degrade old enzyme molecules to their component amino acids. These are synthesised to form new enzyme molecules

Question 33

Advantages of enzyme degradation

Answer 33

• Elimination of abnormally shaped proteins that might otherwise accumulate and harm the cell
• Regulation of metabolism in the cell by eliminating any superfluous enzymes (surplus to requirements)

Question 34

As enzyme conc increases…

Answer 34

• More active sites on the enzyme become available
• more successful collisions between an enzyme and substrate
• more ES complexes form per unit time so ror increases
• enzyme concentration is limiting factor - as it increases so does ror

Question 35

Explain why ror reaches a max as enzyme conc increases

Answer 35

If substrate conc is fixed/limited, all the S-molecules will be occupying an active site or released as P-molecules -> maximum rate for the fixed substrate conc
- If enzyme conc increases further, no increase in ror because the active site of the extra enzyme molecules will not be occupied
- enzyme conc is no longer limiting factor
- substrate conc is limiting factor as it prevent rate of reaction from increasing. If substrate conc is increased and ror can increase

Question 36

Why is the initial ror always fastest

Answer 36

• Beginning of reaction: enzyme and substrate molecules are mixed and moving randomly - high chance of substrate molecule successfully colliding with an enzymes active site
• Reaction proceeds: substrate molecules used up and converted into product molecules, so substrate conc drops
• Frequency of successful collisions decreases because some enzymes may collide with P molecules, so ror decreases.
• Initial reaction gives maximum reaction rate for an enzyme under a particular experimental situation

Question 37

How to work out rate of reaction from graph

Answer 37

change in y / change x