S1-L12: Enzymes 2

Question 1

Figure 1 and 2 show enzyme catalysed reactions

Answer 1

• figure 1: substrate binds to enzyme and is then broken down after forming E-S complex
• ->enzyme can be reused
• figure 2: activation energy reduced with use of enzyme (catalyst)

Question 2

What is the effect of substrate concentration on enzyme-catalysed reactions? (figure 3)

Answer 2

• increasing substrate conc increases reaction rate
• at optimum conc of substrate all active sites full AND working at max efficiency
• any increase in conc beyond optimum will have no added effect as no extra a. sites to be used

Question 3

Describe the effect of enzyme concentration on rate of enzyme-catalysed reactions (refer to figure 4)

Answer 3

• increase amount of enzyme
• ->increases rate of reaction
• more enzyme–>more frequently collide with substrate
• reaches saturation point where increasing conc not have affect on reaction rate as all a. sites occupied by substrate (figure 5)

Question 4

In appropriate detail explain the effect of temperature & pH on enzyme-catalysed reactions

Answer 4

-TEMP: increase in heat energy causes more collisions between enzyme and substrate
–>optimum temp reached
–>enzyme denatured at high temp so rate falls rapidly
(figure 6)
-pH: similar as temperature
–>each enzyme also has optimum pH works best at
(figure 7)

Question 5

What is Enzyme Kinetics?

Answer 5

• study of rate of enzyme controlled reaction

- ->to see substrate conc effect of different inhibitors/drugs or different isozymes/ mutations

Question 6

Outline the Michaelis-Menten equation (study of enyzme kinetics) and the different parts of the equation

Answer 6

• Equation: Vo= Vmax (S)/Km + (S)
• Vmax: represents max velocity achieved by system at saturating substrate conc’s
• Km (Michaelis constant): substrate conc at which reaction velocity 50% of V max
• (S): conc of substrate S

Question 7

Using figure 8 describe basic enzyme kinetics based on the structure of the graph

Answer 7

• amount of P produced/unit time start of reaction–> initial velocity (V0)
• V0 increases until E saturated–> plateau produced
• max reaction rate- Vmax- point at which graph plateaus out
• (S) which gives half vmax is Km
• Km–>useful measure of affinity of E has for the S

Question 8

What value of Vmax and Km does a fast, efficient and specific enzyme have? (refer to figure 9- Velocity VS substrate graph)

Answer 8

• Km: measures affinity for it’s substrate
• Low Km value: indicates high affinity of enzyme for it’s substrate
• ->means reaction going quickly even at low (S)
• High Km: corresponds to lower affinity for substrate

Question 9

How do enzyme inhibitors work? (figure 10)

Answer 9

• many enzymes can be inhibited by binding small ions AND molecules
• many drugs work this way
• ->drug and substrate compete to for active site of enzyme
• ->drug blocks active site of enzyme

Question 10

Outline how enzyme inhibitors may cause toxic effects (figure 11)

Answer 10

• inhibiting compounds block drug metabolsim enzymes

- depending on drug- inhibition can lead to reduced therapeutic effects OR toxic buildup of unmetabolised compounds

Question 11

In what way may enzyme inhibitors have therapeutic effects? (figure 12)

Answer 11

• ACE inhibitors
• ->inhibit formation of angiotension II
• ->and thereby lower blood pressure

Question 12

How does competitive inhibition work? (refer to figure 13)

Answer 12

• competitive inhibitor has structure similar to substrate
• competes with substrate for active site
• has it’s effect reversed by increasing (S)
• active site blocked by inhibitor so substrate conc increases

Question 13

Example of competitive inhibition:

Outline the way in which Disulfiram (Antabuse) acts as a competitive inhibitor
figure 14

Answer 13

• drug which inhibits Aldehyde Oxidase

- causes accumulation of acetaldehyde with subsequent unpleasant side effects of nausea AND vomiting

Question 14

What is Disulfiram used for? (figure 14)

Answer 14

-to support treatment of chronic alcoholism by producing an acute sensitivity (sensitive to details) to ethanol (alcohol)

Question 15

What is non-competitive inhibition? (figure 15)

Answer 15

• non-competitive inhibitors can alter configuration of active site
• ->rendering enzyme inactive so no competition

Question 16

Non-competitive inhibitor example- Nifedipine. How does it work? (figure 16)

Answer 16

• Nifedipine is calcium channel blocker
• ->inhibits Ca2+ dependent ATPase
• prevents Ca uptake into cardiac cells
• ->Ca needed for cardiac muscle excitation
• it’s main uses are to treat angina AND high B. pressure

Question 17

What are Irreversible Inhibitors? (figure 17)

Answer 17

• bind so tightly to enzyme they are unable to remove

- ->often via covalent bond

Question 18

Outline the way in which the following irreversible inhibitors work:

1-Sarin
2-Aspirin

Answer 18

1-binds to serin residue in acetylcholinesterase
–>prevents nerve impulse transmission
2-covalently modifies prostaglandin H2 synthase
–>reducing synthesis of inflammatory signals

Question 19

Describe uncompetitive inhibition (figure 18)

Answer 19

• uncompetitive inhibitor binds to enzyme-substrate complex BUT not to free enzyme
• ->stops enzyme reacting with substrate to form product
• ->also results in decrease in Vmax AND Km
• uncompetitive inhibitor effect is that it can not be overcome by high conc’s of substrate

Question 20

Brief end-product inhibition (figure 19)

Answer 20

• substrate broken down through sequence of enzymes
• ->final substrate formed at final enzyme binds to first enzyme
• ->changes active site shape and so initial substrate no longer able to bind to first enzyme and form E-S complex
• figure 20 shows an example of this process occurring

Question 21

Define Zymogens

Answer 21

-some enzymes initially produced as inactive precursor form

Question 22

How may Zymogens become active forms of enzymes?

refer to figure 21

Answer 22

• zymogens form typically longer polypeptide which must be hydrolysed at specific location to produce active form of enzyme
• cleavage at activation site(s) may release polypeptide known as “pro-sequence” and result in activation of enzyme

Question 23

How may further enzyme activity be regulated?

Answer 23

• through gene expression/ phosphorylation OR proteolysis
• living cells & organisms can’t function without enzyme controlled reactions
• ->greater understanding of enzyme function + reactions they control the better we can use machinery of nature to benefit human needs