Unit 2 - Enzymes

Question 1

what are enzyme commission numbers?

Answer 1

classification of enzymes based on reaction catalysed - 4 numbers
1st - general type of reaction catalysed
2nd - sub-class/ 3rd - sub-subclass - highlights compounds/groups/ bonds/ product involved in reaction
4th - serial identifier - highlights metabolites/ co-factors involved

Question 2

apart from EC numbers what other names do enzymes have?

Answer 2

recommended name (everyday use)/ systematic name (substrate followed by enzyme specifying type of reaction catalysed

Question 3

what are the defined conditions under which specified activity of catalytic proteins are measured?

Answer 3

temp/ pH/ buffer/ substrate/ co-enzyme concentration

Question 4

what is the rate of an enzymatic reaction?

Answer 4

rate of conversion of substrate to product per unit of time

Question 5

what does 1 IU equal?

Answer 5

amount of enzyme that catalyses conversion of 1 micromole of substrate to product per minute

Question 6

what does 1 katal equal?

Answer 6

amount of enzyme that catalyses conversion of 1 mole of substrate to product per second

Question 7

what unit given for specific activity?

Answer 7

IU/ mg

Question 8

what is difference between holoenzyme and apoenzyme?

Answer 8

holoenzyme has non-/covalently bound enzyme but apoenzyme does not

Question 9

distinguish between soluble co-enzymes/ prosthetic groups/ cofactors

Answer 9

soluble: bind reversibly to protein moiety of enzyme but during reaction they are modified, dissociated and recycled by another enzyme
prosthetic groups: covalently linked to enzyme and remain associated during entire catalytic cycle
co-factors: insoluble inorganic ions required for enzymatic activity

Question 10

what are the assumptions for the michael-menten equation?

Answer 10

• E, S, ES in rapid equilibrium
• conversion of ES to E + P - rate-limiting step
• there are no other forms of the enzyme present apart from E and ES

Question 11

what is Kcat?

Answer 11

turnover number - rate constant showing the number of substrate molecules converted to product per enzyme molecule per unit time

Question 12

what is velocity limited by? - give an equation

Answer 12

limited by ratio of enzyme substrate complexes to total number of enzymes
equation: v= Kcat [ES]

Question 13

what is Km

Answer 13

kinetic constant showing substrate affinity

- on graph it is point corresponding to 1/2 Vmax ([ES] = 1/2 [E]t)

Question 14

what is the relationship of lineweaver-burk plot to michael-menten equation? what is the significance of this?

Answer 14

double reciprocal plot of the menten equation so puts the equation in form y = mx + c
-this makes it easier to obtain values for Vmax and K

Question 15

what is represented by the following on a lineweaver plot:
x
y
x- intercept
y- intercept
slope

Answer 15

x= 1/ [S]
y = 1/ v
x-intercept: -1/ Km
y-intercept: 1/ Vmax
slope: Km/ Vmax

Question 16

what are the limitations of the lineweaver-burk plot?

Answer 16

small experimental error at low Km so results in larger error for graphically determining Km and Vmax
- data at higher substrates are concentrated in narrow region

Question 17

what is the other transformation of michael-menten equation? what is represented by:
x
y
x- intercept
y- intercept
slope

Answer 17

eadie-hofstee plot - no data compression
x = v/ [s]
y = v
x-intercept: Vmax/Km
y-intercept: Vmax
slope: - Km

Question 18

how do competitive inhibitors affect an enzyme?

Answer 18

increase in Km but no change in V max

Question 19

how do non-competitive inhibitors affect an enzyme?

Answer 19

decrease in V-max and no change in Km

Question 20

how do uncompetitive inhibitors affect an enzyme?

Answer 20

may alter both Km and V max