4 - Enzyme kinetics

Question 1

what are the facts you need to know about enzymes?

Answer 1

1. they’re catalysts
2. they’re specific
3. they control well defined chemical reactions (they can be used in lab to investigate & develop a wide variety of diagnostic kits & therepeutic drugs)

Question 2

what effect to enzymes can have major implications for disease?

Answer 2

small changes in their abundance, efficiency or distribution in tissue

Question 3

what can enzymes catalytic behaviour be used for?

Answer 3

to diagnose disease (e.g. isoforms)

Question 4

what are Vmax and Km used to describe?

Answer 4

they’re parameters to help describe how concentration of a substrate affects rate of catalysed reaction

Question 5

what is Km?

Answer 5

equivalent to the substrate concentration where the initial reaction rate is half-maximal
(concentration of substrate which permits the enzyme to achieve half Vmax)

Question 6

what is the point of max energy & least stability?

Answer 6

enzyme substrate complex at activation energy barrier -> point at which reaction could go either way (equillibrium)

Question 7

what is Michaelis-Menten model used to explain?

Answer 7

explains the relationship between Vmax and Km & describes the rate of catalysis as a function of substrate concentration

Question 8

how do you calculate Km?

Answer 8

k-1 + k2 / k1

Question 9

is k-1?

Answer 9

backwards rate constant for enzyme dissociation from the substrate

Question 10

what is k1?

Answer 10

the forward rate constant for enzyme association with the substrate.

Question 11

what is k2?

Answer 11

the forward rate constant of enzyme conversion of substrate to product (P)

Question 12

how are Vmax and Km measured?

Answer 12

-by measuring initial reaction velocity at V0 (at max - known - substratecapacity)
-then repeat at increasing substrate concentrations

Question 13

what is Vmax?

Answer 13

a theoretical maximum - at infinite substrate concentration the initial reaction rate approaches Vmax

Question 14

how can you accurately determine Vmax and Km?

Answer 14

by turning equation of V= Vmax[S] / Km +[S] into straight line equation y=mx+c

Question 15

where is Vmax on Lineweaver Burk plot?

Answer 15

intersection of straight line with y - axis

Question 16

where is Km on liineweaver Burk plot?

Answer 16

at intersection of straight line with x-axis

Question 17

what do you use Vmax and Km for? (in simple terms)

Answer 17

to determine how good an enzyme is at doing it’s job (how fast it works)

Question 18

what does a low Km value mean?

Answer 18

that enzyme only needs a little substrate to work at 1/2 max (so it’s efficient)

Question 19

what does a high Km value mean?

Answer 19

that enzyme needs a lot of substrate to work at 1/2 max velocity (so less efficient)

Question 20

do 2 different enzymes that have same Vmax always have same Km too?

Answer 20

NO - they could have same Vmax and different Km

Question 21

how can you regulate different enzymes?

Answer 21

by activating different isozymes

Question 22

what effect does competitive inhibiton have on Vmax and Km?

Answer 22

-Vmax does not change (as infinite substrate so substrates outcompete competitive inhibitors)
-Km varies (as depends on specific enzyme since substrate con 1/2 of max doesn’t necessarily outcompete number of competitive inhibitors, but it could)

Question 23

what is effect on Vmax and Km in non-competitive inhibiton?

Answer 23

-Vmax varies (as inhibitor binds to different site so reaction rate not affected by substrate concentration)
-Km does not change (substrate not converted, not really affects)

Question 24

what is feedback inhibition?

Answer 24

inhibition of rate limiting enzymes by end products - common mechanism of allosteric control

Question 25

do allosteric enzymes follow Michaelis- Menten model?

Answer 25

No

Question 26

what does increasing substrate concentration of allosteric enzymes result in on graph?

Answer 26

sigmoidal curve

Question 27

what do allosteric factors affect enzymes?

Answer 27

modulate enzyme kinetic behaviour

Question 28

what does lineweaver-Burk plot do?

Answer 28

enables accurate determination of Km and Vmax

Question 29

what is orthosteric inhibition?

Answer 29

binding at same site (competitive)

Question 30

how can allosteric enzymes be controlled?

Answer 30

by allosteric inhibitors and activators

Question 31

competitive&non-competitive inhibitors are mostly reversible - but when are they irreversible?

Answer 31

some non competitive irreversible when involve formation or breakage of covalent bonds in enzyme complex