steady state kinetics

Question 1

why is rate of a reaction proportional to [S] at low [S]?

Answer 1

equilibrium is pushed towards formation of more ES

Question 2

when is Vmax observed?

Answer 2

when virtually all enzyme is present as ES, and [E] is very small

Question 3

what is the steady state assumption?

Answer 3

rate of breakdown of ES = rate of formation of ES

Question 4

why are studies of enzyme kinetics useful?

Answer 4

• provide evidence for intermediates in enzyme catalysed reactions
• suggest how modulators work and provide clues about enzyme structure
• suggest points of metabolic regulation

Question 5

what does Km represent?

Answer 5

measure of affinity of an enzyme for its substrate

Question 6

what does Kcat represent?

Answer 6

the rate limiting step of an enzyme catalysed reaction

Question 7

define turnover number

Answer 7

the number of substrate molecules converted to product in a given unit of time on a single enzyme molecule when the enzyme is saturated with substrate

Question 8

what is ideal for an efficient enzyme?

Answer 8

large Vmax, large K2, small Km

Question 9

what does K2&raquo_space; K-1 indicate?

Answer 9

reaction is going forwards, many productive collisions

Question 10

what is the upper limit for efficiency of an enzyme?

Answer 10

diffusion

Question 11

what is the best way to analyse enzyme efficiency?

Answer 11

compare kcat/km ratio

Question 12

give an example of an efficient enzyme

Answer 12

acetylcholinesterase - has a relatively small Km but very large kcat. 1 in 10 collisions are productive

Question 13

give an example of why efficiency may not be a priority for an enzyme

Answer 13

where specificity is more important than efficiency, e.g. clotting cascade or complement proteases

Question 14

what is the role of physical factors?

Answer 14

stabilise transition state

Question 15

what is the role of chemical factors?

Answer 15

provide chemical groups to facilitate the reaction; covalent intermediaries, charge stabilisation, metal ions and coenzymes

Question 16

how do enzymes lower activation energy?

Answer 16

stabilise transition state

Question 17

why do enzymes not bind tightly to substrate?

Answer 17

as the enzyme binds the substrate, its stabilised. this causes a decrease in free energy, and increases the energy barrier

Question 18

what kind of compound does bind tightly to the substrate?

Answer 18

inhibitor

Question 19

how do serine proteases use transition state theory?

Answer 19

the substrate is planar, the transition state is tetrahedral. the enzyme is configured to recognise the transition state