Lecture 11: HOW DOES AN ENZYME CATALYSE A REACTION?2

Question 1

What does a progress curve do?

Answer 1

Measure the appearance of product or disappearance of substrate with time at a steady state

Question 2

When is initial reaction velocity measured?

Answer 2

At/near time zero

Question 3

What happens if there is sufficient excess of substrate?

Answer 3

As the amount of enzyme increases, the rate of reaction increases

Question 4

What is initial velocity proportional to?

Answer 4

Enzyme concentration when substrate is in excess

Question 5

How is data usually collected?

Answer 5

With a fixed amount of enzyme and variable amount of substrate

Question 6

What happens to the initial rate as substrate concentration is increased?

Answer 6

At first when there is a low substrate concentration, it increases linearly and then as all the enzyme active sites become occupied, the rate of reaction stops increasing

Question 7

What kinetic parameters can be identified on a velocity vs substrate concentration graph?

Answer 7

maximum velocity and Km

Question 8

What is maximum velocity?

Answer 8

The maximum velocity possibly when substrate concentration is infinity

Question 9

What is Km?

Answer 9

The substrate concentration at which velocity=maximum velocity/2. The Michaelis constant

Question 10

What does a better enzyme have?

Answer 10

A smalle Km

Question 11

What did enzymes evolve?

Answer 11

A Km which matched the substrate concentration in the cell

Question 12

How I binding and catalyses described?

Answer 12

An enzyme, E, converts a single substrate, S, to a single product P that is instantly released.

Question 13

What do we assume is irreversible?

Answer 13

Conversion of products back to the enzyme substrate complex

Question 14

What do the relative speeds of K1 and K-1 do?

Answer 14

Define how tightly substrate binds

Question 15

What is the rate of catalysis?

Answer 15

K2

Question 16

What does K2 relate to?

Answer 16

Energy of activation for the transition state

Question 17

What does the Michaelis-menten equation describe?

Answer 17

The V-obs vs substrate concentration curve

Question 18

What does [ES]/[E] total describe?

Answer 18

The fraction of vmax the enzyme is running at

Question 19

What are the assumptions to do with the Michaelis-menten equation?

Answer 19

Product is not converted back to substrate, Haldane’s steady state assumption, measuring initial rate insures [S] does not change significantly, [S] is much greater than [E] and all ES complexes have the same rate of reaction

Question 20

What is Haldane’s steady state assumption?

Answer 20

The rate of ES formation equals the rate of its breakdown

Question 21

What is necessary for the reaction?

Answer 21

The complex ES

Question 22

What governs the rate of the reaction at any time?

Answer 22

[ES]

Question 23

What does steady state refer to?

Answer 23

Time when [ES] does not change

Question 24

What is [ES]+[E]?

Answer 24

Always constant

Question 25

What kinetics does ES convert to E+P with?

Answer 25

First order

Question 26

If each ES complex has the same chance of going through the transition state...

Answer 26

The ES>>>E+P step will follow first order kinetics

Question 27

What does cellular activity of enzyme activity often do?

Answer 27

Violate Michaelis menten

Question 28

At any given time...

Answer 28

Some enzymes need to turn on and others need to drop out

Question 29

What are the control mechanisms for enzymes?

Answer 29

Enzyme amount, allosteric control, cell location, proteolytic activation and post-translational modification

Question 30

What do allosteric enzymes respond to?

Answer 30

Effectors binding away from the active site

Question 31

What does binding accompany in allosteric enzymes?

Answer 31

A change of shape, which in turn changes enzymatic activity

Question 32

What do allosteric enzymes often have?

Answer 32

Multiple subunits and display cooperative behaviour

Question 33

What do co-operativity and allostery depend on?

Answer 33

The enzyme switching between active and inactive forms

Question 34

Do cooperative enzymes follow Michaelis-menten equation?

Answer 34

No

Question 35

What is the graph of v-obs vs [S] in cooperative enzymes?

Answer 35

Sigmoidal, not hyperbolic

Question 36

What do cooperative enzyme respond to?

Answer 36

More steeply to intermediate changes in[S]

Question 37

Where do cooperative enzymes evolve?

Answer 37

At regulatory points in metabolic pathways

Question 38

What are some allosteric enzymes?

Answer 38

Aspartate transcrabamylase (ATCase) and phosphofructokinase

Question 39

What does phosphofructokinase control?

Answer 39

Glycolysis

Question 40

What is the action of phosphofructokinase?

Answer 40

It phosphorylates fructose-6-phosphate (F6P) to fructose biphosphate

Question 41

When is phosphofructokinase inhibited?

Answer 41

If cell has plenty of ATP and glycolysis is not needed for energy

Question 42

What type of protein is phosphofructokinase?

Answer 42

A homotetramer

Question 43

When is phosphofructokinase cooperative?

Answer 43

When inhibited by ATP or phosphoenolpyruvate (PEP)

Question 44

How does the T state of phosphofructokinase react?

Answer 44

Slowly

Question 45

How does the R state of phosphofructokinase react?

Answer 45

Quickly

Question 46

How is the T state in phosphofructokinase?

Answer 46

More compact, stabilised by PEP, an abundant intermediate of glycolysis

Question 47

What is the R state of phosphofructokinase stabilised by?

Answer 47

Substrate FGP anf ADP

Question 48

What changes when switching between T and R state of phosphofructokinase?

Answer 48

Arginine 162 and glutamic acid 161 swap positions in the active site

Question 49

Where are zymogens secreted from?

Answer 49

The pancreas in inactive form

Question 50

What does cleavage of zymogens by proteases in the gut do?

Answer 50

Produces active enzymes

Question 51

What control are pancreatic digestive enzymes under?

Answer 51

Temporal and spatial