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bioc192 > Lecture 11: HOW DOES AN ENZYME CATALYSE A REACTION?2 > Flashcards

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

1
Q

What does a progress curve do?

A

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

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2
Q

When is initial reaction velocity measured?

A

At/near time zero

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3
Q

What happens if there is sufficient excess of substrate?

A

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

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4
Q

What is initial velocity proportional to?

A

Enzyme concentration when substrate is in excess

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5
Q

How is data usually collected?

A

With a fixed amount of enzyme and variable amount of substrate

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6
Q

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

A

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

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7
Q

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

A

maximum velocity and Km

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8
Q

What is maximum velocity?

A

The maximum velocity possibly when substrate concentration is infinity

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9
Q

What is Km?

A

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

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10
Q

What does a better enzyme have?

A

A smalle Km

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11
Q

What did enzymes evolve?

A

A Km which matched the substrate concentration in the cell

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12
Q

How I binding and catalyses described?

A

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

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13
Q

What do we assume is irreversible?

A

Conversion of products back to the enzyme substrate complex

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14
Q

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

A

Define how tightly substrate binds

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15
Q

What is the rate of catalysis?

A

K2

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16
Q

What does K2 relate to?

A

Energy of activation for the transition state

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17
Q

What does the Michaelis-menten equation describe?

A

The V-obs vs substrate concentration curve

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18
Q

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

A

The fraction of vmax the enzyme is running at

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19
Q

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

A

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

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20
Q

What is Haldane’s steady state assumption?

A

The rate of ES formation equals the rate of its breakdown

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21
Q

What is necessary for the reaction?

A

The complex ES

22
Q

What governs the rate of the reaction at any time?

A

[ES]

23
Q

What does steady state refer to?

A

Time when [ES] does not change

24
Q

What is [ES]+[E]?

A

Always constant

25
Q

What kinetics does ES convert to E+P with?

A

First order

26
Q

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

A

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

27
Q

What does cellular activity of enzyme activity often do?

A

Violate Michaelis menten

28
Q

At any given time…

A

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

29
Q

What are the control mechanisms for enzymes?

A

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

30
Q

What do allosteric enzymes respond to?

A

Effectors binding away from the active site

31
Q

What does binding accompany in allosteric enzymes?

A

A change of shape, which in turn changes enzymatic activity

32
Q

What do allosteric enzymes often have?

A

Multiple subunits and display cooperative behaviour

33
Q

What do co-operativity and allostery depend on?

A

The enzyme switching between active and inactive forms

34
Q

Do cooperative enzymes follow Michaelis-menten equation?

A

No

35
Q

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

A

Sigmoidal, not hyperbolic

36
Q

What do cooperative enzyme respond to?

A

More steeply to intermediate changes in[S]

37
Q

Where do cooperative enzymes evolve?

A

At regulatory points in metabolic pathways

38
Q

What are some allosteric enzymes?

A

Aspartate transcrabamylase (ATCase) and phosphofructokinase

39
Q

What does phosphofructokinase control?

A

Glycolysis

40
Q

What is the action of phosphofructokinase?

A

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

41
Q

When is phosphofructokinase inhibited?

A

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

42
Q

What type of protein is phosphofructokinase?

A

A homotetramer

43
Q

When is phosphofructokinase cooperative?

A

When inhibited by ATP or phosphoenolpyruvate (PEP)

44
Q

How does the T state of phosphofructokinase react?

A

Slowly

45
Q

How does the R state of phosphofructokinase react?

A

Quickly

46
Q

How is the T state in phosphofructokinase?

A

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

47
Q

What is the R state of phosphofructokinase stabilised by?

A

Substrate FGP anf ADP

48
Q

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

A

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

49
Q

Where are zymogens secreted from?

A

The pancreas in inactive form

50
Q

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

A

Produces active enzymes

51
Q

What control are pancreatic digestive enzymes under?

A

Temporal and spatial

bioc192 (38 decks)

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