Enzyme Kinetics

Question 1

What does the initial velocity (V₀) of an enzymatic reaction represent?

Answer 1

The rate of substrate consumption or product formation measured at the beginning of the reaction when [S] is constant

Question 2

Why is the initial velocity (V₀) used in enzyme kinetics?

Answer 2

Because substrate concentration has not significantly changed and reverse reactions are minimal

Question 3

What is the steady-state assumption?

Answer 3

The assumption that the concentration of the enzyme-substrate complex [ES] remains constant during the initial phase of the reaction

Question 4

What is the Michaelis–Menten equation?

Answer 4

V₀ = (Vmax × [S]) / (Km + [S])

Question 5

What does Km represent in the Michaelis–Menten model?

Answer 5

[S] at which the reaction velocity is half of Vmax

Question 6

What is Vmax?

Answer 6

The maximum reaction velocity when all enzyme active sites are saturated with substrate

Question 7

How is Vmax related to enzyme concentration?

Answer 7

Vmax = k₂ × [E]total

Question 8

What is the turnover number?

Answer 8

• k or k₂
  ᶜᵃᵗ
• refers to the number of substrate molecules converted to product per enzyme per unit time when fully saturated

Question 9

How is kcat calculated?

Answer 9

kcat = Vmax / [ Eₜₒₜₐₗ ]

Question 10

What kinetic behavior is observed when [S] ≪ Km?

Answer 10

First-order kinetics: velocity is proportional to substrate concentration

Question 11

What kinetic behavior occurs when [S] ≫ Km?

Answer 11

Zero-order kinetics: velocity is independent of substrate concentration

Question 12

What happens to V₀ when [S] = Km?

Answer 12

V₀ = Vmax / 2

Question 13

What is the Lineweaver–Burk equation?

Answer 13

1/V₀ = (Km/Vmax)(1/[S]) + 1/Vmax

Question 14

What does the y-intercept in a Lineweaver–Burk plot represent?

Answer 14

1/Vmax

Question 15

What does the x-intercept in a Lineweaver–Burk plot represent?

Answer 15

-1/Km

Question 16

What type of inhibition occurs when an inhibitor competes with substrate for the active site?

Answer 16

Competitive inhibition

Question 17

How does competitive inhibition affect Km and Vmax?

Answer 17

Increases apparent Km, Vmax remains unchanged

Question 18

What type of inhibition involves inhibitor binding to a site other than the active site?

Answer 18

Non-competitive inhibition

Question 19

How does non-competitive inhibition affect Km and Vmax?

Answer 19

Km may remain unchanged (pure), Vmax decreases

Question 20

What is uncompetitive inhibition?

Answer 20

Inhibitor binds only to the enzyme-substrate complex, reducing both Km and Vmax

Question 21

What is the inhibition constant (Kᵢ)?

Answer 21

Kᵢ = [E][I]/[EI], representing the affinity of inhibitor for the enzyme

Question 22

What does a lower Ki value indicate?

Answer 22

A stronger binding affinity of the inhibitor for the enzyme

Question 23

What are the units of reaction velocity in enzyme kinetics?

Answer 23

Typically mmol·L⁻¹·min⁻¹

Question 24

How does temperature affect enzyme activity?

Answer 24

Activity increases with temperature until a peak, after which denaturation reduces activity

Question 25

What pH range do most enzymes operate optimally in?

Answer 25

Between pH 5 and 9

Question 26

What shape does enzyme activity vs. pH typically display?

Answer 26

A bell-shaped curve

Question 27

What is allosteric regulation?

Answer 27

Regulation of enzyme activity through binding of effectors at sites other than the active site

Question 28

How do allosteric enzymes differ from Michaelis–Menten enzymes?

Answer 28

They often exhibit sigmoidal (S-shaped) rather than hyperbolic kinetics

Question 29

What is cooperativity in enzyme kinetics?

Answer 29

The interaction where substrate binding to one subunit affects the binding affinity of other subunits

Question 30

What kind of enzymes typically exhibit cooperativity?

Answer 30

Multimeric enzymes with quaternary structure

Question 31

What is covalent modification in enzyme regulation?

Answer 31

The addition or removal of chemical groups like phosphates that modulate enzyme activity

Question 32

Which residues are commonly phosphorylated in covalent enzyme regulation?

Answer 32

Serine, threonine, and tyrosine

Question 33

What enzymes mediate phosphorylation and dephosphorylation?

Answer 33

Kinases (phosphorylation) and phosphatases (dephosphorylation)

Question 34

What is proteolytic activation?

Answer 34

Irreversible activation of enzymes by limited proteolysis (cleavage of peptide bonds)

Question 35

What is an example of an enzyme activated by proteolysis?

Answer 35

Caspases and digestive enzymes like trypsin

Question 36

How does substrate concentration affect enzyme velocity?

Answer 36

Velocity increases with [S] until enzyme saturation (Vmax)

Question 37

What is the significance of Km in clinical enzymology?

Answer 37

It helps compare enzyme affinity for different substrates or among isoenzymes

Question 38

What defines the efficiency of an enzyme?

Answer 38

The specificity constant: kcat/Km

Question 39

What does a high kcat/Km value indicate?

Answer 39

High catalytic efficiency of the enzyme

Question 40

How can enzyme inhibitors be used in medicine?

Answer 40

To regulate enzymatic activity in diseases (e.g., ACE inhibitors, statins)

Question 41

What is the impact of enzyme saturation on velocity?

Answer 41

Velocity plateaus at Vmax when all active sites are occupied

Question 42

How do reversible inhibitors differ from irreversible ones?

Answer 42

Reversible inhibitors bind non-covalently and can dissociate; irreversible inhibitors form permanent covalent bonds

Question 43

What is the role of the ES complex in catalysis?

Answer 43

It allows substrate orientation, transition state formation, and catalysis

Question 44

What is the shape of a Michaelis–Menten velocity vs. [S] curve?

Answer 44

Hyperbolic

Question 45

What does a sigmoidal curve in enzyme kinetics suggest?

Answer 45

Cooperative binding or allosteric regulation

Question 46

What is the effect of increasing [S] on competitive inhibition?

Answer 46

High [S] can overcome competitive inhibition

Question 47

How is Vmax affected in competitive inhibition?

Answer 47

It remains unchanged

Question 48

How does non-competitive inhibition affect the ES complex?

Answer 48

It can bind to both free enzyme and ES complex, affecting turnover without changing Km (in pure cases)

Question 49

What is Kₘ and what are its units?

Answer 49

The amount of substrate required to obtain the semi-maximum velocity of an enzyme, given in units of concentration