# Enzyme Technology Questions and Answers – Kinetics of Enzyme Catalysed Reaction – 1

This set of Enzyme Technology Multiple Choice Questions & Answers (MCQs) focuses on “Kinetics of Enzyme Catalysed Reaction – 1”.

1. Which of the following assumption was not made in rapid equilibrium model?
a) Flexible nature of enzymes
b) [S] >> [E]
c) [E]+[S]$$\rightleftharpoons$$[ES]
d) It is the initial substrate concentration [So] that determines efficacy of an enzyme to mediate a reaction

Explanation: The flexible nature of enzymes was not dealt by lock and key model, but the induced fit model. This model dealt with change in shape after enzyme substrate interaction. The following assumptions were made in rapid equilibrium model:
* The substrate concentration will always be higher than enzyme concentration. [E] >> [S]
* An equilibrium state is assumed between reactants and products. [E]+[S]$$\rightleftharpoons$$[ES]
* It is the initial substrate concentration [So] that determines the efficacy of an enzyme to mediate a reaction.

2. Which of these is referred to as Kcat?
a) Miachelis Menten constant
b) Catalytic efficiency
c) Substrate concentration
d) Turn over number

Explanation: Kcat is referred to as turn over number which denotes the number of catalytic turn over events that happens per unit time. Catalytic efficiency is given by Kcat/Km. Miachelis Menten constant is given by Km. Substrate concentration is the amount substrate used in an enzyme catalyzed reaction.

3. What does the following equation represent?
$$\frac{1}{V_0} = \frac{K_m}{V_{max}} \frac{1}{[S]}+\frac{1}{V_{max}}$$
b) Lineweaver Burk equation
c) Miachelis Menten equation
d) Hanes plot equation

Explanation: The Miachelis Menten equation is proved to be unsatisfactory for the following reasons:
The graph proposed is not following a proper mathematical model.
The graph is following a tangent model and hence it is difficult to calculate an exact Km and Vmax.
Hence Lineweaver and Burk came up with a suitable solution to solve this problem. The equation
$$\frac{1}{V_0} = \frac{K_m}{V_{max}} \frac{1}{[S]}+\frac{1}{V_{max}}$$ represents Lineweaver Burk equation where,
V0 = initial velocity, Vmax = maximum rate, Km = Miachelis Menten constant and [S] = substrate concentration.

4. Which of the following is based on rearrangement of Lineweaver Burk plot equation by multiplication of [So] factor?
a) Hanes plot equation
c) Miachelis Menten equation
d) Eisenthal and Cornish and Bowden plot equation

Explanation: Hanes plot equation is based on rearrangement of the Lineweaver Burk plot equation by multiplication of [S0] factor and is given by $$\frac{[S_0]}{V_0} = \frac{1}{V_{max}} [S_0]+\frac{K_m}{V_{max}}$$
where,
V0 = initial velocity, Vmax = maximum rate, Km = Miachelis Menten constant and [S] = substrate concentration.
The graphical representation of [S0]/V0 versus [S0] is called the Hanes plot. One of the drawbacks of this approach is that neither the ordinate nor the abscissa represents independent variable, both are dependent on substrate concentration.

5. Miachelis Menten model is also referred to as “rapid equilibrium model”.
a) True
b) False

Explanation: The main difference between Henri and Miachelis Menten model is that Miachelis and Menten did a strong experimental work to prove that any enzyme catalyzing single substrate reaction will possess a hyperbolic curve. The unique feature of this model is assumption of rapid equilibrium between reactants and ES complex, where the ES complex will slowly decompose to form free enzyme and products. Hence it is appropriate to term this model as “rapid equilibrium model”. Hence the above statement is true.

6. What does the following plot represent?

a) Miachelis Menten plot
b) Hanes plot
c) Lineweaver Burk plot

Explanation: Miachelis Menten equation is given by $$V_0 = \frac{V_{max}[S]}{K_m+[S]}$$, where
V0 = initial velocity, Vmax = maximum rate, Km = Miachelis Menten constant and [S] = substrate concentration.
According to this equation, Vo versus [S] plot giving a hyperbolic curve is referred to as Miachelis Menten plot. The tangent to the curve at the origin goes through the point (V0 = Vmax), ([S] = Km). The substrate concentration, which gives a rate of half the maximum reaction velocity, is equal to the Km.

7. What does the following graph represent?

a) Miachelis Menten plot
b) Lineweaver Burk plot
d) Hanes plot

Explanation: Hanes plot is a graphical representation of [S0]/V0 is plotted against [S0] which is based on the equation, $$\frac{[S_0]}{V_0} = \frac{1}{V_{max}} [S_0]+\frac{K_m}{V_{max}}$$, where
V0 = initial velocity, Vmax = maximum rate, Km = Miachelis Menten constant and [S] = substrate concentration.
This plot yields a straight line of slope 1/Vmax, a y-intercept of Km/Vmax and an x-intercept of -Km.

8. ___________ is defined as the enzyme’s binding efficiency with its appropriate substrate through optimized multiple non-covalent interactions.
a) Km
b) Kcat
c) Catalytic efficiency
d) Vmax

Explanation: Km is defined as the enzyme’s binding efficiency with its appropriate substrate through optimized multiple non-covalent interactions. Km is also referred to as the substrate concentration at which the reaction occurs at half of the maximum rate. Kcat is referred to as turnover number that denotes the number of catalytic turn over events that happen per unit time. Catalytic efficiency is referred to Kcat/Km. The high value of this indicates collision frequencies between enzyme and substrate molecule and low value indicates the equilibrium assumption status. Vmax is the maximum rate of the reaction.

9. 4*107: Catalase :: 25: ___________
a) Protease
b) Amylase
c) Glucose isomerase
d) Creatinine kinase

Explanation: 4*107 is the value of catalytic efficiency of catalase. Catalytic efficiency is referred to as Kcat/Km. High value indicates that the collision frequencies between enzyme and substrate molecule and low value indicates that the equilibrium assumption status. As 4*107 M-1s-1 is the value of catalytic varies from less than 1 M-1s-1 to greater than 108 M-1s-1.

10. What is the ratio of V0/Vmax when [S] = 10Km?
a) 1/2
b) 10/11
c) 4/5
d) 5/6

Explanation: Miachelis Menten equation is given by $$V_0=\frac{V_{max} [S]}{K_m+[S]}$$
Rearranging, $$\frac{V_0}{V_{max}} = \frac{[S]}{[S]+K_m}$$
$$\frac{V_0}{V_{max}} = \frac{10K_m}{10K_m+K_m}$$
$$\frac{V_0}{V_{max}} = \frac{10 K_m}{11 K_m}$$
$$\frac{V_0}{V_{max}} = \frac{10}{11}$$

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