Catalytic Efficiency Definition
Catalytic efficiency measures how effectively an enzyme converts substrate to product. For enzymes following Michaelis-Menten kinetics, this efficiency is determined by combining the enzyme's turnover rate and its affinity for the substrate.
Key Kinetic Parameters
- $k_{Cat}$ (Turnover Number): Represents the maximum rate of catalysis per enzyme active site. It indicates how many substrate molecules one enzyme molecule can convert into product per unit time.
- $K_M$ (Michaelis Constant): Represents the substrate concentration at which the reaction rate is half of $V_{max}$. It often reflects the enzyme's affinity for its substrate; a lower $K_M$ suggests higher affinity.
Calculating Catalytic Efficiency
Catalytic efficiency is defined as the ratio of $k_{Cat}$ to $K_M$. This measure combines how fast the enzyme works ($k_{Cat}$) with the substrate concentration it requires to function effectively ($K_M$).
Formula: Catalytic Efficiency $\boldsymbol{=} \frac{k_{Cat}}{K_M}$
Significance of the Ratio
A higher value of $\frac{k_{Cat}}{K_M}$ indicates greater catalytic efficiency. This means the enzyme has a high turnover rate (large $k_{Cat}$) and/or binds strongly to its substrate (small $K_M$), allowing it to efficiently process the substrate even at low concentrations.
Therefore, the catalytic efficiency of an enzyme following Michaelis-Menten kinetics is given by the term $\frac{k_{Cat}}{K_M}$.
Important Questions from Enzyme Kinetics and Michaelis Menten Equation
You are characterizing a new enzyme isolated and purified in the laboratory. If the maximum velocity of the enzyme is $1800 \text{ } \mu moles \text{ L}^{-1} \text{min}^{-1}$ and the total concentration of the enzyme in the reaction mixture is $1.5 \mu \text{M}$, then the turnover number of the enzyme is _______ $\text{s}^{-1}$. (answer in integer)
You have purified an enzyme using a series of chromatographic methods. It was observed that a $10 \mu \text{ g mL}^{-1}$ of this purified enzyme converted $10 \text{ mM}$ substrate per hour at 25$^{\circ}$C and pH 7. Its specific activity is _______ $\text{IU } \mu\text{g}^{-1}$. (rounded off to three decimal places)
Within the Michaelis-Menten framework, the ratio of $v_0/V_{max}$
when $[S] = 20 \times K_m$ is _________.
(Round off to two decimal places)
The activity of lactate dehydrogenase can be measured by monitoring the following reaction:
Pyruvate + NADH $ \longrightarrow $ Lactate + $NAD^+$
The molar extinction coefficient of NADH at 340 nm is $6220 \ M^{-1}.cm^{-1}$. $NAD^+$ does not absorb at this wavelength. In an assay, $25 \ \mu L$ of a sample of enzyme (containing $5 \ \mu g$ protein per mL) was added to a mixture of pyruvate and NADH to give a total volume of 3 mL in a cuvette of 1 cm pathlength. The rate of decrease in absorbance at 340 nm was $0.14 \ min^{-1}$. The specific activity of the enzyme will be ____________________ $ \mu mol.min^{-1}.mg^{-1}$.
- An enzyme (E) catalyzes the biochemical reaction $A \rightarrow B$ with $k_{cat}$ equal to $500 s^{-1}$. If the initial reaction velocity ($V_0$) is $10 \mu M.s^{-1}$ at the total enzyme concentration $[E_t]$ of 30 nM and substrate concentration $[A]$ of $40 \mu M$, the value of $K_m$ (in $\mu M$) is ________
