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Cell: Structure and Function - Enzymes (Types, Properties, Enzyme Action)

Grade 11ICSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Enzymes are proteinaceous biocatalysts (except ribozymes made of RNARNA) that accelerate the rate of metabolic reactions by lowering the activation energy (EaE_a).

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The 'Lock and Key' hypothesis suggests a rigid active site, while the 'Induced Fit' hypothesis proposes that the enzyme's active site changes shape slightly to fit the substrate (SS).

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Michaelis-Menten Constant (KmK_m): Defined as the substrate concentration at which the reaction velocity is half of its maximum velocity (VmaxV_{max}). It is inversely proportional to the enzyme's affinity for the substrate.

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Factors affecting enzyme activity include temperature, pHpH, and substrate concentration [S][S]. Most enzymes have an optimum temperature (usually 37∘C37^{\circ}C in humans) and pHpH.

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Enzymes are classified into six classes by the IUBMB: 1. Oxidoreductases, 2. Transferases, 3. Hydrolases, 4. Lyases, 5. Isomerases, and 6. Ligases.

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A Holoenzyme is a complete, catalytically active enzyme consisting of a protein part (Apoenzyme) and a non-protein part (Co-factor).

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Competitive Inhibition occurs when an inhibitor molecule closely resembles the substrate and competes for the active site, increasing the KmK_m but leaving VmaxV_{max} unchanged.

📐Formulae

E+S⇌ES→E+PE + S \rightleftharpoons ES \rightarrow E + P

v=Vmax[S]Km+[S]v = \frac{V_{max} [S]}{K_m + [S]}

Holoenzyme=Apoenzyme+Co−factorHoloenzyme = Apoenzyme + Co-factor

Q10=Rate at (t+10)∘CRate at t∘C≈2Q_{10} = \frac{\text{Rate at } (t + 10)^{\circ}C}{\text{Rate at } t^{\circ}C} \approx 2

ΔGcatalyzed‡<ΔGuncatalyzed‡\Delta G^{\ddagger}_{catalyzed} < \Delta G^{\ddagger}_{uncatalyzed}

💡Examples

Problem 1:

If an enzyme-catalyzed reaction reaches its maximum velocity VmaxV_{max} at a substrate concentration of 10 mmol/L10 \text{ mmol/L}, what is the KmK_m if the velocity is 12Vmax\frac{1}{2} V_{max} at 2 mmol/L2 \text{ mmol/L}?

Solution:

The KmK_m is 2 mmol/L2 \text{ mmol/L}.

Explanation:

By definition, the Michaelis-Menten constant (KmK_m) is the substrate concentration [S][S] at which the reaction rate is exactly half of the maximum velocity (V=12VmaxV = \frac{1}{2} V_{max}). Since the velocity is 12Vmax\frac{1}{2} V_{max} when [S]=2 mmol/L[S] = 2 \text{ mmol/L}, then Km=2 mmol/LK_m = 2 \text{ mmol/L}.

Problem 2:

Describe the chemical reaction involving Carbonic Anhydrase in the blood.

Solution:

CO2+H2O→Carbonic AnhydraseH2CO3CO_2 + H_2O \xrightarrow{\text{Carbonic Anhydrase}} H_2CO_3

Explanation:

Carbonic anhydrase is one of the fastest enzymes known, increasing the reaction rate of carbon dioxide (CO2CO_2) and water (H2OH_2O) to form carbonic acid (H2CO3H_2CO_3) by about 10710^7 times compared to the uncatalyzed reaction.

Problem 3:

Identify the components of the enzyme system for Succinate Dehydrogenase and Malonate.

Solution:

Substrate = Succinate; Inhibitor = Malonate.

Explanation:

Malonate acts as a competitive inhibitor for Succinate Dehydrogenase because it structurally resembles the substrate Succinate. This competition for the active site can be overcome by increasing the concentration of Succinate.