Catalytic Efficiency (kcat /Km )
Key Concepts
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Relationship: $\mathbf{k_{cat} / K_m}$ combines catalytic turnover ($k_{cat}$) with substrate affinity ($K_m$) to give a holistic measure of efficiency.
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Physiological Relevance: It is particularly relevant at low (non-saturating) substrate concentrations, reflecting the enzyme's performance under typical cellular conditions.
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Units & Interpretation: A second-order rate constant (M⁻¹s⁻¹), indicating how rapidly an enzyme can find and convert substrate. Higher values mean greater efficiency and specificity.
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"Perfect" Enzymes: Enzymes with $k_{cat} / K_m$ near the diffusion limit ($10^8$ - $10^9$ M⁻¹s⁻¹) are considered optimally efficient.
Examples & Applications
Comparing Enzymes for a Substrate:
If Enzyme A has a $k_{cat} / K_m$ of $10^6 \text{ M}^{-1}\text{s}^{-1}$ for substrate X, and Enzyme B has $10^5 \text{ M}^{-1}\text{s}^{-1}$ for the same substrate, Enzyme A is 10 times more catalytically efficient for substrate X.
Comparing Substrates for an Enzyme:
An enzyme might act on two different substrates. If $k_{cat} / K_m$ for substrate A is $5 \times 10^7 \text{ M}^{-1}\text{s}^{-1}$ and for substrate B is $5 \times 10^5 \text{ M}^{-1}\text{s}^{-1}$, the enzyme shows a much higher specificity and efficiency for substrate A.
"Catalytically Perfect" Enzyme Example:
Carbonic anhydrase catalyzes the hydration of CO$2$ with a $k{cat} / K_m$ value around $8.3 \times 10^7 \text{ M}^{-1}\text{s}^{-1}$, which is very close to the diffusion-controlled limit. This means that as soon as CO$_2$ molecules diffuse to the active site, they are almost immediately converted to bicarbonate.
Drug Design Application:
If a drug aims to inhibit a specific enzyme, researchers would look for inhibitors that significantly decrease the enzyme's $k_{cat} / K_m$ for its natural substrate, effectively reducing its catalytic efficiency in vivo.
Memory Aids
Interactive tools to help you remember key concepts
Memory Tools
Think of it as "How good is the enzyme overall?" It's a comprehensive report card.
Memory Tools
"Low Substrate Conditions": This ratio tells you how well the enzyme works when there's not a lot of food (substrate) around, which is typical in cells.
Memory Tools
"Mighty Second": M⁻¹s⁻¹** helps remember it's a second-order rate constant, showing efficiency per amount of substrate and per time.
Memory Tools
Diffusion Limit": When an enzyme's $k_{cat} / K_m$ is super high, it means the enzyme is so incredibly fast that the only thing slowing the reaction down is literally how quickly the substrate can diffuse to the enzyme's active site.
Flash Cards
Glossary
- Enzyme Kinetics
The study of the rates of enzyme-catalyzed reactions and the factors affecting them.