Enzyme Kinetics and Regulation — Revision Notes

⚡ 30-Second Revision

Michaelis-Menten Equation — $V_0 = \frac{V_{max}[S]}{K_m + [S]}$\n- **$K_m$**: Substrate concentration at $0.5 \times V_{max}$. Lower $K_m$ = higher apparent affinity.\n- **$V_{max}$: Maximum reaction velocity at saturating [S]. Proportional to enzyme concentration.\n- Competitive Inhibition**: Inhibitor binds active site. $\uparrow K_m$, $V_{max}$ unchanged. Lineweaver-Burk: lines intersect on y-axis.\n- Non-competitive Inhibition: Inhibitor binds allosteric site. $\downarrow V_{max}$, $K_m$ unchanged (pure). Lineweaver-Burk: lines intersect left of y-axis.\n- Uncompetitive Inhibition: Inhibitor binds ES complex. $\downarrow K_m$, $\downarrow V_{max}$ proportionally. Lineweaver-Burk: parallel lines.\n- Allosteric Regulation: Effectors bind allosteric sites, causing conformational change. Sigmoidal kinetics, cooperative binding.\n- Feedback Inhibition: End-product inhibits early enzyme in pathway.\n- Covalent Modification: Phosphorylation/dephosphorylation to activate/inactivate.

2-Minute Revision

Enzyme kinetics studies reaction rates, governed by the Michaelis-Menten equation, which defines $V_{max}$ (maximum velocity) and $K_m$ (substrate concentration at half $V_{max}$, indicating apparent affinity).

Factors like temperature and pH significantly influence activity, with optimal ranges and denaturation occurring at extremes. Enzyme inhibitors reduce reaction rates. Competitive inhibitors bind to the active site, increasing apparent $K_m$ but leaving $V_{max}$ unchanged, and can be overcome by high substrate concentrations.

Non-competitive inhibitors bind to allosteric sites, decreasing $V_{max}$ but not affecting $K_m$ (for pure non-competitive). Uncompetitive inhibitors bind only to the ES complex, proportionally decreasing both $K_m$ and $V_{max}$.

These inhibitions are visually distinct on Lineweaver-Burk plots. Enzyme regulation is vital for metabolic control. Allosteric regulation involves effectors binding to non-active sites, leading to conformational changes and often sigmoidal kinetics.

Feedback inhibition is a common allosteric mechanism where an end-product inhibits an early enzyme. Covalent modification, like phosphorylation, reversibly switches enzyme activity. Zymogen activation involves proteolytic cleavage of inactive precursors.

5-Minute Revision

Enzyme kinetics is the quantitative analysis of enzyme-catalyzed reaction rates. The Michaelis-Menten model is fundamental, describing how initial velocity ($V_0$) varies with substrate concentration ($[S]$).

Key parameters are $V_{max}$, the maximum velocity when the enzyme is saturated, and $K_m$, the substrate concentration at which $V_0 = 0.5 \times V_{max}$. A low $K_m$ generally indicates a high apparent affinity of the enzyme for its substrate.

The Lineweaver-Burk plot (double reciprocal plot) linearizes the Michaelis-Menten equation, allowing for easier determination of $K_m$ and $V_{max}$ and differentiation of inhibition types.\n\nFactors affecting enzyme activity include substrate concentration (following Michaelis-Menten kinetics), enzyme concentration (directly proportional to $V_0$), temperature (optimal range, denaturation at high temperatures), and pH (optimal range, affecting active site ionization).

\n\nEnzyme inhibition can be reversible or irreversible. Reversible inhibition types are:\n1. Competitive: Inhibitor resembles substrate, binds active site. $\uparrow K_m$, $V_{max}$ unchanged.

Lineweaver-Burk: lines intersect on y-axis. Overcome by $\uparrow [S]$.\n2. Non-competitive: Inhibitor binds allosteric site. $\downarrow V_{max}$, $K_m$ unchanged (pure non-competitive). Lineweaver-Burk: lines intersect left of y-axis.

\n3. Uncompetitive: Inhibitor binds only to ES complex. $\downarrow K_m$, $\downarrow V_{max}$ proportionally. Lineweaver-Burk: parallel lines.\n\nEnzyme regulation ensures metabolic control:\n1.

Allosteric Regulation: Effectors bind to allosteric sites, causing conformational changes that alter active site activity. Allosteric enzymes often show sigmoidal kinetics and cooperative binding.

\n2. Feedback Inhibition: The end-product of a pathway inhibits an enzyme early in the same pathway, preventing overproduction.\n3. Covalent Modification: Reversible attachment/removal of chemical groups (e.

g., phosphorylation by kinases, dephosphorylation by phosphatases) to switch enzyme activity.\n4. Zymogen Activation: Inactive precursors (zymogens) are activated by proteolytic cleavage (e.g., digestive enzymes).

\n5. Isozymes: Different forms of an enzyme with varying kinetic properties, expressed in different tissues.\n\nUnderstanding these concepts is crucial for interpreting experimental data and predicting enzyme behavior in biological systems.

Prelims Revision Notes

1
Enzyme Kinetics Basics:\n * Enzymes are biological catalysts, lowering activation energy.\n * Michaelis-Menten Equation: $V_0 = \frac{V_{max}[S]}{K_m + [S]}$.\n * **$V_{max}$:** Maximum velocity, achieved at substrate saturation. Proportional to total enzyme concentration ($V_{max} = k_{cat}[E]_T$).\n * **$K_m$:** Michaelis constant, substrate concentration at $0.5 \times V_{max}$. Lower $K_m$ implies higher apparent affinity for substrate.\n * **Turnover Number ($k_{cat}$):** Number of substrate molecules converted to product per enzyme active site per unit time at saturation.\n * Catalytic Efficiency: $k_{cat}/K_m$, a measure of how efficiently an enzyme converts substrate to product.\n2. Factors Affecting Enzyme Activity:\n * Substrate Concentration: Increases $V_0$ until $V_{max}$ is reached.\n * Enzyme Concentration: Directly proportional to $V_0$ (assuming non-limiting substrate).\n * Temperature: Optimal temperature for maximal activity. High temperatures cause denaturation (loss of 3D structure and activity).\n * pH: Optimal pH for maximal activity. Deviations alter ionization states of active site residues, affecting binding/catalysis.\n3. Enzyme Inhibition (Reversible):\n * Competitive:\n * Inhibitor (I) resembles substrate (S), binds active site.\n * Effect: $\uparrow$ apparent $K_m$, $V_{max}$ unchanged.\n * Lineweaver-Burk: Lines intersect on y-axis.\n * Overcome by $\uparrow [S]$.\n * Non-competitive (Pure):\n * Inhibitor binds allosteric site (E or ES complex equally). Does not affect S binding.\n * Effect: $\downarrow V_{max}$, $K_m$ unchanged.\n * Lineweaver-Burk: Lines intersect on x-axis.\n * Uncompetitive:\n * Inhibitor binds *only* to ES complex.\n * Effect: $\downarrow$ apparent $K_m$, $\downarrow V_{max}$ proportionally.\n * Lineweaver-Burk: Parallel lines.\n4. Enzyme Regulation:\n * Allosteric Regulation:\n * Effectors bind to allosteric sites, causing conformational changes.\n * Often multi-subunit enzymes, exhibit sigmoidal kinetics (not hyperbolic).\n * Show cooperative binding (binding of one S affects others).\n * Feedback Inhibition (End-product Inhibition):\n * End-product of a pathway inhibits an enzyme early in the same pathway (often allosterically).\n * Covalent Modification:\n * Reversible attachment/removal of groups (e.g., phosphorylation by kinases, dephosphorylation by phosphatases) to activate/inactivate.\n * Proteolytic Activation (Zymogen Activation):\n * Inactive precursors (zymogens) activated by specific proteolytic cleavage (e.g., pepsinogen to pepsin).\n * Isozymes: Different forms of an enzyme catalyzing the same reaction, but with different kinetic/regulatory properties, found in different tissues.

Vyyuha Quick Recall

Can Not Understand Kinetics Very Well: \n\n* Competitive: Km $\uparrow$, Vmax same. Well (y-intercept) same. \n* Non-competitive: Km same, Vmax $\downarrow$. Well (y-intercept) different. \n* Uncompetitive: Km $\downarrow$, Vmax $\downarrow$. Well (y-intercept) different, Parallel lines.