Enzyme Kinetics & Catalysis
โญ CSIR NET / NEET / MCAT High-Yield
6 Parameters
Enzyme Inhibition: Competitive vs Non-Competitive vs Uncompetitive vs Mixed
Crucial distinction matrix comparing reversible enzyme inhibition modes, binding targets, kinetic parameters (Km & Vmax), and Lineweaver-Burk double-reciprocal plot behaviors.
| Kinetic Parameter | Competitive | Non-Competitive | Uncompetitive | Mixed |
|---|---|---|---|---|
| Inhibitor Binding Site | Active site (structurally mimics substrate) | Allosteric site (free E or ES complex equally) | Only to Enzyme-Substrate [ES] complex | Allosteric site (unequal affinity for E vs ES) |
| Apparent Km | Increased (Km' = ฮฑยทKm) | Unchanged (Km' = Km) | Decreased (Km' = Km / ฮฑ') | Increased (ฮฑ > ฮฑ') or Decreased (ฮฑ < ฮฑ') |
| Apparent Vmax | Unchanged (overcome by high [S]) | Decreased (Vmax' = Vmax / ฮฑ) | Decreased (Vmax' = Vmax / ฮฑ') | Decreased (Vmax' = Vmax / ฮฑ') |
| Lineweaver-Burk Plot | Intersect on 1/v axis (same y-intercept) | Intersect on 1/[S] axis (same x-intercept) | Parallel lines (same slope Km/Vmax) | Intersect in 2nd or 3rd quadrant |
| Reversible by Excess [S]? | Yes (100% reversible) | No (Vmax never recovered) | No (cannot be overcome by excess [S]) | No (cannot be completely overcome) |
| Clinical / Classic Example | Malonate vs Succinate Dehydrogenase; Statins vs HMG-CoA Reductase | Cyanide vs Cytochrome Oxidase; Lead vs Ferrochelatase | Lithium vs Inositol Monophosphatase; Glyphosate vs EPSP synthase | Non-nucleoside reverse transcriptase inhibitors (NNRTIs) |
Kinetic Shift Summary
Competitive
Km Effect: Km โ
Vmax Effect: Vmax โ
Lineweaver Pattern: Common Y-intercept
Non-Competitive
Km Effect: Km โ
Vmax Effect: Vmax โ
Lineweaver Pattern: Common X-intercept
Uncompetitive
Km Effect: Km โ
Vmax Effect: Vmax โ
Lineweaver Pattern: Parallel Lines
Mixed
Km Effect: Km โ/โ
Vmax Effect: Vmax โ
Lineweaver Pattern: Off-axis Intersection
Metabolic Regulation & Cycles
โญ Medical Biochemistry / USMLE Core
6 Parameters
Hexokinase vs Glucokinase (Isozymic Comparison)
Functional, kinetic, and tissue-specific comparison of glucose-phosphorylating isozymes regulating systemic glucose homeostasis.
| Feature | Hexokinase (Types IโIII) | Glucokinase (Hexokinase IV) |
|---|---|---|
| Tissue Distribution | Ubiquitous (all extrahepatic tissues, brain, RBCs, muscle) | Hepatocytes (liver) and Pancreatic beta-islet cells |
| Km for Glucose (Affinity) | Low Km (~0.05 โ 0.1 mM) โ Extremely High Affinity | High Km (~5 โ 10 mM) โ Low Affinity (active post-prandially) |
| Vmax (Capacity) | Low Vmax (prevents cellular ATP depletion) | High Vmax (clears massive glucose spikes into glycogen) |
| Feedback Inhibition by Glucose-6-Phosphate (G6P) | Strongly inhibited by G6P (allosteric feedback) | NOT inhibited by G6P (regulated by GKRP in nucleus) |
| Induction by Insulin | Constitutive; NOT induced by insulin | Strongly induced at transcriptional level by insulin |
| Biological Role | Basal cellular energy production even under fasting hypoglycemia | Glucose sensing for insulin secretion and hepatic glycogen buffer |
Affinity & Regulation Dynamics
Hexokinase
Km Effect: Low Km (0.1 mM)
Vmax Effect: Low Vmax
G6p Inhibition: Yes (Allosteric)
Glucokinase
Km Effect: High Km (10 mM)
Vmax Effect: High Vmax
G6p Inhibition: No (Insulin Regulated)
Metabolic Regulation & Cycles
โญ Reciprocal Regulation Matrix
6 Parameters
Glycolysis vs Gluconeogenesis: The 3 Irreversible Bypasses
Detailed comparative breakdown of opposing catabolic and anabolic carbohydrate pathways and the three thermodynamic bypass checkpoints.
| Checkpoint / Parameter | Glycolytic Reaction & Enzyme | Gluconeogenic Bypass Enzyme | Allosteric Effectors |
|---|---|---|---|
| Bypass 1 (PEP to Pyruvate) | Pyruvate Kinase (PEP + ADP โ Pyruvate + ATP) | Pyruvate Carboxylase + PEP Carboxykinase (PEPCK) | Activated by Acetyl-CoA (PC); Inhibited by Alanine & ATP (PK) |
| Bypass 2 (F1,6BP to F6P) | Phosphofructokinase-1 (PFK-1: Rate-Limiting) | Fructose-1,6-Bisphosphatase-1 (FBPase-1) | Fructose-2,6-bisphosphate: Activates PFK-1, Strongly Inhibits FBPase-1 |
| Bypass 3 (G6P to Glucose) | Hexokinase / Glucokinase (Glucose + ATP โ G6P + ADP) | Glucose-6-Phosphatase (ER lumen-bound in liver & kidney) | Glucagon induces G6Pase; Insulin suppresses transcription |
| Primary Cellular Location | 100% Cytosolic | Mitochondrial matrix (PC) + Cytoplasm + ER Lumen | Compartmental segregation prevents futile cycling |
| High AMP / Low Energy State | Stimulates Glycolysis (activates PFK-1) | Inhibits Gluconeogenesis (shuts down FBPase-1) | AMP allosterically activates PFK-1 and inhibits FBPase-1 |
| High Citrate / High Energy State | Inhibits Glycolysis (allosterically inhibits PFK-1) | Stimulates Gluconeogenesis (activates FBPase-1) | Citrate signals abundant TCA precursors and high ATP |
Energy Charge Control
Low Energy (High AMP)
Glycolytic Flux: Activated (PFK-1)
Gluconeogenic Flux: Inhibited (FBPase-1)
High Energy (High ATP/Citrate)
Glycolytic Flux: Inhibited
Gluconeogenic Flux: Activated
Biomolecular Architecture
โญ Cooperativity & Oxygen Binding
7 Parameters
Myoglobin vs Hemoglobin: Structure, Kinetics & Allostery
Comparison between monomeric storage globins and tetrameric transport globins, Hill coefficients, and allosteric effectors.
| Feature | Myoglobin (Mb) | Hemoglobin (Hb) |
|---|---|---|
| Quaternary Structure | Monomer (1 heme, 1 polypeptide chain, 153 AA) | Tetramer (2ฮฑ:2ฮฒ chains in adult HbA, 4 hemes) |
| Oxygen Binding Curve | Hyperbolic curve (no cooperativity) | Sigmoidal curve (positive cooperativity, T โ R transition) |
| P50 (Oxygen Partial Pressure at 50% Saturation) | Very low (~2.8 mmHg) โ extremely high affinity | Higher (~26 mmHg) โ facilitates unloading in tissues |
| Hill Coefficient (nH) | nH = 1.0 (non-cooperative independent binding) | nH โ 2.8 โ 3.0 (strong positive cooperativity) |
| 2,3-BPG Regulation | Unresponsive (no central cavity for 2,3-BPG binding) | Strongly binds central cavity of T-state, stabilizing deoxygenation |
| Bohr Effect (H+ and CO2 sensitivity) | Absent (pH does not affect O2 binding curve) | Present (acidosis & high CO2 trigger O2 unloading via salt bridges) |
| Primary Physiological Function | Oxygen storage in skeletal and cardiac muscle for emergency bursts | Oxygen transport from lungs to peripheral tissues |
O2 Affinity Comparison
Myoglobin
P50 Value: 2.8 mmHg
Curve Shape: Hyperbolic
Cooperativity: None (nH=1.0)
Hemoglobin
P50 Value: 26.0 mmHg
Curve Shape: Sigmoidal
Cooperativity: Positive (nH=2.8)
Metabolic Regulation & Cycles
โญ Lipid Metabolism High-Yield
6 Parameters
Fatty Acid Synthesis vs Beta-Oxidation: Compartmental & Enzymatic Contrast
Exhaustive contrast between lipid anabolism and catabolism, carrier proteins, redox cofactors, and shuttle systems.
| Parameter | Fatty Acid Synthesis (Lipogenesis) | Fatty Acid Beta-Oxidation |
|---|---|---|
| Intracellular Compartment | Cytoplasm (Cytosol) | Mitochondrial Matrix (and Peroxisomes for VLCFA) |
| Redox Electron Carrier | NADPH (reducing equivalents from Pentose Phosphate Pathway) | NAD+ and FAD (oxidizing agents yielding NADH and FADH2) |
| Acyl Carrier / Anchor | Acyl Carrier Protein (ACP phosphopantetheine arm) | Coenzyme A (CoA-SH) |
| Key Rate-Limiting Enzyme | Acetyl-CoA Carboxylase (ACC; produces Malonyl-CoA) | Carnitine Palmitoyltransferase-I (CPT-1 / CAT-1) |
| Sequential Reactions | Condensation โ Reduction โ Dehydration โ Reduction | Oxidation โ Hydration โ Oxidation โ Thiolysis |
| Reciprocal Regulator | Citrate allosterically polymerizes and activates ACC | Malonyl-CoA potently inhibits CPT-1, preventing futile cycling |
Metabolic Vector & Cofactor Profile
Synthesis
Compartment: Cytosol
Electron Carrier: NADPH
Rate Limiter: ACC
Beta-Oxidation
Compartment: Mitochondria
Electron Carrier: NAD+ / FAD
Rate Limiter: CPT-1
