Enzymes: Nomenclature, Mechanism, Kinetics, and Regulation

Enzymes: Nomenclature, Properties, Mechanism, Kinetics, Regulation

• Pi value shown at the start (π = 3.141592) appears in the document but is not relevant to enzyme chemistry. It can be ignored for study purposes.

ENZYMES

• Enzymes are protein catalysts that increase the rate of chemical reactions without being consumed in the overall process.
• They direct all metabolic events.
• Most are proteins, but a few are RNA-based (ribozymes).
• In vitro, peptide bonds in peptides/proteins have a half-life of about 20 years; enzymes accelerate peptide bond hydrolysis when rapid turnover is needed to limit biological effects.
• There are 3 general ways to increase the rate of hydrolysis (or any chemical reaction):
  • 1) Increase temperature (add energy as heat)
  • 2) Increase concentrations of reacting substances
  • 3) Add a catalyst (enzyme)

TOPIC OUTLINE: NOMENCLATURE AND PROPERTIES; MECHANISM OF ENZYME ACTION; ENZYME KINETICS

NOMENCLATURE AND PROPERTIES

Nomenclature

• Recommended Name (short, commonly used name): end with “-ase” attached to the substrate (e.g., Glucosidase, Urease) or attached to a description of action (e.g., lactate dehydrogenase, adenylyl cyclase).
• Systematic Name: attaches “-ase” to a complete description of the chemical reaction, including substrates (e.g., lactate:NAD+ oxidoreductase).
• Enzymes are divided into six major classes:
  1) Oxidoreductases
  2) Transferases
  3) Hydrolases
  4) Lyases
  5) Isomerases
  6) Ligases
• Example:
  • Recommended: Lactate Dehydrogenase
  • Systematic: Lactate:NAD+ oxidoreductase

Properties

• Enzymes are protein catalysts that increase the velocity of a chemical reaction and are not consumed after the reaction.
• Key properties include:
  • Active sites
  • Catalytic efficiency
  • Specificity
  • Holoenzymes, apoenzymes, cofactors, and coenzymes
  • Regulation
  • Localization within the cell

Active Site

• The active site is a pocket formed by protein folding, containing amino acid side chains that participate in substrate binding and catalysis.
• Substrate binds to the active site to form an enzyme–substrate (ES) complex; a conformational change may occur to facilitate catalysis.
• ES converts to enzyme–product (EP) complex, which dissociates to enzyme and product.

Visual Concept (simplified)

• Substrate binds → enzyme changes shape slightly → ES complex forms → products released → enzyme free again.

Catalytic Efficiency

• Enzyme-catalyzed reactions are extremely fast, typically 10^3–10^8 times faster than uncatalyzed reactions.
• Turnover number (k_cat): number of substrate molecules converted to product per enzyme molecule per second; typically 10^2–10^4 s^-1.
• Relationship: $k<em>{cat} = \frac{V</em>{max}}{[E]_{total}}$

Specificity

• Enzymes are highly specific for substrates and reactions; many enzymes catalyze only one type of chemical reaction or act on one or a few substrates.
• Isozymes: different enzymes that catalyze the same reaction but have different catalytic properties.
• Models of specificity:
  • Lock-and-key model
  • Induced-fit model (enzyme changes shape upon binding to optimize fit and catalysis)

Holoenzymes, Apoenzymes, Cofactors, and Coenzymes

• Holoenzyme: active enzyme with its nonprotein component.
• Apoenzyme: enzyme without its nonprotein component; inactive.
• Cofactor: non-protein component required for activity.
  • Metal ion (inorganic cofactor): e.g., Zn^2+, Fe^2+.
  • Coenzyme (organic cofactor): small organic molecules, often vitamin-derived (e.g., NAD^+, FAD, CoA).
• Cofactors can be classified as:
  • Cosubstrates: bind temporarily/transiently (e.g., NAD^+).
  • Prosthetic group: permanently bound to the enzyme (e.g., FAD in succinate dehydrogenase).

Regulation

• Enzyme activity can be up- or down-regulated to match cellular needs.
• Regulation occurs via changes in substrate concentration, allosteric effects, covalent modification, and changes in enzyme synthesis/degradation.

Localization within the Cell

• Enzymes are localized to specific organelles, isolating substrates/products and organizing pathways for efficient metabolism.

MECHANISM OF ENZYME ACTION

Perspectives on catalysis

• Perspective 1: Energetic view — enzymes provide an energetically favorable alternative pathway with a lower free energy of activation (ΔG‡) than the uncatalyzed reaction.
• Perspective 2: Active-site chemistry — the active site uses diverse chemical mechanisms to faciliate conversion of substrate to product.

Energy changes and activation energy

• Reactions have an energy barrier between reactants and products: the free energy of activation (ΔG‡).
• Transition from reactants (A) to high-energy intermediate (T*) to products (B).
• In uncatalyzed reactions, the barrier is often high, leading to slower rates.
• Enzymes lower the barrier via an alternate pathway, increasing reaction rate without changing the overall free energies of reactants/products or the reaction equilibrium.

Chemistry of the active site

• The active site acts as a molecular machine employing several catalytic mechanisms:
  • Transition-state stabilization: stabilizes the transition state to increase the concentration of reactive intermediates, accelerating the reaction.
  • Catalytic groups: provide or accept protons (general acid–base catalysis) to influence the transition state.
  • Covalent intermediates: transient formation of a covalent enzyme–substrate (ES) complex in some reactions.
• Visualization concepts (described conceptually): ES forms; transition state T*; progression to EP; products released; enzyme regenerated.

SUBSTRATE CONCENTRATION AND ENZYME KINETICS

Substrate concentration effects

• Enzymes can be studied in vitro where responses to substrate concentration, temperature, and pH vary.
• Maximal velocity (V_max): the rate when all binding sites are saturated with substrate.
• Hyperbolic kinetics (Michaelis–Menten) vs. sigmoidal kinetics (allosteric enzymes).

Michaelis–Menten kinetics (MM)

• MM describes how Vo varies with substrate concentration [S].
• Key equation (standard form):$V<em>0 = \frac{V</em>{max} imes [S]}{K_m + [S]}$
• Km (Michaelis constant): a characteristic of enzyme–substrate pair; equal to the substrate concentration at which Vo = 1/2 V_max.
  • In general, Km = (k-1 + k2)/k_1.
  • Km reflects enzyme affinity for substrate (low Km = high affinity; high Km = low affinity).
• Assumptions for MM derivation:
  1) [S] >> [E] (substrate in excess)
  2) Steady-state: ES is constant over time (formation rate of ES equals its breakdown rate)
  3) Initial velocity: Vo is measured immediately after mixing to avoid product buildup
• Relationship to enzyme concentration: the reaction rate is directly proportional to enzyme concentration at all [S]. If [E] is halved, Vo and V_max halve.
• Order of reaction with respect to substrate:
  • First order ([S] << Km): rate ∝ [S]; many drugs follow first-order kinetics at therapeutic concentrations.
  • Zero order ([S] >> Km): rate is independent of [S] and equals V_max (enzyme sites saturated).
• Lineweaver–Burk plot (double reciprocal): used to linearize MM equation for estimating Km and V_max and to analyze inhibition mechanisms.
  • Transformed form: $\frac{1}{V<em>0} = \frac{K</em>m}{V<em>{max}}\frac{1}{[S]} + \frac{1}{V</em>{max}}$

Kinetic curves

• Hyperbolic MM curve for many enzymes (Vo vs [S]).
• Sigmoidal curves for allosteric enzymes (cooperative binding).
• V_max is the plateau velocity at saturating substrate.
• V_0 is the initial velocity observed when reacting mixture is first combined.

TEMPERATURE AND PH EFFECTS ON ENZYME ACTIVITY

Temperature

• Velocity increases with temperature as more molecules have sufficient energy to cross the activation barrier.
• At high temperatures, enzymes denature, decreasing activity.
• Optimum temperature for most human enzymes: ~35–40 °C.
• Denaturation risk rises above 40 °C.

pH

• Active-site ionization and substrate states depend on proton concentration (H+).
• Enzyme activity often requires specific protonation states (e.g., –NH3+ form).
• Extreme pH can denature enzymes by altering ionic character of amino acid side chains.
• pH optima vary by enzyme; e.g., pepsin has optimum around pH ~2, while others function best near neutral pH and are denatured in strongly acidic environments.

ENZYME INHIBITION

General concept

• An inhibitor is any substance that reduces the velocity of an enzyme-catalyzed reaction.
• Inhibitors can be:
  • Reversible (noncovalent interactions)
  • Irreversible (covalent bonds to the enzyme)
• The two most common reversible types are competitive and noncompetitive inhibition.

Competitive inhibition

• Inhibitor binds reversibly to the same active site as the substrate.
• Effects:
  • V_max: unchanged (can be reached with high [S])
  • Km: increases (apparent affinity decreases); more substrate needed to reach 1/2 V_max
• Lineweaver–Burk plot: inhibited and uninhibited lines intersect on the y-axis (1/V_max); different x-intercepts indicating increased Km.
• Real-world example: Statins (e.g., atorvastatin, pravastatin) competitively inhibit HMG-CoA reductase to reduce cholesterol synthesis by mimicking the natural substrate.

Noncompetitive inhibition

• Inhibitor binds to an enzyme at a site other than the active site; can bind to free enzyme or to ES complex.
• Effects:
  • V_max decreases (cannot be overcome by adding more substrate)
  • Km remains essentially unchanged (substrate binding affinity not affected)
• Lineweaver–Burk plot: inhibited lines intersect on the x-axis (Km unchanged) but have a different y-intercept (1/Vmax increased or decreased, depending on whether inhibition reduces Vmax).

Uncompetitive inhibition

• Inhibitor binds only to the ES complex, not to free enzyme.
• Effects:
  • V_max decreases because ES can't proceed to product
  • Km decreases (apparent higher affinity because ES is stabilized by inhibitor)
• Lineweaver–Burk plot: parallel lines to the uninhibited line; both Km and V_max decrease.

Double reciprocal (Lineweaver–Burke) plots: quick summary

• Competitive: V_max unchanged; Km increased; lines intersect on the y-axis.
• Noncompetitive: V_max decreased; Km unchanged; lines intersect left of the y-axis.
• Uncompetitive: Both V_max and Km decrease; lines are parallel.

REGULATION OF ENZYME ACTIVITY

General regulation

• Enzyme activity is regulated to coordinate metabolic processes.
• Intracellular substrate levels often lie around Km, so changes in substrate concentration modulate rate.
• Increasing substrate concentration generally increases rate to restore substrate levels toward normal.

Allosteric regulation

• Allosteric enzymes are regulated by effectors (molecules that bind sites other than the active site).
  • Negative effectors decrease activity.
  • Positive effectors increase activity.
• Effectors can alter Km (substrate affinity) and/or V_max (maximal catalytic rate) for the enzyme.
• Allosteric behavior is often represented by a sigmoidal response or a shift in apparent K0.5 (the substrate concentration at half-maximal activity for allosteric enzymes).

Covalent modification

• Enzyme activity can be regulated by covalent modification, most commonly phosphorylation/dephosphorylation on serine, threonine, or tyrosine residues.
• Protein kinases catalyze phosphorylation using ATP; phosphatases remove phosphate groups.
• Effects of phosphorylation are enzyme-specific; e.g., glycogen phosphorylase phosphorylation increases activity, while glycogen synthase phosphorylation decreases activity.

Induction and repression of enzyme synthesis

• Cells regulate the amount of enzyme by altering synthesis or degradation rates.
• Induction: increase in enzyme production in response to stimuli (e.g., higher insulin levels stimulating enzymes in glucose metabolism).
• Repression: decrease in enzyme production.
• These changes occur slowly (hours to days) compared with allosteric or covalent regulation (seconds to minutes).

ADDITIONAL RESOURCES AND NOTES

• YouTube links provided as supplementary content:

  • https://www.youtube.com/watch?v=qgVFkRn8f10&ab_channel=AmoebaSisters
  • https://www.youtube.com/watch?v=wpyyDEEC3k&list=PPSV&abchannel=Study.com
  • https://www.youtube.com/watch?v=6EDBlowVST0&t=202s&ab_channel=Amoebasisters

• Motivational quote included in the source: "Sometimes you win, Sometimes you learn." (@REALLYGREATSITE)

KEY FORMULAS AND CONCEPTS (REVIEW)

• Michaelis–Menten equation: $V<em>0 = \frac{V</em>{ ext{max}} [S]}{K_m + [S]}$
• Michaelis constant: $K<em>m = \frac{k</em>{-1} + k<em>2}{k</em>1}$
• Turnover number (catalytic rate): $k<em>{cat} = \frac{V</em>{ ext{max}}}{[E]_{ ext{total}}}$
• Lineweaver–Burk equation: $\frac{1}{V<em>0} = \frac{K</em>m}{V<em>{ ext{max}}}\frac{1}{[S]} + \frac{1}{V</em>{ ext{max}}}$
• Activation energy and transition state: enzymes provide a lower-energy pathway via stabilization of the transition state and catalytic chemistry (acid-base, covalent interactions).
• Enzyme regulation terminology:
  • Allosteric effector: Modulates enzyme affinity and/or activity.
  • Positive/negative effectors: Increase or decrease activity.
  • K0.5: Substrate concentration giving half-maximal allosteric activity.

SUMMARY OF KEY IDEAS

• Enzymes speed up reactions by lowering activation energy without changing the equilibrium, via active-site chemistry and transition-state stabilization.
• Nomenclature distinguishes recommended vs systematic names; enzymes are grouped into six major classes.
• Enzyme properties include active-site architecture, catalytic efficiency (k_cat), substrate specificity, and regulatory mechanisms (allosteric, covalent, synthesis-control).
• MM kinetics describes most enzymes as hyperbolic equilibria between Vo and [S], with Km indicating affinity; lineweaver-burke plots aid in determining Vmax, Km, and inhibitor mechanisms.
• Temperature and pH profoundly affect enzyme activity; most human enzymes function best near body temperature and neutral pH, with some enzymes specialized to acidic environments.
• Inhibitors regulate activity; competitive inhibitors increase apparent Km; noncompetitive inhibitors decrease Vmax; uncompetitive inhibitors reduce both; these patterns are diagnostic via Lineweaver-Burk plots.
• Regulation extends beyond instantaneous activity to control enzyme amounts via induction/repression of synthesis and covalent modification (phosphorylation).
• Spatial organization within cells (compartmentalization) serves to optimize reaction conditions and pathway organization.