Definition: Enzymes are biological catalysts that do not change the thermodynamics (enthalpy or Gibbs free energy) but speed up reactions.
Role: They help reactions proceed faster by affecting the kinetics.
Classification of Enzymes
Oxidor Reductase
Catalyze oxidation reactions by transferring electrons.
Example: Alcohol dehydrogenases (convert alcohols to aldehydes or ketones).
Transferases
Catalyze the transfer of functional groups between molecules.
Example: Aminotransferases (degrade amino acids by removing amino groups).
Hydrolases
Catalyze hydrolysis (breaking bonds with water).
Liases
Catalyze the breakdown of chemical bonds, forming new double bonds or rings.
Example: Pyruvate decarboxylase (removes carbon dioxide from pyruvate).
Isomerases
Catalyze structural shifts within a molecule.
Example: Ribulose phosphate epimerase (converts ribulose biphosphate to xylose 5-phosphate).
Ligase/Synthetases
Catalyze the joining of two molecules by forming new bonds.
How Enzymes Work
Mechanism: Lower the activation energy of a reaction by providing favorable environments, stabilizing transition states, or bringing reactive groups closer.
Protein Nature: All enzymes are proteins, but not all proteins are enzymes.
Specificity: Enzymes are highly specific, only binding to specific substrates that fit their active sites.
Enzyme-Substrate Complex: Formation of this complex lowers activation energy, catalyzing the reaction.
Regulation: Enzymes can be regulated by other molecules (activators or inhibitors) and post-translational modifications (e.g., phosphorylation).
Theories of Enzyme-Substrate Interaction
Lock and Key Theory
Active site is already in the right conformation for the substrate.
Induced Fit Model
Both enzyme and substrate undergo conformational changes to fully interact.
Enzyme Kinetics
Relation to Concentration: The rate of reaction depends on concentrations of both the enzyme and the substrate.
Km: Substrate concentration at which half of the enzyme active sites are occupied.
Michaelis-Menten Curve
X-axis: Substrate concentration.
Y-axis: Reaction rate (velocity).
Vmax: Plateau where increasing substrate does not increase reaction rate.
Km: Substrate concentration at 1/2 Vmax.
Lineweaver-Burk Plot
Plot: Double reciprocal of Michaelis-Menten (
extV vs 1/[S]).
Y-intercept: 1/Vmax.
X-intercept: -1/Km.
Slope: Km/Vmax.
Effects of Local Conditions on Enzyme Activity
Temperature: Reaction rate doubles for every 10°C increase until the optimal temperature, beyond which denaturation occurs.
pH: Optimal pH for human enzymes is 7.4. Changes can lead to denaturation.
Salinity: Affect enzyme activities by disrupting bonds, leading to conformational changes.
Regulation of Enzyme Activity
Feedback Regulation: Allows control of enzyme activity to maintain homeostasis.
Feedback Inhibition:
Irreversible Inhibitors: Form covalent or tight bonds with the enzyme.
Reversible Inhibitors:
Competitive: Inhibitors compete with the substrate for the active site. Can be overcome by increasing substrate concentration. Increases Km, no effect on Vmax.
Non-competitive: Bind to an allosteric site, changing enzyme conformation. Decreases Vmax, no effect on Km.
Uncompetitive: Bind only to enzyme-substrate complexes, locking substrate in place. Decrease both Vmax and Km.