Complexation and Protein Binding Quiz

Complexation and Protein Binding

• Complexation is the interaction between a metal ion and a ligand.
• Polypyridyl ligands fit well with large metal ions.
• Extended aromatic systems are useful sensors for metal ions.
• Benzo groups lead to more preorganized ligands.
• Steric clashes between ortho H atoms and adjacent polypyridyl groups can destabilize the complex.

Analysis of Complexation

• Involves estimating the stoichiometric ratio of ligand:metal and the stability constant of the complex.
• pH titration is a method to determine complexation. The change in pH indicates complex formation.
• Chelation of cupric ion by glycine, for example, can be represented as: Cu² + 2NH3 CH2CH2 COO- = Cu(NH2CH2COO)2 + 2H+.
• Two protons formed during the reaction cause a decrease in pH.

Distribution Method

• The distribution behavior of a solute between two immiscible liquids is expressed by a distribution coefficient or partition coefficient.
• When a solute complexes with an added substance, the solute distribution pattern changes based on the nature of complex.
• The stability constant can be estimated.
• Example: Iodine complex with Potassium iodide.

Solubility Method

• Using the change in solubility profile as a criterion to analyze complexation.
• When components in a mixture form a complex, solubility may increase or decrease.
• Example: Estimation of complexation of para-aminobenzoic acid (PABA) and caffeine.

Continuous Variation Method

• Physical properties, such as UV absorbance, dielectric constant, refractive index, and spectrophotometric extinction coefficient are characteristics of a species.
• A change in property suggests complex formation.
• If no complexation occurs, the property value is additive and shows a linear relationship.
• Complexation leads to a nonlinear relationship in property value.

Spectroscopy Method

• Studying donor-acceptor or charge transfer complexation using absorption spectroscopy in the visible/UV range..
• The complex will have different UV absorbance from its individual components.
• Other methods for determining complexation include nuclear magnetic resonance (NMR), infrared spectroscopy (IR), polarography, and X-ray diffraction.

Question Examples

• Identify the central metal ion, ligand and coordination number in the complex [CO(NH3)6]+3.
• Identify the central metal ion, ligand and coordination number in the complex [Cr(H2O)6]+3.

Protein Binding

• The complex formed by drug interaction with proteins.
• Human plasma is composed of many known proteins.
• Proteins are made of approximately 20 amino acids.
• Functions include maintaining osmotic pressure, proper pH, immune reactions (antibodies), transport, enzymatic function (enzymes or hormones), cell growth/tissue maintenance, and catalyzing metabolism reactions.

Protein Binding - Types

• Protein binding can be reversible or irreversible.
• Reversible binding involves weak bonds like hydrogen bonds, hydrophobic bonds, and van der Waals forces.
• Irreversible binding is rare and results from covalent bonding (can cause toxicity).

Protein-Ligand Binding Kinetics

• Describes protein-ligand association/dissociation.
• Binding is based on protein-ligand affinity and kinetics arising from contacts/collisions and molecular diffusion.
• P + L <=> PL (Protein and ligand can form a complex)

Protein-Ligand Binding Models

• Common models like lock-and-key, induced-fit, and conformational selection are suggested.
• Lock-and-key model: Protein and ligand are rigid, with perfectly matched binding interfaces.
• Induced-fit model: Binding site is flexible and the interacting ligand induces a conformational change in the binding site.
• Conformational selection model: The native state of protein exists in a vast conformational ensemble.

Mechanisms of Interactions (Protein-small molecules)

• Hydrogen bonding between donor and acceptor functional groups in amino acid sequences.
• Electrostatic interactions between charged amino acids and oppositely charged ligand molecules.
• Van der Waals interactions (dipole-dipole, dipole-induced dipole, dispersion forces).
• Hydrophobic interactions between nonpolar groups in the presence of water molecules.

Implications of Protein Binding

• Bound drugs are inactive; free drugs are active.
• Free drugs distribute to organs and tissues.
• Bound drugs act as reservoirs, and a small change in concentration can have a significant impact.
• Drug-related factors (physicochemical characteristics): Lipophilicity increases binding extent. -Concentration of drug in the body changes protein binding process.
• Affinity of the drug to the binding component depends on the degree of attraction or affinity of the protein molecule or tissues towards drug moieties.
• Protein/tissue related factors (physicochemical characteristics): This is related to the characteristics of proteins.

Patient-Related Factors Affecting Protein Binding

• Neonates have lower albumin content; thus, more free drug.
• Elderly patients have lower albumin content; thus, more free drug.
• Inter-subject variability (individual differences)

Disease States Affecting Protein Binding

• Renal failure can decrease albumin content leading primarily to increased binding.
• Hepatic failure can decrease albumin synthesis and thus decreased binding.
• Inflammation can lead to increases in some protein concentrations changing the drug binding capacity.

Protein Binding Locations

• Protein binding can occur intracellularly and extracellularly.
• Common proteins that bind drugs include human serum albumin (HSA), alpha-1-acid glycoprotein, lipoproteins, and blood cells.

Binding of Drug to Specific Proteins

• Drugs can bind to HSA, with albumin possessing four binding sites.
• Alpha-1-acid glycoprotein (orosomucoid) binds by hydrophobic bonds; basic drugs bind in acidic regions.
• Lipoproteins bind by hydrophobic bonds (lipid core).
• Globulins (cortisol binding globulin, transferrin, ceruloplasmin) bind steroids, vitamins, and metals.
• Blood cells (hemoglobin) bind lipophilic ligands.

Significance/Effects of Protein Binding

• Absorption: Protein binding decreases free drug concentration.
• Distribution: Bound drug does not cross barriers (BBB, placental, glomerulus);
• Metabolism: Protein binding can decrease metabolism; unbound drug is metabolized.
• Elimination: Only unbound drug is eliminated.

Other Considerations

• Diseases/disorders change protein status (used as biomarkers).
• Lipoproteins transport hydrophobic drugs.
• Protein binding inactivates drugs; a sufficient concentration at the receptor site is needed for action.
• Drug-protein complex acts as a reservoir; and continuously provides free drug.