Chapter 3: Amino Acids

Amino Acid Titration and pKa Determination

Amino acids possess ionizable groups that can be titrated to determine their pKa values. The titration curve of glycine, a simple amino acid, illustrates the stepwise ionization of its carboxyl and amino groups.

• Isoelectric Point (pI): The pH at which the amino acid has no net charge. For glycine, this occurs when the carboxyl group is deprotonated and the amino group is protonated.

• pKa Values: Glycine has two main pKa values: pKa1 (carboxyl group) and pKa2 (amino group).

• Titration Curve: The curve shows two buffering regions corresponding to the ionization of the carboxyl and amino groups.

• Key Equations:

  • pI for amino acids without ionizable side chains:

• Example: For glycine, pKa1 ≈ 2.34, pKa2 ≈ 9.60, so .

Structure and Properties of Amino Acids

Amino acids are the building blocks of proteins, each containing a central carbon (α-carbon), an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group).

• Zwitterion: At physiological pH, amino acids exist as zwitterions, carrying both positive and negative charges.

• Chirality: Most amino acids (except glycine) are chiral, with the α-carbon being a stereocenter.

• Classification: Amino acids are classified based on the properties of their side chains (nonpolar, polar, acidic, basic).

• Example: Alanine is a nonpolar, aliphatic amino acid. Its structure is:

  • Uncharged: H2N–CH(CH3)–COOH

  • Zwitterionic: H3N+–CH(CH3)–COO−

Ionization of Amino Acid Side Chains

Some amino acids have ionizable side chains, which contribute additional pKa values and affect their behavior in proteins and during purification.

• Functional Groups: Side chains such as those in Asp, Glu, Lys, Arg, and His can ionize, affecting the overall charge of the amino acid.

• pKa Table:

Additional info: The slight decrease in pKa2 with increasing peptide length is due to the influence of neighboring groups.

Peptide Bond and Protein Structure

Peptides are formed by the condensation of amino acids, resulting in a peptide bond. The sequence and composition of amino acids determine the properties and function of proteins.

• Peptide Bond: The covalent bond between the carboxyl group of one amino acid and the amino group of another.

• Primary Structure: The linear sequence of amino acids in a polypeptide.

• Example: The peptide Glu-His-Trp-Ser-Gly-Leu-Arg-Pro-Gly contains both acidic and basic residues, affecting its overall charge and behavior during purification.

Protein Purification Techniques

Proteins can be purified using various chromatographic and biochemical methods, each exploiting different properties such as size, charge, and binding affinity.

• Ion-Exchange Chromatography: Separates proteins based on charge. Cation-exchange columns bind positively charged groups; anion-exchange columns bind negatively charged groups.

• Gel Filtration (Size-Exclusion) Chromatography: Separates proteins based on size; larger molecules elute first.

• Affinity Chromatography: Utilizes specific binding interactions between a protein and a ligand.

• Protein Purification Table:

Additional info: Specific activity increases as purification proceeds, indicating enrichment of the target protein.

Electrophoresis and Protein Analysis

Electrophoresis is a technique used to separate proteins based on their size and charge. SDS-PAGE is commonly used to estimate protein molecular weight.

• SDS-PAGE: Sodium dodecyl sulfate (SDS) denatures proteins and imparts a uniform negative charge, allowing separation by size.

• Isoelectric Focusing: Separates proteins based on their isoelectric points (pI).

• Example: A protein with a molecular mass of 40 kDa can be analyzed by SDS-PAGE to determine its subunit composition.

Peptide Sequencing and Amino Acid Analysis

Determining the sequence of amino acids in a peptide involves hydrolysis, chromatography, and mass spectrometry.

• Edman Degradation: Sequentially removes N-terminal amino acids for identification.

• Hydrolysis: Breaks peptide bonds to release free amino acids for analysis.

• Example: Hydrolysis of a peptide followed by chromatography can reveal its amino acid composition.

Polypeptide Separation and Chromatography

Polypeptides can be separated using ion-exchange or size-exclusion chromatography, depending on their charge and size.

• Ion-Exchange Chromatography: Polypeptides with more positively charged residues migrate slowly through cation-exchange columns.

• Size-Exclusion Chromatography: Smaller peptides elute later than larger ones.

• Example: Given three polypeptide sequences, their migration rates can be predicted based on their charge and size.

Biological Significance of Amino Acids

Amino acids play crucial roles in protein structure, enzyme activity, and cellular function. Their side chains determine protein folding, stability, and interactions with other biomolecules.

• Histones and DNA Binding: Histones are rich in basic amino acids (e.g., lysine, arginine), facilitating strong binding to negatively charged DNA.

• Example: The abundance of basic residues in histones increases their affinity for DNA, promoting chromatin structure and gene regulation.