Affinity Chromatography Explained: His & GST Tag Purification

This lecture covers essential biochemistry concepts including amino acid ionization at physiological pH (histidine with pKa 6), hydrogen bonding capabilities of amino acid side chains, structural disruption by proline in alpha helices and beta sheets, salt bridge formation between oppositely charged residues, optical activity (glycine is optically inactive due to two identical hydrogen atoms), buffer capacity of histidine near physiological pH, calcium binding by aspartic acid, Ramachandran plot preferences (glycine most flexible, proline most restricted), and protein structural levels from primary to quaternary structure.

Amino acids are classified based on their side chain properties. Acidic amino acids have more carboxylic acid groups than amino groups, including aspartic acid and glutamic acid, which have a negative charge at physiological pH. Basic amino acids have more amino groups than carboxylic acid groups, including lysine, arginine, and histidine, which have a positive charge at physiological pH. Basic amino acids are found in basic proteins, particularly histone proteins, and play important roles in DNA binding. Aromatic amino acids contain aromatic ring structures: phenylalanine, tyrosine, and tryptophan. Tryptophan is a precursor for serotonin and melatonin. Sulfur-containing amino acids include cysteine and cystine, which form disulfide bonds important for protein structure stability. Hydroxyl-containing amino acids include serine, threonine, and tyrosine, which participate in hydrogen bonding for protein structure.

Biochemistry studies chemical compounds with biological importance in the human body, including proteins, carbohydrates, lipids, enzymes, and hormones. Proteins are essential for forming cell walls and are found in muscles, hormones, and enzymes. Every protein is composed of 20 standard amino acids, which connect through peptide bonds. All amino acids share a universal structure with a central alpha carbon bonded to four groups: a carboxylic acid group (COOH), an amino group (NH2), a hydrogen atom, and a variable R group (side chain). The carboxylic acid group is acidic and can lose a hydrogen atom, while the amino group is basic and can gain one, creating a zwitterion. Each amino acid has a specific mRNA codon for protein synthesis and a transfer RNA for cellular transport.

This comprehensive section establishes the foundational framework for understanding amino acid chemistry. It begins with structural concepts: alpha amino acids have the amino group attached to the alpha carbon (adjacent to the carboxyl group), with beta-alanine as the primary exception. Chiral carbons are explained as carbon atoms with four different functional groups, enabling optical isomerism; glycine uniquely lacks a chiral center and is optically inactive. Imino acids differ by having their amino group bound within a ring structure (proline as the key example). The classification system organizes 22 amino acids into seven categories based on side chain properties: simple (glycine, alanine), branched-chain (leucine, isoleucine, valine), sulfur-containing (methionine, cysteine), hydroxyl-containing (serine, threonine, tyrosine), acidic (aspartate, glutamate), amide-containing (asparagine, glutamine), and basic (histidine, lysine, arginine). Polar/hydrophilic amino acids include those with hydroxyl, acidic, basic, and amide groups, while nonpolar/hydrophobic amino acids lack these functional groups. Arginine is the most hydrophilic, while isoleucine is the most hydrophobic.

Proteins are essential nitrogen-containing macromolecules forming cellular structure and performing diverse biological functions including enzymatic catalysis, hormone signaling, oxygen transport (hemoglobin, myoglobin), and immune defense (antibodies, antigens). Proteins consist of approximately 20 amino acids linked by peptide bonds, with four structural levels: primary (amino acid sequence), secondary (alpha-helices, beta-sheets via hydrogen bonds), tertiary (3D folding), and quaternary (multiple subunits). Denaturation disrupts higher-order structures through heat, pH extremes, or solvents. Proteins classify by shape (globular vs fibrous) and composition (simple vs conjugated). Amino acids contain a central alpha-carbon bonded to hydrogen, amino group, carboxyl group, and variable R side chain determining properties. Twenty standard amino acids exist, classified by polarity. Nine are essential (must be dietary): histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine. Key metabolic amino acids include glycine (heme and glutathione synthesis), tyrosine (melanin, thyroid hormones, catecholamines, dopamine precursor), tryptophan (serotonin, niacin precursor), and histidine (histamine precursor).

Expression vectors require several additional components beyond basic cloning vector features: (1) Promoter region to initiate transcription, (2) Transcriptional start and stop sites to define mRNA boundaries, (3) Translational start and stop codons for proper protein synthesis, and (4) Affinity tags (such as His-tag or GST-tag) that enable easy purification of expressed proteins using specific binding techniques. These tags contain repeated amino acid sequences that can be recognized by purification resins or antibodies.

An expression vector is a plasmid or virus designed for protein expression inside host cells, containing essential elements such as a strong inducible promoter (like lac, tryptophan, or arabinose promoters), translation initiation sequences, termination codons, host-specific elements (Shine-Dalgarno or Kozak sequences), purification tags (such as 6xHis tag), and optionally fusion proteins (like GFP or GST) for solubility and detection; unlike cloning vectors which only transfer genes, expression vectors utilize the host cell's machinery to transcribe DNA into mRNA and translate it into proteins, requiring multi-layered regulatory systems for precise control over gene expression.

Cloning vectors require six essential features: origin of replication for autonomous replication, multiple cloning site (MCS) with restriction enzyme sites for gene insertion, genetic selection markers (like antibiotic resistance genes) for transformant selection, high copy number for abundant gene amplification, small size to accommodate larger DNA fragments, and MCS disruption markers to distinguish recombinant from non-recombinant vectors; expression vectors additionally require strong inducible promoters for high-level transcription, strong terminators to stop transcription, ribosome binding sites for translation initiation, protein purification tags (such as His-tag or GST-tag) for single-step purification, F1 origin for sequencing, secretion signals (like alpha-factor) for extracellular protein release, and fluorescent protein tags (like GFP) for protein localization visualization.

The five main tools in recombinant DNA technology are: (1) Restriction Enzymes (molecular scissors) - cut DNA at specific sequences, (2) Polymerase Enzymes - synthesize DNA strands, (3) DNA Ligase - joins DNA fragments, (4) Cloning Vectors - vehicles for carrying DNA, and (5) Host Organisms - cells that receive and replicate DNA. Restriction enzymes are classified as exonucleases (cut from ends) or endonucleases (cut internally). The first restriction endonuclease discovered was Hind II. Restriction enzymes are named by convention: genus (Eco), species (coli), strain (RY13), and isolation order (13). Palindromic DNA sequences read the same forward and backward on complementary strands, creating sticky ends that allow DNA fragments to join together. A cloning vector is a DNA molecule used to carry foreign DNA into host cells, commonly bacterial plasmids. Three essential features are: (1) Origin of Replication (ori) for DNA replication, (2) Selectable markers (antibiotic resistance genes like ampicillin and tetracycline resistance) for identifying transformed cells, and (3) Cloning sites where foreign DNA can be inserted. pBR322 is the first artificially constructed cloning vector.

Two primary methods exist for adding protein tags to expression vectors. For short tags like His-tags, PCR primers can be designed to include the tag sequence immediately after the ATG start codon, followed by the gene of interest sequence. The reverse primer must remove the natural stop codon so the stop codon resides at the tag's end. For larger tags like MBP or GFP, pre-designed vectors containing the tag sequence are used; the gene of interest is cloned into the vector, and the natural stop codon is removed. Both approaches create fusion proteins where the tag and protein of interest are physically linked, enabling the desired functional properties of the tag.

Molecular recognition is the process by which molecules identify and bind to each other through non-covalent interactions such as hydrogen bonding, van der Waals forces, pi-pi interactions, electrostatic attractions, or metal coordination, following the lock and key concept where a host molecule acts as a lock and a guest molecule acts as a key; this phenomenon is central to supramolecular chemistry, which studies molecular assemblies formed through non-covalent interactions rather than covalent bonding.

Molecular recognition is the specific interaction between host and guest molecules through non-covalent forces such as hydrogen bonding, van der Waals forces, and electrostatic interactions, which enables the formation of host-guest complexes and is fundamental to understanding molecular binding in organic chemistry.

Molecular recognition refers to the specific binding between two molecules (ligand and receptor) through non-covalent interactions such as ionic bonds, hydrogen bonds, and hydrophobic interactions, where the strength of this binding is quantified by affinity (measured by the free energy change ΔG)—more negative ΔG indicates stronger binding—and the relative binding strengths between different ligands and receptors determine specificity.

Supramolecular chemistry studies molecular aggregation through non-covalent interactions. Host molecules (large entities with binding sites/cavities) recognize and bind guest molecules (smaller entities with complementary features) through molecular recognition—a purposeful binding process based on the lock-and-key principle. Two complementarity types govern these interactions: size complementarity (geometric matching, e.g., crown ethers binding K⁺ ions) and electronic complementarity (charge-based attraction). Key non-covalent interactions include ion-ion, ion-dipole, dipole-dipole, π-π stacking, cation-π, and van der Waals forces. Hydrogen bonding (D-H...A) serves as the master key interaction, classified by George Jeffrey into strong (180° angle, short X-H), medium (178-140° angle, longer X-H, stabilizes proteins), and weak (poor donors like C-H) categories, each contributing uniquely to molecular recognition and structural stability across chemical and biological systems.

Supramolecular chemistry is the study of organized chemical systems composed of distinct molecules held together by non-covalent interactions such as electrostatic forces, ion-dipole interactions, dipole-dipole interactions, hydrogen bonding, metal coordination, hydrophobic effects, π-π interactions, and van der Waals forces; these interactions enable molecular recognition and self-assembly processes that form the basis of many biological phenomena like enzyme catalysis and DNA structure, and have earned Nobel Prizes in 1987 and 2016 for pioneering research in this field.