Amino Acids & Protein Structure

There are 20 standard amino acids. All have an amino group (NH₂), carboxyl group (COOH), hydrogen, and R group (side chain) attached to a central alpha carbon.

Categories of amino acids (by R group):

Nonpolar/hydrophobic: Glycine, Alanine, Valine, Leucine, Isoleucine, Proline, Phenylalanine, Tryptophan, Methionine

Polar/uncharged: Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine

Positively charged (+): Lysine, Arginine, Histidine

Negatively charged (−): Aspartate, Glutamate

Protein structure levels:

Primary — amino acid sequence (peptide bonds)

Secondary — alpha helix, beta sheet (hydrogen bonds between backbone)

Tertiary — 3D folding of one polypeptide (R group interactions)

Quaternary — multiple polypeptide subunits (e.g., hemoglobin = 4 subunits)

Isoelectric point (pI): pH at which amino acid has no net charge. Below pI = positive charge; above pI = negative charge.

Reference:

Wikipedia: Amino Acid

https://en.wikipedia.org/wiki/Amino_acid