Summary
Highlights
A protein is a polymer made up of many amino acids. Each amino acid is a monomer, and when many come together, they form a polypeptide. The bond connecting individual amino acid residues is called a peptide bond.
An amino acid features a chiral carbon with a hydrogen, an amine group (N-terminal), an 'R' group, and a carboxyl group (C-terminal). Two amino acids react in a condensation or dehydration reaction, losing water to form a covalent peptide bond between the carbon of one and the nitrogen of the other, creating a dipeptide.
Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. The primary structure is determined by the sequence of amino acids, which dictates the protein's overall shape and function. Even a single amino acid change can alter the protein's shape.
The secondary structure describes the localized shape, primarily the alpha helix and beta pleated sheet. Both are stabilized by hydrogen bonds between the NH group of one amino acid and the carbonyl group of another. An alpha helix turns every 3.6 amino acid residues.
The tertiary structure is the complete three-dimensional folding pattern of a single protein chain, including straight chains, alpha helices, and beta pleated sheets. The quaternary structure arises when multiple individual protein subunits combine, such as hemoglobin, which has four subunits (two alpha and two beta).