Proteins are large, complex molecules that perform a vast array of functions in living organisms. Their function is intimately linked to their structure, which is organized into four distinct levels: primary, secondary, tertiary, and quaternary.

The Four Levels of Protein Structure

Primary Structure

The primary structure is the linear sequence of amino acids in a polypeptide chain. Each protein has a unique sequence determined by the corresponding gene. Even a single amino acid change can alter the protein’s function, as seen in sickle cell anemia where a single valine replaces a glutamic acid.

Secondary Structure

The secondary structure refers to local folded structures that form within the polypeptide chain due to hydrogen bonding between backbone atoms. The two most common types are the alpha-helix, a right-handed coil stabilized by hydrogen bonds between every fourth amino acid, and the beta-sheet, a flat, pleated structure formed by hydrogen bonds between adjacent polypeptide segments running parallel or antiparallel.

Tertiary Structure

The tertiary structure is the overall three-dimensional shape of a single polypeptide chain. It is stabilized by several types of interactions between side chains: hydrophobic interactions (nonpolar side chains cluster in the protein’s interior), hydrogen bonds between polar side chains, ionic bonds between oppositely charged side chains, and disulfide bridges (covalent bonds between cysteine residues).

Quaternary Structure

The quaternary structure describes how multiple polypeptide chains assemble into a functional protein complex. Hemoglobin, for example, is composed of four polypeptide subunits — two alpha and two beta chains — that work together to transport oxygen.

Protein Folding

Proteins fold into their native three-dimensional structures spontaneously or with the help of molecular chaperones. Misfolded proteins can form aggregates and are associated with diseases such as Alzheimer’s and Parkinson’s.