Proteins have evolved to form quaternary structures and multi-unit complexes when larger molecules are needed by the cell, rather than having longer amino acid chains. Most proteins fall into one of two categories: globular and fibrous.

Globular proteins are compact, with their amino acid chain wound into a spheroid shape. Their secondary structures are usually a mixture of alpha-helices and beta-sheets.  

Most intracellular proteins are globular and water-soluble, such as many enzymes and transcription factors. In these structures, the hydrophobic amino acids pack tightly into the middle of the spheroid structure, while the hydrophilic amino acids are found on the outer surface. 

Globular proteins interact with each other in a variety of configurations, forming different types of structures including filaments or multimeric complexes.

Many form quaternary structures, a single functional unit composed of more than one amino acid chain. For example, hemoglobin functions as a tetramer, composed of two alpha and two beta subunits.  

Even larger structures can be built when a globular protein associates with one or more additional proteins. For example, an actin filament is formed when many globular actin monomers join together to create long helical protein strands. 

Unlike compact globular proteins, fibrous proteins are often located in the extracellular matrix, provide structure, and form extended shapes.

Fibrous proteins are usually composed of either alpha-helices or beta-sheets, but rarely a mix of both. They often have hydrophobic amino acids on their outer surfaces that interact with other monomers to form larger structures.

For example, collagen is a fibrous protein composed of an extended alpha helix.  Three helices wind around each other to make a structure called a coiled-coil. Collagen fibrils provide structure and flexibility in connective tissues.

The protein fibroin is an example of a fibrous protein composed of beta-sheets and is the protein in silk which allows the silk strands to be both flexible and strong.