Proteins are large, complex macromolecules present in all living organisms, performing nearly every function necessary for life, from catalyzing metabolic reactions to maintaining cell structure. The full understanding of these substances was a multi-stage process spanning over a century of investigation, not the achievement of a single person. This journey began with the identification of nitrogen-containing compounds, progressed through determining their linear sequence, and culminated in mapping their intricate three-dimensional shapes.
Coining the Term and Early Chemical Identification
The initial recognition of a unique substance occurred in the 1830s through detailed chemical analysis. Dutch chemist Gerardus Johannes Mulder conducted extensive elemental studies on substances isolated from plants and animals, such as fibrin from blood and albumin from egg whites. He observed a commonality in their composition, finding they all contained carbon, hydrogen, oxygen, and a high proportion of nitrogen.
Mulder concluded they were built around a common, massive core molecule. He reached out to his colleague, the Swedish chemist Jöns Jacob Berzelius, for naming advice. Berzelius suggested the term “protein” in 1838, deriving it from the Greek word proteios, meaning “of the first rank” or “primary.” Mulder adopted the term to describe the basic organic material of living tissue. While the specific empirical formula Mulder proposed was later shown to be incorrect, his work established a distinct chemical class for these nitrogenous compounds.
Establishing the Primary Structure: The Peptide Bond
The focus of research shifted from elemental composition to internal organization around the turn of the 20th century. Scientists knew that proteins could be broken down into smaller components called amino acids, but the exact nature of the bond holding them together remained a mystery. Two scientists, German chemist Emil Fischer and Austrian physiologist Franz Hofmeister, independently provided the answer in 1902. They proposed that amino acids were linked together in long chains.
This linkage occurred when the carboxyl group of one amino acid reacted with the amino group of the next, resulting in the removal of a water molecule. Fischer later named this chemical linkage the “peptide bond.” The peptide bond theory established the concept of the protein’s primary structure, defining it as the precise, linear sequence of amino acids in the chain. Fischer’s subsequent work involved the synthesis of small chains of amino acids, known as polypeptides, which demonstrated the stability and feasibility of the peptide bond. The peptide bond itself possesses partial double-bond character, which limits rotation around the bond and keeps the connecting atoms in a rigid, planar arrangement. This structural detail proved to be an important constraint on the overall folding of the protein chain.
Defining the Molecular Architecture: Sequencing and 3D Form
Proving the existence of a specific, defined sequence of amino acids was the next step in understanding proteins. British biochemist Frederick Sanger achieved this feat by determining the complete amino acid sequence for the hormone insulin in the early 1950s. Insulin, a relatively small protein, consists of two chains—one with 21 amino acids and the other with 30—held together by disulfide bridges. Sanger’s accomplishment, which took over a decade, was the first time any protein had its entire primary structure mapped, definitively proving the Fischer-Hofmeister hypothesis. His work utilized specialized chemical methods to break the protein into smaller, overlapping fragments, allowing him to piece the sequence back together.
The final piece of the structural puzzle involved determining how the linear chain folds into a functional three-dimensional shape. This was achieved using X-ray crystallography, a technique that bombards crystallized proteins with X-rays to generate a diffraction pattern that can be computationally interpreted. In 1958, John Kendrew and Max Perutz used this method to map the first atomic-resolution structures of proteins—myoglobin and hemoglobin, respectively. These structures revealed that the amino acid chain folds into complex, specific shapes, such as alpha helices and beta sheets, which are stabilized by weak chemical forces. The determination of these three-dimensional structures cemented the understanding that the sequence dictates the final, functional architecture of the molecule.