The Chemistry and Biology of Proteins - F. Haurowitz 1953

The Role of Proteins in Biology

Proteins are of paramount importance in biology as the indispensable foundation of living matter. While living organisms do contain Introduction/36.html">CARBOHYDRATES and Lipids alongside proteins—often in even greater quantities, as seen in green plants that are richer in carbohydrates (Cellulose) than in proteins—fundamental differences set proteins apart from all other cellular constituents. Wherever we encounter the phenomena of GROWTH AND REPRODUCTION, we find that proteins play a leading role. In nucleated Cells, division is intimately linked to the presence in The Nucleus of proteins combined with Nucleic Acids, known as Nucleoproteins. In Bacteria, which lack a visible nucleus, proteins and nucleoproteins make up the bulk of living matter. Moving further down the scale of living things, we find that Viruses consist primarily of proteins and nucleoproteins, while the simplest among them contain no lipids or carbohydrates at all.

Another key feature distinguishing proteins from all other cellular components is their Specificity. For instance, while we have no evidence of species specificity in lipids, it is well established that every species of animal, plant, bacterium, and virus possesses its own distinct, specific proteins. In some cases, variations can even be detected among individuals belonging to the same species. There is no doubt that proteins must be regarded as the crucial factors responsible for the transmission of hereditary traits to offspring.

A third characteristic property of protein substances, Setting them apart from all Other Compounds present in The Cell, is their susceptibility to Denaturation. Native proteins are denatured by the very same physical or chemical agents that kill or damage living cells. However, Protein Denaturation can be triggered not only by harsh treatments such as heating or exposure to acids and alkalis, but also at room Temperature under METABOLISM/18.html">The Influence of aqueous urea solutions and other mild-acting agents. These findings lead to the Conclusion that the molecular Structure of native proteins is so labile that it can be disrupted merely by collisions with urea molecules present in solution. No other cellular components exhibit a comparable degree of lability.

The Functions of Proteins within cells and the Organism as a whole are remarkably diverse. Proteins form such inert structural Materials as Hair, horn, and bone, and they also comprise the contractile substance of Muscle fibers. These latter proteins possess a distinctive feature—The ability to convert chemical energy into mechanical work; consequently, they are responsible for the motility of higher organisms. Similarly, in lower organisms, mechanical movement (flagellar motion, coordinated ciliary beating, amoeboid movement) is intimately bound up with the presence of contractile proteins. Turning to other functions, it must be noted that Enzymes (vital biological catalysts), respiratory pigments, Hormones from certain glands (such as the Pancreas, thyroid, and pituitary), Antibodies, and Bacterial toxins are all proteins. From the foregoing, it is clear that The Significance of proteins to a living organism cannot be overstated.

The sheer variety of functions and the extreme lability of proteins point to the extraordinary complexity of their molecular chemical structure. We are still a long way from being able to describe the details of this architecture. Only in a few instances has it been possible to probe The structure of specific segments of the protein molecule. The most significant gap in our knowledge of proteins is our inability to correlate the enzymatic or hormonal activity of various proteins with the specifics of their chemical makeup. For example, we still do not know which molecular groups are responsible for the proteolytic action of crystalline Trypsin or the hormonal activity of crystalline Insulin.

These gaps in our understanding of proteins are closely related to the fact that protein properties depend not only on Chemical Composition and The sequence of amino acid residues in the polypeptide chain, but also on the spatial arrangement of these chains. The properties of a molecule containing compactly folded long peptide chains differ from those of the same molecule when its peptide chains are uncoiled. In other words, the properties of protein molecules largely depend on the three-dimensional structure of their peptide chains. Unlike most other fibrillar macromolecules, protein chains are able to maintain their specific folding pattern over extended periods.

The specific spatial arrangement of peptide chains exerts a profound influence on the properties of protein macromolecules. Many biological Properties of Proteins—such as solubility, serological behavior, and enzymatic or hormonal activity—depend on the molecular groups exposed on the protein surface; in other words, on the spatial conformation and the mode of folding of the peptide chains. Consequently, one of the primary objectives of Protein Chemistry is to elucidate the Internal Structure of protein macromolecules and the distribution of functional groups within them. Only through this approach can we hope to correlate the biological properties of proteins with their specific molecular structure.