Chemistry and Biology of Proteins - F. Haurowitz 1953

Protein Synthesis
Proteins of Cellular Structural Elements

Before moving on to the Mechanisms of Protein synthesis, we must examine current understanding of the physicochemical state of Proteins within living Cells. Some of these proteins exist in a soluble state within The Cell fluid, while others are integrated into cellular structures, such as fibers or granules [84].

The high viscosity of cell sap indicates that the proteins dissolved in this fluid possess elongated, thread-like molecules. Using a polarizing Microscope, it is sometimes possible to determine the orientation of these protein filaments [80].

This same method is used to study Fibrillar Proteins in cell membranes, Muscles, nerves, and other tissues. In many cell membranes, proteins are bound to Lipids, forming organized layers. Research on the cortical layer of the sea urchin egg [82] and Nervous Tissue [83] has demonstrated that lipid molecules are arranged radially, with their long axes directed outward from the center of the cell toward its surface. In contrast, protein fibers are oriented tangentially, forming a network parallel to the cell surface [83, 85]. A similar arrangement of lipids and proteins has been discovered in the Plastids of green plants. When examined under polarized light, these plastids exhibit double refraction (birefringence) of their layers [86].

Not only Lipoproteins but also Glycoproteins possess The ability to form elongated particles within cells. Fertilizin, a substance involved in the Fertilization of the sea urchin egg [87], serves as a classic example of such a fibrous glycoprotein.

Minor alterations in the physicochemical state of Fibrous proteins are exceedingly difficult to detect with currently available instruments. These instruments are generally limited to studying fiber orientation or, in some cases, determining their shape. One of the few tissues where Changes in the state of protein fibers can be observed with relative ease is the transparent lens of the eye. Physicochemical alterations in lens protein fibers induced by salt solutions or acids lead to tissue opacity [88]. Furthermore, distinct differences have been observed in how these agents affect the lenses of calves versus adult cattle. Exposing a calf lens to a hypertonic salt solution or low Temperature induces a central cataract, whereas in the bovine lens it produces peripheral opacification [88].

Cellular permeability and excitability [89], the capacity to form complexes with other cells [90], and a variety of other properties depend directly on the physicochemical state of structural proteins [91]. Fibrillar protein particles can be effectively studied using electron micrography. Cytology/cytology/93.html">ELECTRON MICROGRAPHS OF Muscle Proteins [92, 93], Skin proteins [94], and other Organs [95] have revealed a distinct fibrillar Structure within their protein components. It should be borne in mind, however, that while a fibrillar structure is by no means universal to all proteins, certain Globular proteins—such as Insulin, hemocyanin, or viral proteins—can also form fibrillar particles. Such transformations from a globular to a fibrillar state are frequently reversible [96].

Over the past few years, biochemists and cytologists have focused heavily on The Study of submicroscopic granules within cells. These granules can be extracted from cells using Water and are responsible for the turbidity of the resulting extracts.

To isolate these granules as an analyzable pellet, differential centrifugation of the extracts is performed after adding salt or sucrose solutions of appropriate concentration and specific density. Brief, low-speed centrifugation removes unbroken cells, cell nuclei, and larger granules known as Mitochondria. Subsequent high-speed centrifugation sediments the submicroscopic granules referred to as microsomes [97–100]. Cytoplasmic granules can also be separated using Chromatography [101].

Chemical analysis of mitochondria has shown that they contain large amounts of ribonucleic acid, protein, and lipids [102]. The presence of Ribonucleic acid in mitochondria has been confirmed by numerous investigators [103–105], whereas deoxyribonucleic acid has never been detected in them. Mitochondria harbor a wide array of vital Enzymes, such as cytochrome oxidase, peroxidase [103], fatty acid dehydrogenase [106], enzymes of The Tricarboxylic Acid Cycle [107], as well as hydrolytic enzymes including Trypsin, cathepsin, amylase, and Ribonuclease [62]. Smaller granules appear to be particularly rich in enzymatic activity [97].

All of the above evidence suggests that submicroscopic cytoplasmic granules represent the fundamental structural units of living organisms and possess the capacity for self-Replication [108, 109]. The size and properties of these granules closely resemble those of Viruses, often making it difficult to determine whether one is dealing with a virus or a subcellular particle [110]. Therefore, before addressing the primary question of self-replication and, consequently, the Synthesis of specific living cell proteins, it is necessary to provide a Brief Overview of current knowledge regarding viruses—the smallest known living entities to date.