Chemistry and Biology of Proteins - F. Haurowitz 1953

Sources of amino acids for protein biosynthesis

Although Protein Synthesis is a fundamental manifestation of life processes, we still know very little about the pathways of this process, which represents a crucial problem in biochemistry and biology as a whole. GROWTH AND REPRODUCTION are inextricably linked to an increased rate of cellular protein synthesis. Proteins consist of peptide chains formed by Amino Acids, which is why Amino acids are regarded as protein precursors. Plants, certain Bacteria, and Molds can synthesize the amino acids necessary for protein building from inorganic nitrogen compounds, and sometimes even from molecular nitrogen. In the latter case, the initial step evidently involves the reduction of N₂ to ammonia. Experiments using the nitrogen isotope N¹⁵ have shown that glutamic acid is the primary product of nitrogen assimilation [1]. The energy required for Nitrogen Fixation is supplied by coupled oxidative processes. This is supported by the fact that nitrogen assimilation is inhibited by Carbon Monoxide and sulfides—typical poisons of respiratory Enzymes [1].

Unlike bacteria capable of fixing molecular nitrogen and plants that synthesize all their Amino Acids and Proteins from Inorganic Compounds, higher animals require a supply of specific amino acids. These amino acids enter their bodies in the form of dietary protein; the consumed proteins are broken down in the digestive tract by Proteolytic Enzymes, while the liberated amino acids are absorbed and incorporated into species-specific body proteins.

Thus, the preliminary phase of protein formation in the animal Organism is the proteolytic breakdown of ingested proteins. Subsequent phases of this process involve the synthesis of Certain amino acids from other Organic compounds and the assembly of proteins from the available pool of amino acids.