Chemistry and Biology of Proteins - F. Haurowitz 1953

Protein Synthesis
Enzymatic Peptide Synthesis

There are numerous reports indicating that Amino Acids and Peptides can be converted into Proteins through the action of Proteolytic Enzymes. Synthetic proteins obtained in this manner have been termed plasteins. Careful Replication of these experiments using advanced techniques [12] and immunological Analytical Methods [13] has demonstrated that some of the substances formed during such Treatment are low molecular weight peptides or cyclopeptides [14]. In several published experiments, protein formation was actually caused by the proliferation of Bacteria or Molds. This is evidenced by the fact that the Amino Acid Composition of the proteins formed under these conditions differs from that of proteins subjected to proteolytic treatment. On the other hand, true Protein Synthesis is observed when Chymotrypsin acts on a mixture of peptides present in Witte's peptone [15]. Chymotrypsin also catalyzes The formation of peptides from amino acid esters [16]. The energy required for this endergonic reaction is supplied by a concurrently occurring process in which a portion of The amino acid esters is converted into free amino acids [16].

The linking of amino acids to one another, leading to the formation of peptides or proteins, is accompanied by the release of Water; for this reason, the synthesis of such compounds should theoretically be accompanied by an increase in the volume of the solution, whereas Hydrolysis should result in a decrease (see Chap. III). Consistent with this, one would expect Protein Hydrolysis to be accelerated under high pressure. Contrary to this expectation, recent reports have emerged stating that tryptic hydrolyzate of serum albumin is converted back into serum albumin under a pressure of 6,000 atm at 38° [17]. It must be noted that reports in the literature concerning the enzymatic synthesis of proteins are highly contradictory and require further verification. True enzymatic synthesis of peptides was achieved by Bergmann [18], who synthesized acylamino acid anilides from acylamino acids and substituted anilines using Papain or chymotrypsin. Hippurylanilide was obtained by this method from hippurylamide and aniline under the action of papain

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and benzoyltyrosylglycylanilide from benzoyltyrosine and glycylanilide under the action of chymotrypsin [20]

¹ The author devotes only very minor attention here to the work of our Soviet researchers S. E. Bressler and his coworkers on the resynthesis of proteins from the products of their enzymatic Cleavage under high pressure. Meanwhile, these studies are of tremendous importance for The Development of our understanding of protein synthesis processes. For more details on this work, see the review by S. E. Bressler (Some Considerations on METABOLISM/35.html">Protein Biosynthesis, Usp. sovr. biol., XXX, 1 (4), 90, 1950).

What is most surprising in these experiments is that the substrates used for synthesis are not Natural Amino Acids, but rather anilides and acetylated amino acids. The ease with which peptides can be synthesized from such compounds is explained by the fact that the products resulting from this reaction are insoluble in water and precipitate out of the system as insoluble sediments. Therefore, equilibrium (1)

is continuously shifted to the right, which, According to the law of mass action, favors synthesis.

By analogy, it was hypothesized that Protein synthesis in the animal Organism would also proceed more favorably if the resulting protein were insoluble. This assumption, however, had to be discarded, as it was found that the enzymatic synthesis of aniline peptides does not depend on their solubility [19].

Another hypothesis proposed to explain The Mechanism of the enzymatic synthesis of peptides from anilides and acetylated amino acids is based on the fact that none of the substances participating in this synthesis can form zwitterions. The synthesis of peptides from compounds in the deionized form (reaction 1) requires significantly less energy than the synthesis of peptides from compounds in the ionized form (reaction 2) [21]:

This is explained by the fact that a certain amount of Energy is required to convert the ionized amine and carboxyl groups of amino acids present In aqueous solutions into their non-ionized forms (COOH and NH₂).

When writing reactions (1) and (2), it is assumed that protein synthesis is a process inverse to the enzymatic Hydrolysis of Proteins. However, the validity of this assumption has never been proven conclusively. It is quite possible that the pathways of PROTEIN SYNTHESIS AND protein breakdown are different.

In recent years, several hypotheses have been put forward regarding possible pathways of protein synthesis. One such pathway involves coupling protein synthesis with The oxidation of certain organic substances. For example, it has been shown that when p-aminohippuric acid and Glycine are added to a Liver homogenate, the corresponding dipeptide is formed. The synthesis of this peptide is associated with the structural granules of liver homogenates and is inhibited by Calcium Ions [22]. Similarly, hippuric acid is formed in liver homogenates from benzoic acid and glycine; the energy required for this endergonic reaction is supplied by The breakdown of adenosine triphosphate [23]. It is possible that Phosphorylated Amino Acids are formed as intermediates in these reactions [24, 25]. This is supported by the fact that hippuric acid forms spontaneously from glycine and dibenzoyl phosphate in a phosphate buffer at pH 4.7 and a Temperature of 37° [26]. In this case, no enzyme is required for the formation of hippuric acid, and the energy necessary for synthesis is presumably released upon the Cleavage of the anhydride bond between phosphoric and benzoic acids. It is highly probable that the reaction described above is the result of Amino Acid Activation occurring during phosphorylation, although conclusive evidence for this theory has not yet been obtained.

According to another hypothesis, amino acid activation may be the result of their Acetylation [27] or reduction to amino aldehydes; it is known that the Condensation of amino aldehydes to form peptide chains releases energy [28].