Biochemistry of Amino Acids - A. Master 1961

Intermediary Metabolism of Amino Acids
Proline
Biosynthesis in Microorganisms

When discussing the Intermediary METABOLISM of Proline, it is essential to examine the relationship between this amino acid, Ornithine, and glutamic acid. Therefore, this section includes relevant data concerning the Synthesis and degradation of ornithine and glutamic acid; other metabolic reactions of these Two Amino Acids are discussed in the respective sections of this chapter.

The proline Biosynthesis pathway in Escherichia coli was investigated by Vogel and Davis [345]. They used mutant strains of this Organism in which various stages of biosynthesis are blocked. Three types of mutants were isolated that could grow only in the presence of: a) proline, b) proline or glutamic acid y-semialdehyde, and c) proline, glutamic acid y-semialdehyde, or glutamic acid. Cultures of mutant (a) accumulated glutamic acid y-semialdehyde. This compound was isolated from the culture medium and also synthesized by Vogel and Davis [345]. Glutamic acid y-semialdehyde exists in equilibrium with its cyclic form, ∆¹-pyrroline-5-carboxylic acid; the latter is converted into proline via enzymatic reduction. This pathway of proline biosynthesis is also found in Neurospora crassa [346] and Torula utilis [347], and can be represented by the following reaction sequence:

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The conversion of glutamic acid into its y-semialdehyde presumably occurs via reactions analogous to those established by Black in his Study of the conversion of aspartic acid to aspartic acid ß-semialdehyde (p. 333). The reduction of ∆¹-pyrroline-5-carboxylic acid to proline was studied in experiments using enzyme preparations from N. crassa and E. coli; this reaction requires the presence of diphosphopyridine nucleotide or triphosphopyridine nucleotide [348].

In microorganisms, glutamic acid y-semialdehyde and its a-N-acetyl derivative also serve as precursors of ornithine. Vogel [349–351] studied an E. coli mutant requiring ornithine, which accumulates N-acetylglutamic acid y-semialdehyde during growth. This compound underwent a Transamination reaction with glutamic acid to yield a-N-acetylornithine, which was subsequently converted into ornithine via Hydrolysis. Isotope data confirmed the hypothesis that the Acetylation of glutamic acid is the initial step in the reaction sequence presented below:

The enzymatic acetylation of glutamic acid mediated by coenzyme A has been described. The aforementioned transamination reaction (p. 226) and the acylase participating in The final stage of the process have also been studied [351]. In N. crassa and T. utilis, acetylglutamic acid y-semialdehyde is not involved in ornithine biosynthesis; this compound cannot support the growth of mutants of these organisms. In these species, ornithine is formed from glutamic acid and glutamic acid y-semialdehyde [352]; however, this may not be the sole pathway for ornithine formation. Extracts from Neurospora catalyze a reversible transamination between glutamic acid y-semialdehyde and glutamic acid, yielding a-ketoglutaric acid and ornithine (p. 226). If this were the only pathway for ornithine formation, a Neurospora mutant requiring ornithine but not proline should lack ornithine transaminase. Upon investigating such a mutant, it was found to possess active transaminase. It follows that the ornithine synthesis pathway described above may not be the primary one [353]. At the same time, transamination reactions of ornithine allow for its reversible conversion into proline without the intermediacy of glutamic acid: glutamic acid y-semialdehyde, formed during ornithine transamination, can be converted into proline via cyclization and reduction. Structure/149.html">The problem of ornithine synthesis in microorganisms has been reviewed in detail by Davis [118] and Vogel [352].