Biosynthesis of Tetrapyrrole Pigment Precursors

Abstract

The universal tetrapyrrole precursor delta-aminolevulinic acid (ALA) is formed from glutamate (Glu) in algae and higher plants. In the postulated reaction sequence, Glu-tRNA is produced by a Glu-tRNA synthetase, and the product serves as a substrate for a reduction step catalyzed by a pyridine nucleotide-requiring Glu-tRNA dehydrogenase. The reduced intermediate is then converted into ALA by a transaminase. An RNA and three enzyme fractions required for ALA formation from Glu have been isolated from soluble Chlorella extracts. The recombined fractions catalyzed ALA production from Glu or Glu-tRNA. The fraction containing the synthetase produced Glu-tRNA from Glu and tRNA in the presence of ATP and Mg(2+). The isolated product of this reaction served as substrate for ALA production by the partially reconstituted enzyme system lacking the synthetase fraction and incapable of producing ALA from Glu. The production of ALA from Glu-tRNA by this partially reconstituted system did not require free Glu or ATP, and was not affected by added ATP. These results show that (a) free Glu-tRNA is an intermediate in the formation of ALA from Glu, (b) ATP is required only in the first step of the reaction sequence, and NADPH only in a later step, (c) Glu-tRNA production is the essential reaction catalyzed by one of the enzyme fractions, (d) this enzyme fraction is active in the absence of the other enzymes and is not required for activity of the others. The specific Glu-tRNA synthetase required for ALA formation has an approximate molecular weight of 73,000 +/- 5,000 as determined by Sephadex G-100 gel filtration and native polyacrylamide gel electrophoresis. Other Glu-tRNA synthetases were present in the cell extracts but were ineffective in the the ALA-forming process.