Biosynthesis of Protoheme and Heme a from Glutamate in Maize

Abstract

The heme and chlorophyll precursor Saminolevulinic acid (ALA) can be formed by two biosynthetic routes: from the intact carbon skeleton of glutamate via a five-carbon pathway, which occurs in chloroplasts and bluegreen algae, and by ALA synthase-catalyzed condensation of succinyl-CoA and glycine, which occurs in bacteria and animal mitochondria. The biosynthetic route of plant mitochondrial heme a was determined by incubating terminal epicotyl sections of 8-day-old etiolated Zea mays seedlings in the dark with L-1-14"Cglutamate (which can be incorporated into ALA only via the five-carbon route) or 2-1[4Cjglycine (which would be incorporated via ALA synthase). Label incorporation was measured in highly purified protoheme and heme a. In 12-hour incubations, label uptake was greater than 70%. Total cellular protoheme was labeled 29.7 times more effectively by glutamate than glycine. Heme a was labeled 4.1 times more effectively by glutamate than by glycine. To assess the relative ability of the two amino acids to contribute label to the farnesyl moiety of heme a, label incorporation into total cellular nonsaponifiable lipids was measured. Glycine labeled this fraction 11.3 times more effectively than glutamate. Thus, a contribution by glycine to the farnesyl moiety may account for the small amount of label appearing in heme a. Our results indicate that in etiolated maize, noncovalently bound hemes, including mitochondrial heme a, are made mostly, and possibly entirely, from ALA synthesized via the five-carbon pathway. There is little or no contribution from ALA formed via ALA synthase, and no evidence was found for the operation of this enzyme in maize.

Antisense HEMA1 RNA Expression Inhibits Heme and Chlorophyll Biosynthesis in Arabidopsis

“…Therefore, it was concluded that the protocol used in the present study measures mostly protoheme. However, heme a could not be measured satisfactorily as we did not detect peaks at 429 nm and 588 nm as shown previously (Schneegurt and Beale, 1986).…”

“…The authentic protoheme when processed similarly also gave two peaks at 418 and 557 nm. Furthermore, it was previously reported that the protoheme purified from the maize seedlings showed two peaks at 418 and 557 nm (Schneegurt and Beale, 1986). Therefore, it was concluded that the protocol used in the present study measures mostly protoheme.…”

Tetrapyrrole Metabolism inArabidopsis thaliana

“…Protoheme is further used as substrate for heme a and heme c biosynthesis, as well as bilin biosynthesis, by oxidative cleavage. Although the precise subcellular localization of heme a biosynthesis has not been confirmed in plants, it is hypothesized to occur in mitochondria (Schneegurt and Beale 1986). …”

Expression of the Soybean (Glycine max) Glutamate 1-Semialdehyde Aminotransferase Gene in Symbiotic Root Nodules

“…ALA is synthesized in chloroplasts from glutamate by the C5 pathway in leaves ( Fig. 1; reviewed by Beale, 1990;Beale and Weinstein, 1991;Jahn et al, 1992), and ALA made in this way is incorporated into hemes as well as Chl (Schneegurt and Beale, 1986). The Cs pathway enzyme GSA aminotransferase has been purified from leaves of barley and pea (Grimm et al, 1989;Pugh et al, 1992), and a cDNA encoding the barley enzyme has been cloned (Grimm, 1990).…”