Oxo3methylvalerate

Branched chain amino acid transaminase

L-isoleucine

L-leucine

FIGURE 14.4. Enzymes of the branched-chain amino acid pathway identified in the wheat amyloplast proteome preparation: Acetolactate synthase 4.1.3.18; branched-chain amino acid transaminase 2.6.1.42; dihydroxy-acid dehydratase 4.2.1.9; ketol-acid reductoisomerase 1.1.1.86; 3-isopropylmalate dehydratase, large subunit 4.2.1.33; 3-isopropylmalate dehydratase, small subunit 4.2.1.33; 3-isopropylmalate dehydrogenase 1.1.1.85; 2-isopropylmalate synthase 2.3.3.13. Abbreviations: G, glutamate; OG, 2-oxyglutarate; G/OG, conversion of glutamate to 2-oxyglutarate; TRX, enzymes identified as thioredoxin targets.

three of the eight enzymes in Figure 14.4 were also identified in the salt extract of total endosperm, namely, ketol-acid reducto-isomerase, 2-isopropyl malate syn-thase, and lipoamide dehydrogenase (LDH). All of the proteins except ketol-acid reducto-isomerase and 2-isopropyl malate synthase are recognized as plastid proteins in the NCBI database and gene sequences for the latter two proteins have N-terminal sequences that are predicted to be plastid transit peptides by the program Target P [67]. Other pathways likely to operate in the wheat endosperm amyloplast include those for glycolysis, starch biosynthesis, pentose phosphate cycle, citric acid cycle, folate 1-carbon metabolism, biosynthesis of most amino acids, nucleotide biosynthesis, fatty acid biosynthesis, and vitamin biosynthesis. This finding indicates that amyloplasts share many of the metabolic functions found in chloroplasts of green tissue [68].

Recent proteomic studies of the amyloplast suggest that it plays a major role in the synthesis of amino acids as well as starch. Within this organelle these are a number of intersections of the pathways for the synthesis of carbohydrates and amino acids appear to reside within the organelle amyloplast. These intersections may represent control points that regulate the balance between starch and protein and thus may be important in understanding the trade-off between grain yield and protein content. For example, the enzyme LDH (Figure 14.4) is essential in pathways converting pyruvate to sugars or amino acids. Furthermore, this enzyme, like others in Figure 14.4, may be regulated by thioredoxin. It is thus clear that recent proteomics studies have provided new evidence for the role of amyloplasts in a spectrum of biosynthetic pathways, a finding in keeping with earlier results obtained with chloroplasts.

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