A notable characteristic of acyl-ACP thioesterases is their substrate selectivity, which determines the acyl-moiety that is
released from the intraplastidial pool and, therefore, has an important impact on the final composition of the oil. The FatA and FatB
thioesterases display a high degree of divergence that is reflected
in their different selectivity [36]. Thus, FatA thioesterases display
high selectivity toward oleoyl-ACP, indicating that they are responsible for much of the export of de novo synthesized oleate which is
then used in the endoplasmic reticulum for glycerolipid synthesis.
FatB, on the contrary, also has high activity toward saturated fatty
acids, mainly palmitic acid, and is, therefore, responsible for a large
proportion of the saturated fatty acid present in plant glycerolipids
[37]. In sunflower only one copy coding for each of the FatA and
FatB genes has been cloned and characterized [38]. It is very likely
that these two were the only copies present in the plant, but this
will have to be confirmed by sunflower genome sequencing. The
FatA from sunflower (HaFatA) was phylogenetically very similar
to the gene in Carthamus tinctorius, whereas, the sunflower FatB
(HaFatB) was quite distinct from these forms in the cluster shown
in Fig. 3, being more similar to the gene found in Capsicum annuum.
Both enzymes are chloroplast-targeted proteins and carry transit
peptides in their N-terminal domains. As with the other enzymes
of these families HaFatA was a soluble protein, whereas HaFatB
showed a membrane anchorage domain, suggesting a stromal
localization for the former enzyme and a possible binding of the
latter to plastid membranes.
A notable characteristic of acyl-ACP thioesterases is their substrate selectivity, which determines the acyl-moiety that isreleased from the intraplastidial pool and, therefore, has an important impact on the final composition of the oil. The FatA and FatBthioesterases display a high degree of divergence that is reflectedin their different selectivity [36]. Thus, FatA thioesterases displayhigh selectivity toward oleoyl-ACP, indicating that they are responsible for much of the export of de novo synthesized oleate which isthen used in the endoplasmic reticulum for glycerolipid synthesis.FatB, on the contrary, also has high activity toward saturated fattyacids, mainly palmitic acid, and is, therefore, responsible for a largeproportion of the saturated fatty acid present in plant glycerolipids[37]. In sunflower only one copy coding for each of the FatA andFatB genes has been cloned and characterized [38]. It is very likelythat these two were the only copies present in the plant, but thiswill have to be confirmed by sunflower genome sequencing. TheFatA from sunflower (HaFatA) was phylogenetically very similarto the gene in Carthamus tinctorius, whereas, the sunflower FatB(HaFatB) was quite distinct from these forms in the cluster shownin Fig. 3, being more similar to the gene found in Capsicum annuum.Both enzymes are chloroplast-targeted proteins and carry transitpeptides in their N-terminal domains. As with the other enzymesof these families HaFatA was a soluble protein, whereas HaFatBshowed a membrane anchorage domain, suggesting a stromallocalization for the former enzyme and a possible binding of thelatter to plastid membranes.
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