Evolutionary origins of enzymes of amylopectin biosynthesis

Both plant- and animal-like metabolisms are probably involved in amylopectin biosynthesis in T. gondii. This is likely a signature of the evolutionary origin of apicomplexan parasites. These parasites contain a vestigial plastid 'apicoplast' that is derived from a secondary endosymbiosis with the engulfment of a unicellular alga (McFadden et al.,

FIGURE 8.2 Proposed metabolic pathways involved in the biosynthesis of semi-crystalline polysaccharide storage amylopectin of Toxoplasma gondii. The biosynthetic pathway and enzymes involved therein (glyco-genin, UDP-glucose pyrophosphorylase, amylopectin synthase, branching enzymes, indirect debranching enzymes) are shown in black and red arrows, respectively (see color plate). The enzymes (a-amylase, R1 protein, phosphorylase and a-glucosidase) involved in the degradation pathway (green arrow) are indicated in brown. The putative genes encoding these enzymes have been identified in the genome sequence of T. gondii (http://www.ToxoDB.org/). The presence of active amylopectin synthase using UDP-glucose as a substrate has been assayed in tachyzoite and bradyzoite crude extracts. This figure is reproduced in color in the color plate section.

Glucose-6-P

Phosphoglucomutase ||

Glucose-1-P

UDP-glucose pyrophosphorylase

UDP-glucose pyrophosphorylase

-UTP PPi

UDP-glucose Glycogenin Amylopectin-synthase

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