ADP-Glucose Pyrophosphorylase: A Regulatory Enzyme for Plant Starch Synthesis

Abstract

In plants, the synthesis of starch occurs by utilizing ADP-glucose as the glucosyl donor for the elongation of alpha-1,4-glucosidic chains. In photosynthetic bacteria the synthesis of glycogen follows a similar pathway. The first committed step in these pathways is the synthesis of ADP-glucose in a reaction catalyzed by ADP-glucose pyrophosphorylase (ADPGlc PPase). Generally, this enzyme is allosterically regulated by intermediates of the major carbon assimilatory pathway in the respective organism. In oxygenic photosynthesizers, ADPGlc PPase is mainly regulated by 3-phosphoglycerate (activator) and inorganic orthophosphate (inhibitor), interacting in four different patterns. Recent reports have shown that in higher plants, some of the enzymes could also be redox regulated. In eukaryotes, the enzyme is a heterotetramer comprised of two distinct subunits, a catalytic and a modulatory subunit. The latter has been proposed as related to variations in regulation of the enzyme in different plant tissues. Random and site-directed mutagenesis experiments of conserved amino acids revealed important residues for catalysis and regulation. Prediction of the ADPGlc PPase secondary structure suggests that it shares a common folding pattern to other sugar-nucleotide pyrophosphorylases, and they evolved from a common ancestor.

The Sink-Specific Plastidic Phosphate Transporter PHT4;2 Influences Starch Accumulation and Leaf Size in Arabidopsis

“…Confirmation of this effect through direct measurement in isolated plastids was compromised by ongoing metabolism during the isolation procedure. However, reduced starch accumulation in pht4;2 roots was consistent with an increase in stromal Pi concentrations, as Pi is an inhibitor of starch biosynthesis (Preiss, 1982;Ballicora et al, 2004).…”

“…Pi is an allosteric inhibitor of ADP-Glc pyrophosphorylase (AGPase), which catalyzes the first committed step in starch biosynthesis within plastids (Preiss, 1982;Ballicora et al, 2004). Therefore, we reasoned that starch accumulation in roots could serve as an in vivo indicator for defects in plastidic Pi homeostasis associated with the absence of PHT4;2.…”

Isolation and characterization of cDNAs and genomic DNAs encoding ADP-glucose pyrophosphorylase large and small subunits from sweet potato

“…An amino acid sequence comparison of AGPases from different plant species has revealed 85-95 % identity among all SSs and 50-60 % identity among LSs. Although LS and SS amino acid sequences share relatively fewer residues, significant similarity still exists between them; for example, potato tuber AGPase SS and LS sequences are 53 % identical (Ballicora et al 2004). In light of the high degree of sequence similarity, higher plant AGPase SS-and LSencoding genes are thought to have evolved from a common ancestor through gene duplication followed by divergence.…”

“…Starch biosynthesis in plants involves three main enzymes: ADP-glucose pyrophosphorylase (AGPase; EC 2.7.7.27), starch synthase, and starch branching enzyme. AGPase catalyzes the first, rate-limiting step in the starch biosynthesis pathway, namely, the conversion of glucose 1-phosphate and ATP to ADP-glucose and pyrophosphate (Ballicora et al 2004). In view of this vital role in starch synthesis, AGPases of main crops such as rice (Dawar et al 2013), wheat (Danishuddin et al 2011), and potato (Barıs et al 2009) and the model plant Arabidopsis (Bahaji et al 2011) have been extensively studied.…”

“…In higher plants, AGPase is a heterotetramer (L 2 S 2 ) consisting of two identical large subunits (LSs) and two identical small subunits (SSs) (Ballicora et al 2004). Different approaches have been used in attempts to reveal the role of the two AGPase subunit types in higher plants.…”