Starch biosynthetic pathway

It has, thus, been demonstrated that redirecting the poly(3HB) biosynthetic pathway from the cytoplasm to the plastid resulted in an approximate 100-fold increase in poly(3HB) production [24]. However, it must be kept in mind that the rate of poly(3HB) biosynthesis in A thaliana leaves was relatively low, since poly(3HB) accumulated progressively over 40-60 days to reach 10-14% of the dry weight, whereas synthesis of starch can reach 17% dry weight for a 12 h photoperiod and seed storage lipids can reach 8% dry weight per day. 

Poly(3HB) synthesis in various subcellular compartments could be used to study how plants adjust their metabolism and gene expression to accommodate the production of a new sink, and how carbon flux through one pathway can affect carbon flux through another. For example, one could study how modifying the flux of carbon to starch or lipid biosynthesis in the plastid affects the flux of carbon to acetyl-CoA and poly(3HB). Alternatively, one could study how plants adjust the activity of genes and proteins involved in isoprenoid and flavonoid biosynthesis to the creation of the poly(3HB) biosynthetic pathway in the cytoplasm, since these three pathways compete for the same building block, i. e., acetyl-CoA. 

The mechanism of synthesis of polysaccharides is a controversial issue. After discovery, by Cardini s group, that starch may be polymerized on a protein,152,153 it was suggested153 that all nascent, polysaccharide chains might be covalently associated with a protein. Connected with the formation of glycoprotein is the involvement of lipid intermediates. We shall analyze the biosynthetic pathways of polysaccharides where partial or complete evidence of this kind of mechanism has been educed. 

Interaction of these mutants further clarifies the biosynthetic pathway. For example, the wx mutant is epistatic to all other known maize endosperm mutants and no amylose accumulates (Table 3.6). Mutants such as sh2, bt2 and sit cause major reductions in starch accumulation, but when in combination with wx, the starch produced is all amylopectin.271 In the double mutant ae wx, wx prevents the production of amylose and ae reduces the degree of branching, resulting in the accumulation of a loosely-branched polysaccharide.88 The su mutant is epistatic to du, su2 and wx relative to accumulation of phytoglycogen, but ae and sh2 are partially epistatic to su, causing a marked reduction in the su stimulated phytoglycogen accumulation (Table 3.6). The addition of du or wx to ae su partially overcomes the ae inhibitory effect, and phytoglycogen accumulates. 

Figure 1 The retrobiosynthetic principle. Labeling patterns of central metabolic intermediates (shown in yellow boxes) are reconstructed from the labeling patterns of sink metabolites, such as protein-derived amino acids, storage metabolites (starch and lipids), cellulose, isoprenoids, or RNA-derived nucleosides. The reconstruction is symbolized by retro arrows following the principles of retrosynthesis in synthetic organic chemistry. The figure is based on known biosynthetic pathways of amino acids, starch, cellulose, nucleosides, and isoprenoids in plants. The profiles of the central metabolites can then be used for predictions of the labeling patterns of secondary metabolites. In comparison with the observed labeling patterns of the target compounds, hypothetical pathways can be falsified on this basis.

The simple carbohydrates and monosaccharides produced by photosynthetic reactions serve as precursors of more complex molecules produced from them by a variety of biosynthetic pathways. Carbohydrates such as sucrose and the more complex polysaccharide starch are usually considered as direct products of photosynthesis and are synthesised in reactions involving nucleotide sugars as intermediates. 

Because glyceraldehyde-3-phosphate and dihydroxyacetone phosphate are readily interconverted, these two molecules (referred to the triose phosphates) are both considered to be Calvin cycle products. The synthesis of triose phosphate is sometimes referred to as the C3 pathway. Plants that produce triose phosphates during photosynthesis are called C3 plants. Triose phosphate molecules are used by plant cells in such biosynthetic processes as the formation of polysaccharides, fatty acids, and amino acids. Initially, most triose phosphate is used in the synthesis of starch and sucrose (Figure 13A). The metabolism of each of these molecules is briefly discussed below. 

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