Starch Synthesis and Catabolism

In my last post, I gave an overview of plant carbohydrate metabolism highlighting the general architecture of the metabolic pathways in the chloroplast and cytoplasm of plant cells and pointing out that the concentration of phosphate (Pi) plays a critical regulatory role in directing the flow of carbon through the different branches (Carbohydrate Metabolism in Plants - Overview). For simplicity, I did this without citing any references, but probably this was a violation of #steemstem rules, and for this I apologize.

Here I would like to focus specifically on the upper-left quadrant of the "Overview" image dealing with starch synthesis and catabolism in the chloroplast:

Overview of Starch, Sucrose and Cellulose Synthesis in Plants and its Regulatory Architecture (D. Rhodes)

Abbreviations used: ADP = adenosine-diphosphate; ADP-G = ADP-glucose; ATP = adenosine-triphosphate; E4P = erythrose-4-phosphate; F = fructose; F1,6bP = fructose-1,6-bisphosphate; F6P = fructose-6-phosphate; G = glucose; G6P = glucose-6-phosphate; G1P = glucose-1-phosphate; Pi = inorganic phosphate; PPi = pyrophosphate; 3-PGA = 3-phosphoglycerate; 2-PGA = 2-phosphoglycerate; PEP = phosphoenolpyruvate; 1,3-bPGA = 1,3-bisphosphoglycerate; RuBP = ribulose-1,5-bisphosphate; R5P = ribulose-5-phosphate; S7P = sedoheptulose-7-phosphate; sucrose-P = sucrose-phosphate; triose-phosphate = glyceraldehyde-3-phosphate + dihydroxyacetone phosphate; UDP = uridine-diphosphate; UDP-G = UDP-glucose; UTP = uridine-triphosphate; X5P = xylulose-5-phosphate.

Starch Synthesis and Degradation in Chloroplasts of Plant Leaves

The key enzyme regulating starch biosynthesis in plants is the enzyme catalyzing the conversion of ATP and G1P into PPi and ADP-G. This enzyme is called ADP-glucose pyrophosphorylase (also known as glucose-1-phosphate adenylyltransferase). In principle this enzyme is freely reversible:

ATP + G1P <---> PPi + ADP-G

However, chloroplasts/plastids of plant cells express an enzyme called pyrophosphatase that rapidly converts PPi to 2 x Pi, rendering the above reaction essentially irreversible (Jones et al. (2002)).

ADP-glucose pyrophosphorylase is activated by 3PGA, the first product of CO₂ fixation by RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), and inhibited by inorganic phosphate (Pi). The ratio of 3PGA:Pi is a crucial regulatory parameter for this enzyme and is directly related to its activation state (Kleczkowski, 1999).

Thus, in an illuminated chloroplast when the light reactions of photosynthesis occur, ATP levels rise, Pi levels fall, 3PGA levels rise due to activation of RuBisCO (Michelet et al., (2013)) and conditions are set for starch accumulation, particularly if Pi levels are low in the cytoplasm. Low Pi levels in the cytoplasm would permit retention of triose-phosphates (triose-P) in the chloroplast rather than export to the cytoplasm in exchange for Pi via the triose-phosphate/phosphate translocator.

The enzyme catalyzing the reaction triose-P --> F1,6bP (FBA, fructose-1,6-bisphosphate aldolase) uses 2 x triose-P as substrates. Other enzymes in the Calvin cycle use only a single triose-P as substrate. Thus, the reaction 2 x triose-P --> F1,6bP depends upon the square of the triose-P concentration, and is more sensitive to changes in triose-P levels than other enzymes consuming triose-P in the chloroplast. Build-up of triose-P in the chloroplast therefore favors increased flux to F1,6bP (see bold black arrow in the upper-left quadrant of the image above) (Jones et al. (2002)).

Once ADP-glucose (ADP-G) has been formed, the enzyme starch synthase then adds the ADP-G to a growing chain of glucose residues, releasing ADP and forming amylose. The glucose molecules are linked via 1,4-alpha glycosidic bonds in the unbranched starch, amylose. Starch branching enzyme may then introduce 1,6-alpha glycosidic bonds between these chains, creating the branched starch, amylopectin.

Structure of the amylose molecule. Image Source:

Structure of the amylopectin molecule. Image Source:

Starch is typically synthesized during the day by leaves and stored as granules in the chloroplast stroma. At night, starch is then catabolized and used as an energy source (Starch- Wikipedia). Notably, starch catabolism is dependent upon free Pi which typically increases in chloroplasts in darkness when the light reactions of photosynthesis cease. "Starch phosphorylase catalyzes the reversible transfer of glucosyl units from glucose-1-phosphate to the nonreducing end of alpha-1,4-D-glucan chains with the release of phosphate" (Rathor et al. (2009)). At night this enzyme essentially catalyzes the reaction:

starch + Pi ---> glucose-1-phosphate

A number of glucan, water dikinases (GWDs) may also be involved in starch phosphorylation, facilitating its degradation (Hejazi et al. (2012)).

Starch Synthesis and Degradation in Plastids (Amyloplasts)

In roots and tubers, and in non-photosynthetic plant organs such as seeds, the plastids do not develop into chloroplasts capable of photosynthesis. They lack expression of enzymes required for chlorophyll biosynthesis, and do not express a full complement of Calvin cycle enzymes. Yet they are capable of starch synthesis, relying upon sucrose as a carbon source, imported via the phloem from photosynthetic leaves (I will talk more about sucrose biosynthesis in leaves and phloem loading of sucrose in a future article in this series). These starch-storing plastids of plants are known as amyloplasts.

The amyloplasts of plants typically express a translocator that is capable of exchanging glucose-6-phosphate for Pi on the inner chloroplast membrane (Jones et al. (2002)). This facilitates more ready metabolism to starch of the G6P derived from sucrose metabolism in the cytosol:

Amyloplasts Lack the Full Complement of the Calvin Cycle Enzymes and Express a Glucose-6-Phosphate/Phosphate Translocator (D. Rhodes)

Because amyloplasts do not express RuBisCO and therefore do not synthesize 3PGA by CO₂ fixation, starch synthesis (via ADP-glucose pyrophosphorylase) in amyloplasts is less sensitive to fluctuations in 3PGA levels than in photosynthetic chloroplasts, and is more dependent upon relative levels of ATP, Pi and G1P.

Corn (Zea mays) (maize) accumulates large amounts of starch in the amyloplasts of the endosperm cells of its seeds/kernels. Corn starch has many uses, including as a thickening agent in food preparation, as a precursor for preparation of corn syrup (including high-fructose corn syrup), as a precursor for paper products, bioplastics, various adhesives, and for making ethanol (Corn starch- Wikipedia).

The shrunken-2 (sh2) mutant of maize lacks a functional ADP-glucose pyrophosphorylase in the amyloplast, and so is incapable of making starch. The developing ears of the shrunken-2 (sh2) maize mutant accumulate sucrose to high levels instead, and are known as "supersweet" sweet corn varieties (List of sweetcorn varieties - Wikipedia).

Sweet Corn. Image Source:

When mature, the seeds of these supersweet varieties become very shriveled (shrunken) because they lack starch. These varieties are more difficult to grow because, upon planting, sucrose in the kernels is rapidly leached from the seed to the soil, depleting seed storage reserves, and promoting the growth of bacteria and fungi. The seeds are often coated with a fungicide to limit fungal pathogen growth:

Mature sh2 sweetcorn kernels coated with fungicide (D. Rhodes)

Starch in barley endosperm amyloplasts is the major carbon source for ethanol production by yeast in the beer/ale brewing industry (see: What is the difference between ale and beer?). Starch accumulated in the amyloplasts of potato tubers is the principal source of carbon for brewing alcoholic beverages such as vodka, poitín, or akvavit (Potato - Wikipedia). During seed germination and alcoholic beverage preparation, starch is converted to maltose and glucose by enzymes called amylases (Amylase- Wikipedia).

Selected References

Hejazi, M., Steup, M., Fettke, J. The plastidial glucan, water dikinase (GWD) catalyses multiple phosphotransfer reactions. FEBS J. 279: 1953-1966 (2012)

Jones, R.L., Buchanan, B.B., Guissem, W. Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists (2002)

Kleczkowski, L.A. A phosphoglycerate to inorganic phosphate ratio is the major factor in controlling starch levels in chloroplasts via ADP-glucose pyrophosphorylase regulation. FEBS Lett. 448: 153-156 (1999)

Michelet, L., Zaffagnini, M., Morisse, S., Sparla, F., Pérez-Pérez, M.E., Francia, F., Danon, A., Marchand, C.H., Fermani, S., Trost, P., Lemaire, S.D. Redox regulation of the Calvin-Benson cycle: something old, something new. Front. Plant Sci. 4: 470 (2013)

Rathore, R.S., Garg, N., Garg, S., Kumar, A. Starch phosphorylase: role in starch metabolism and biotechnological applications. Crit. Rev. Biotechnol. 29: 214-224 (2009)

Additional Reading

For additional references on ADP-pyrophosphorylase in plants, see: ADP-glucose pyrophosphorylase and plant

For additional references on starch phosphorylase in plants, see: Starch phosphorylase and plant

Please feel free to ask questions or comment below. I will attempt to answer as soon as possible.