Tag Archives: amylopectin

Digestion of starch and alpha-amylase

Factors affecting relationship between starch and alpha-amylase

alpha-amylase
Fig. 1 – Spaghetti

Amylose and amylopectin, the two families of homopolysaccharides constituting starch, during their biosynthesis within vegetable cells, are deposited in highly organized particles called granules.
Granules have a partially crystalline structure and diameter ranging from 3 to 300 µm.
The access of the alpha-amylase, the enzyme that catalyzes the breakdown of amylose and amylopectin into maltose, maltotriose, and alpha-dextrins or alpha-limit dextrins, to carbohydrates making up granules varies as a function of:

  • amylose-amylopectin ratio;
  • temperature and packaging of amylose and amylopectin;
  • granules-associated proteins;
  • presence of fibers.

Amylose-amylopectin ratio

Starch for foodstuff use is obtained from various sources, the most important of which are corn (normal, waxy or high amylose content), potatoes, rice, tapioca and wheat.
Depending on botanical origin, molecular weight, degree of branching, and amylose-amylopectin ratio will vary.
Generally, there is 20-30% amylose and 70-80% amylopectin, even if there are starches with high amylose or amylopectin content (e.g. waxy corn). These differences justify the existence of starches with different chemical-physical characteristics and, to a certain extent, different digestibility.

  • corn: 24% amylose, 76% amylopectin;
  • waxy corn: 0,8% amylose, 99.2% amylopectin;
  • Hylon VII corn: 70% amylose, 30% amylopectin;
  • potatoes: 20% amylose, 80% amylopectin;
  • rice: 18.5 amylose, 81.5% amylopectin;
  • tapioca: 16.7% amylose, 83.3% amylopectin;
  • wheat: 25% amylose, 75% amylopectin.

Temperature and packaging of amylose and amylopectin

The chains of amylose, and to a lesser extent ramifications of amylopectin, thanks to the formation of hydrogen bonds with neighboring molecules and within the same molecules, have the tendency to aggregate. For this reason, pure amylose and amylopectin are poorly soluble in water at below 55 °C (131°F), and are more resistant to alpha-amylase action (resistant starch).
However, in aqueous solution, these granules hydrate increasing in volume of about 10%.
Above 55°C (131°F), the partially crystalline structure is lost, granules absorb further water, swell and pass to a disorganized structure, that is, starch gelatinization occurs, by which starch assumes an amorphous structure more easily attachable by alpha-amylase.

Granules-associated proteins

In granules, starch is present in association with proteins, many of which are hydrophobic, that means with low affinity for water. This association have the effect to hinder the interaction, in the intestinal lumen, between alpha-amylase, a polar protein, and the polysaccharides making up starch granules.
The physical processes to which cereals undergo before being eaten, such as milling or heating for several minutes, change the relationship between starch and the associated proteins, making it more available to α-amylase action.

Fibers

Alpha-amylase activity may also be hindered by the presence of nondigestible polysaccharides, the fibers: cellulose, hemicellulose and pectin.

Conclusions

The presence of inhibitors, of both chemical and physical type, hinders starch digestion, even when pancreatic α-amylase secretion is normal. This means that a part of starch, ranging from 1% to 10%, may escape the action of the enzyme, being then metabolized by colonic bacteria.
Refined starch is instead hydrolyzed efficiently, even when there is an exocrine pancreatic insufficiency (EPI), condition in which alpha-amylase concentration in gut lumen may be reduced to 10% of the normal.

References

Arienti G. “Le basi molecolari della nutrizione”. Seconda edizione. Piccin, 2003

Belitz .H.-D., Grosch W., Schieberle P. “Food Chemistry” 4th ed. Springer, 2009

Bender D.A. “Benders’ Dictionary of Nutrition and Food Technology”. 8th Edition. Woodhead Publishing. Oxford, 2006

Cozzani I. and Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Giampietro M. “L’alimentazione per l’esercizio fisico e lo sport”. Il Pensiero Scientifico Editore, 2005

Mahan LK, Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

Mariani Costantini A., Cannella C., Tomassi G. “Fondamenti di nutrizione umana”. 1th ed. Il Pensiero Scientifico Editore, 1999

Osorio-Dıaz P., Bello-Perez L.A., Agama-Acevedo E., Vargas-Torres A., Tovar J., Paredes-Lopez O. In vitro digestibility and resistant starch content of some industrialized commercial beans (Phaseolus vulgaris L.). Food Chem 2002;78:333-7 [Abstract]

Shils M.E., Olson J.A., Shike M., Ross A.C. “Modern nutrition in health and disease” 9th ed., by Lippincott, Williams & Wilkins, 1999

Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders Company-An imprint of Elsevier Science, 2000

Glycogen: definition, structure and functions

What is glycogen?

Glycogen Structure
Fig. 1 – Glycogen Structure

Glycogen is an homopolysaccharide formed by units of glucose. Chemically similar to amylopectin, and therefore sometimes referred to as animal starch, compared to the latter it is more compact, extensively branched and larger, reaching a molecular weight up to 108 Da corresponding to about 600000 glucose molecules.
As in the amylopectin, glucose units in the main chain and in the lateral chains are linked by α-(1→4) glycosidic bonds. Lateral chains are joined to the main chain by an α-(1→6) glycosidic bond; unlike amylopectin branches are more frequent, approximately every 10 glucose units (rather than every 25-30 as in amylopectin) and are formed by a smaller numbers of glucose units.
Glycogen is located in the cytosol of the cell in the form of hydrated granules of diameter between 1 to 4 µm and forms complexes with regulatory proteins and enzymes responsible for its synthesis and degradation.

Functions of glycogen

Glycogen, discovered in 1857 by French physiologist Claude Bernard, is the storage form of glucose, and therefore of energy, in animals in which it is present in the liver, muscle (skeletal and heart muscle) and in lower amounts in nearly all the other tissues and organs.
In humans it represents less than 1% of the body’s caloric stores (the other form of caloric reserve, much more abundant, is triacylglycerols stored in adipose tissue) and is essential for maintaining normal glycemia too.
It represents about 10% of liver weight and 1% of muscle weight; although it is present in a higher concentration in the liver, the total stores in muscle are much higher thanks to its greater mass (in a non-fasting 70 kg adult male there are about 100 g of glycogen in the liver and 250 g in the muscle).

  • Liver glycogen stores is a glucose reserve that hepatocyte releases when needed to maintain a normal blood sugar levels: if you consider glucose availability (in a non-fasting 70 kg adult male) there is about 10 grams or 40 kcal in body fluids while hepatic glycogen can supply, also after a fasting night, about 600 kcal.
  • In skeletal and cardiac muscle, glucose from glycogen stores remains within the cell and is used as an energy source for muscle work.
  • The brain contains a small amount of glycogen, primarily in astrocytes. It accumulates during sleep and is mobilized upon waking, therefore suggesting its functional role in the conscious brain. These glycogen reserves also provide a moderate degree of protection against hypoglycemia.
  • It has a specialized role in fetal lung type II pulmonary cells. At about 26 weeks of gestation these cells start to accumulate glycogen and then to synthesize pulmonary surfactant, using it as a major substrate for the synthesis of surfactant lipids, of which dipalmitoylphosphatidylcholine is the major component.
Glycogen: Dipalmitoylphosphatidylcholine
Fig. 1 – Dipalmitoylphosphatidylcholine

Glycogen and foods

It is absent from almost all foods because after an animal is killed it is rapidly broken down to glucose and then to lactic acid; it should be noted that the acidity consequently to lactic acid production gradually improves the texture and keeping qualities of the meat. The only dietary sources are oysters and other shellfish that are eaten virtually alive: they contain about 5% glycogen.

In humans, accumulation of glycogen is associated with weight gain due to water retention: for each gram of stored glycogen 3 grams of water are retained.

References

Arienti G. “Le basi molecolari della nutrizione”. Seconda edizione. Piccin, 2003

Cozzani I. and Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Giampietro M. “L’alimentazione per l’esercizio fisico e lo sport”. Il Pensiero Scientifico Editore, 2005

Mahan LK, Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

Mariani Costantini A., Cannella C., Tomassi G. “Fondamenti di nutrizione umana”. 1th ed. Il Pensiero Scientifico Editore, 1999

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders Company-An imprint of Elsevier Science, 2000