STARCH AND GLYCOGEN

NAME : T.L.V.PEIRIS DEPARTMENT:FOOD SCIENCE UNIVERSITY: UNIVERSITY OF SRI JAYAWARDENAPURA YEAR : AUGUST 2009 STUDENT NUMBER: GS/MSc/Food/3630/08

Summery

This report gives you what is starch and what is glycogen. It includes the molecular structures of starch and glycogen and what are the components that it is made of, their industrial applications and how the synthesis and breakdown of glycogen occur. Further this report contains identification tests for starch and about starch granules. It further explains the hormonal activities of glycogen synthesis and characteristics which made these two molecules good storage molecules.

Key words: amylopectin, amylase, crystallinity, glycogen metabolism

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Contents page number

Introduction 04

Structural units 04-09

Molecular structure

Amylose

Amylopectin

Functionality 09-10

Industrial applications 10- 12

Identification tests 12-13

Starch is a good for storage carbohydrate 13

Glycogen 13-14

Function and regulation of liver glycogen 14-15

In liver

In muscle and other cells

Synthesis

Breakdown

Glycogen debt and endurance exercise 18

Disorders of glycogen metabolism 18

Why Glycogen as an Energy Storage Molecule 19

Reference

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Introduction

Starch or amylum is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. Starch is the major carbohydrate reserve in plant tubers and seed endosperm where it is found as granules, each typically containing several million amylopectin molecules accompanied by a much larger number of smaller amylose molecules. By far the largest source of starch is corn (maize) with other commonly used sources being wheat, potato, tapioca and rice. Amylopectin (without amylose) can be isolated from 'waxy' maize starch whereas amylose (without amylopectin) is best isolated after specifically hydrolyzing the amylopectin with pullulanase . Genetic modification of starch crops has recently led to the development of starches with improved and targeted functionality.

Structural units

Starch consists of two types of molecules, amylose (normally 20-30%) and amylopectin (normally 70-80%). Both consist of polymers of α-D-glucose units in the 4C1

conformation. In amylose these are linked - (1 4)-, with the ring oxygen atoms all on the same side, whereas in amylopectin about one residue in every twenty or so is also linked - (1 6)- forming branch-points. The relative proportions of amylose to amylopectin and - (1 6) - branch-points both depend on the source of the starch, for example, amylomaizes contain over 50% amylose whereas 'waxy' maize has almost none (~3%)

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Representative partial structure of amylase

Representative partial structure of amylopectin

Molecular structure

Amylose and amylopectin are inherently incompatible molecules; amylose having lower molecular weight with a relatively extended shape whereas amylopectin has huge but compact molecules. The presence of amylose tends to reduce the crystallinity of the amylopectin and influence the ease of water penetration into the granules. Most of their structure consists of α-(1 4)-D-glucose units. Although the α-(1 4) links are capable of relatively free rotation around the (φ) phi and (ψ) psi torsions, hydrogen bonding between the O3' and O2 oxygen atoms of sequential residues tends to encourage a helical conformation. These helical structures are relatively stiff and may present contiguous hydrophobic surfaces.

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Amylose

Amylose molecules consist of single mostly-unbranched chains with 500-20,000 α-(14)-D-glucose units dependent on source (a very few α-1 6 branches and linked phosphate groups may be found .But these have little influence on the molecule's behavior. Amylose can form an extended shape (hydrodynamic radius 7-22 nm) but generally tends to wind up into a rather stiff left-handed single helix or form even stiffer parallel left-handed double helical junction zones . Single helical amylose has hydrogen-bonding O2 and O6 atoms on outside surface of the helix with only the ring oxygen pointing inwards. Hydrogen bonding between aligned chains causes retrogradation and releases some of the bound water (syneresis). The aligned chains may then form double stranded crystallites that are resistant to amylases. These possess extensive inter- and intra-strand hydrogen bonding, resulting in a fairly hydrophobic structure of low solubility. The amylose content of starches is thus the major cause of resistant starch formation .

Single helix amylose behaves similarly to the cyclodextrins by possessing a relatively hydrophobic inner surface that holds a spiral of water molecules, which are relatively easily lost to be replaced by hydrophobic lipid or aroma molecules. It is also responsible for the characteristic binding of amylose to chains of charged iodine molecules (for example, the polyiodides; chains of I3

- and I5- forming structures such as I9

3- and I153-; note

that neutral I2 molecules may give polyiodides in aqueous solution and there is no interaction with I2 molecules except under strictly anhydrous conditions) where each turn of the helix holds about two iodine atoms and a blue color is produced due to donor-acceptor interaction between water and the electron deficient polyiodides.

Amylopectin

Amylopectin is formed by non-random α-1 6 branching of the amylose-type α-(14)-D-glucose structure. This branching is determined by branching enzymes that leave each chain with up to 30 glucose residues. Each amylopectin molecule contains a million or so residues, about 5% of which form the branch points. There are usually slightly more 'outer' unbranched chains (called A-chains) than 'inner' branched chains (called B-chains). There is only one chain (called the C-chain) containing the single reducing group

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A-chains generally consist of between 13-23 residues. There are two main fractions of long and short internal B-chains with the longer chains (greater than about 23-35 residues) connecting between clusters and the shorter chains similar in length to the terminal A-chains

Each amylopectin molecule contains up to two million glucose residues in a compact structure with hydrodynamic radius 21-75 nm. The molecules are oriented radially in the starch granule and as the radius increases so does the number of branches required to fill up the space, with the consequent formation of concentric regions of alternating amorphous and crystalline structure. In the diagram below: A - shows the essential features of amylopectin. B - shows the organization of the amorphous and crystalline regions (or domains) of the structure generating the concentric layers that contribute to the “growth rings“ that are visible by light microscopy. C - shows the orientation of the amylopectin molecules in a cross section of an idealized entire granule. D - shows the likely double helix structure taken up by neighboring chains and giving rise to the extensive degree of crystallinity in granule. There is some debate over the form of the crystalline structure but it appears most likely that it consists of parallel left-handed helices with six residues per turn. An alternative arrangement of interconnecting clusters has been described for some amylopectins.

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Some amylopectin (for example, from potato) has phosphate groups attached to some hydroxyl groups, which increase its hydrophilicity and swelling power. Amylopectin double-helical chains can either form the more open hydrated Type B hexagonal crystallites or the denser Type A crystallites, with staggered monoclinic packing, dependent on the plant source of the granules. Type A, with unbroken chain lengths of about 23-29 glucose units is found in most cereals.

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Type B, with slightly longer unbroken chain lengths of about 30-44 glucose units is found in banana, some tubers such as potato and high amylose cereal starches. There is also a type C structure, which is a combination of types A and B and found in peas and beans.

Functionality

Starch is a versatile and cheap, and has many uses as thickener, water binder, emulsion stabilizer and gelling agent. Its form and functionality have recently been reviewed. Starch is often used as an inherent natural ingredient but it is also added for its functionality. It is naturally found tightly and radially packed into dehydrated granules (about one water per glucose) with origin-specific shape and size (maize, 2-30 μm; wheat, 1-45 µm; potato, 5-100 μm). The size distribution determines its swelling functionality with granules being generally either larger and lenticular (lens-like, A-starch) or smaller and spherical (B-starch) with less swelling power. Granules contain 'blocklets' of amylopectin containing both crystalline (~30%) and amorphous areas. As they absorb water, they swell, lose crystallinity and leach amylose. The higher the amylose content, the lower is the swelling power and the smaller is the gel strength for the same starch concentration. To a certain extent, however, a smaller swelling power due to high amylose content can be counteracted by a larger granule size. Although the properties of starch are naturally inconsistent, being dependent on the vagaries of agriculture, there are several suppliers of consistently uniform starches as functional ingredients.

Of the two components of starch, amylose has the most useful functions as a hydrocolloid. Its extended conformation causes the high viscosity of water-soluble starch and varies relatively little with temperature. The extended loosely helical chains possess a relatively hydrophobic inner surface that is not able to hold water well and more hydrophobic molecules such as lipids and aroma compounds can easily replace this. Amylose forms useful gels and films. Its association and crystallization (retrogradation) on cooling and storage decreases storage stability causing shrinkage and the release of water (syneresis). Increasing amylose concentration decreases gel stickiness but increases gel firmness. Retrogradation is affected by lipid content, amylose/amylopectin ratio, chain length of amylose and amylopectin, and solid concentration. Amylopectin interferes with the interaction between amylose chains (and retrogradation) and its solution can lead to an initial loss in viscosity and followed by a more slimy consistency. Mixing with κ-carrageenan, alginate, xanthan gum and low molecular weight sugars can

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also reduce retrogradation. At high concentrations, starch gels are both pseudoplastic and thixotropic with greater storage stability. Their water binding ability (high but relatively weak) can provide body and texture to foodstuffs and is encouraging its use as a fat replacement.

A significant proportion of starch in the normal diet escapes degradation in the stomach and small intestine and is labeled 'resistant starch' (for a recent review see [991]), but this portion is difficult to measure and depends on a number of factors including the form of starch and the method of cooking prior to consumption. Nevertheless resistant starch serves as a primary source of substrate for colonic microflora, and may have several important physiological roles (see hydrocolloids and health). Resistant starch has been categorized as physically inaccessible (RS1), (raw) ungelatinized starch (for example, in banana; RS2 b ), thermally stable retrograded starch (for example, as found in bread, especially stale bread, mainly amylose; RS3) and chemically modified starch (RS4). Resistant starch should be considered a dietary fiber. Although not exactly quantifiable due to its heterogeneous nature, some is determined by the official Association of Official Agricultural Chemists (AOAC) method. Starch with structure intermediate between the more crystalline resistant starch (for example, RS3 in staled bread) and more amorphous rapidly digestible starch (for example, in boiled potato) is slowly digestible starch [293] (for example, in boiled millet). Slowly digestible starch gives reduced postprandial blood glucose peaks and is therefore useful in the diabetic diet

Many functional derivatives of starch are marketed including cross-linked, oxidized, acetylated, hydroxypropylated and partially hydrolyzed material. For example, partially hydrolyzed (that is, about two bonds hydrolyzed out of eleven) starch (dextrin) is used in sauces to control viscosity.

Industrial applications

Papermaking is the largest non-food application for starches globally, consuming millions of metric tons annually. In a typical sheet of copy paper for instance, the starch content may be as high as 8%. Both chemically modified and unmodified starches are used in papermaking. In the wet part of the papermaking process, generally called the “wet-end”, the starches used are cationic and have a positive charge bound to the starch polymer. These starch derivatives associate with the anionic or negatively charged paper fibers / cellulose and inorganic fillers. Cationic starches together with other retention and internal sizing agent help to give the necessary strength properties to the paper web to be formed in the papermaking process (wet strength), and to provide strength to the final paper sheet (dry strength).

In the dry end of the papermaking process the paper web is rewetted with a starch based solution. The process is called surface sizing. Starches used have been chemically, or enzymatically depolymerized at the paper mill or by the starch industry (oxidized starch). The size - starch solutions are applied to the paper web by means of various mechanical presses (size press). Together with surface sizing agent the surface starches impart additional strength to the paper web and additionally provide water hold out or “size” for

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superior printing properties. Starch is also used in paper coating as one of the binders for the coating formulation a mixture of pigments, binders and thickeners. Coated paper has improved smoothness, hardness, whiteness and gloss and thus improves printing characteristics.

Corrugated board adhesives are the next largest application of non-food starches globally. Starch glues are mostly based on unmodified native starches plus some additive such as borax and caustic soda. Part of the starch is gelatinized to carrier slurry of uncooked starches and prevent sedimentation. This opaque glue is called a SteinHall adhesives. The glue is applied on tips of the fluting. The fluted paper is pressed to paper called liner. This is then dried under high heat, which causes the rest of the uncooked starch in glue to swell/gelatinize. This gelatinizing makes the glue a fast and strong for corrugated board production.

Another large non-food starch application is in the construction industry where starch is used in the gypsum wall board manufacturing process. Chemically modified or unmodified starches are added to the stucco containing primarily gypsum. Top and bottom heavyweight sheets of paper are applied to the formulation and the process is allowed to heat and cure to form the eventual rigid wall board. The starches act as a glue for the cured gypsum rock with the paper covering and also provide rigidity to the board.

Adhesives - Starch is used in the manufacture of various glues for book-binding, wallpaper adhesives, paper sack production, tube winding, gummed paper, envelop adhesives, school glues, bottle labeling.

Starch derivatives as yellow dextrins can be modified by addition of some chemical forms to be a hard glue for paper work, some of those forms are Borax, Soda Ash, which mixed with the starch solution at 50-70 °C to gain a very good adhesive, Sodium Silicate can be added to reinforce this formula.

Clothing starch or laundry starch is a liquid that is prepared by mixing a vegetable starch in water (earlier preparations also had to be boiled), and is used in the laundering of clothes. Starch was widely used in Europe in the 16th and 17th centuries to stiffen the wide collars and ruffs of fine linen which surrounded the necks of the well-to-do. During the 19th century and early 20th century, it was stylish to stiffen the collars and sleeves of men's shirts and the ruffles of girls' petticoats by applying starch to them as the clean clothes were being ironed. Aside from the smooth, crisp edges it gave to clothing, it served practical purposes as well. Dirt and sweat from a person's neck and wrists would stick to the starch rather than to the fibers of the clothing, and would easily wash away along with the starch. After each laundering, the starch would be reapplied. Today the product is sold in aerosol cans for home use.Starch is also used to make some packing peanuts, and some dropped ceiling tiles.

Textile chemicals - To reduce breaking of yarns during weaving, the warp yarns are sized. Starch is one of the main agents used for cotton sizing. Starch is also used as printing thickener.

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Printing industry - in the printing industry food grade starch[9] is used in the manufacture of anti-set-off spray powder used to separate printed sheets of paper to avoid wet ink being set off.

Bioplastics - starch is used to produce various bioplastics, synthetic polymers that are biodegradable. An example is polylactic acid.

Body powder - Powdered corn starch is used as a substitute for talcum powder in many health and beauty products.

Oil exploration - starch is used to adjust the viscosity of drilling fluid which is used to lubricate the drill head in (mineral) oil extraction.

Biofuel - Glucose from starch can be further fermented to ethanol.

Hydrogen production - Starch can be used to produce hydrogen, using enzymes.

Identification Tests

Iodine solution is used to test for starch; a darkblue color indicates the presence of starch. The details of this reaction are not yet fully known, but it is thought that the iodine (I3

−

and I5− ions) fits inside the coils of amylose, the charge transfers between the iodine and

the starch, and the energy level spacings in the resulting complex correspond to the absorption spectrum in the visible light region. The strength of the resulting blue color depends on the amount of amylose present. Waxy starches with little or no amylose present will color red.

Starch indicator solution consisting of water, starch and iodine is often used in redox titrations: in the presence of an oxidizing agent the solution turns blue, in the presence of reducing agent the blue color disappears because triiodide (I3

−) ions break up into three iodide ions, disassembling the starch-iodine complex. A 0.3% w/w solution is the standard concentration for a starch indicator. It is made by adding 4 grams of soluble starch to 1 litre of heated water; the solution is cooled before use (starch-iodine complex becomes unstable at temperatures above 35 °C).

Microscopy of starch granules - Each species of plant has a unique shape of starch granules in granular size, shape and crystallisation pattern. Under the microscope, starch

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grains stained with iodine illuminated from behind with polarized light show a distinctive Maltese cross effect (also known as extinction cross and birefringence).

Starch is a good storage of carbohydrates because:

1. It can fold up to take up less space inside the plant.

2. It is insoluable in water, meaning, it can stay inside the plant without dissolving.

3. It can be digested as "back up" energy in case there is less limiting factors like; Carbon Dioxide, light, temperature or nutrients in the soil.

Glycogen

A core protein of glycogenin is surrounded by branches of glucose units. The entire globular granule may contain approximately 30,000 glucose units.

Glycogen is the molecule which functions as the secondary short term energy storage in animal cells. It is made primarily by the liver and the muscles, but can also be made by glycogenesis within the brain and stomach. Glycogen is the analogue of starch, a less branched glucose polymer in plants, and is commonly referred to as animal starch, having a similar structure to amylopectin. Glycogen is found in the form of granules in the cytosol in many cell types, and plays an important role in the glucose cycle. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of triglycerides (fat). In the liver hepatocytes, glycogen can compose up to 8% of the fresh weight (100–120 g in an adult) soon after a meal. Only the glycogen stored in the liver can be made accessible to other organs. In the muscles, glycogen is found in a much lower concentration (1% to 2% of the muscle mass), but the total amount exceeds that in the liver. However the amount of glycogen stored in the body, especially within the red blood cells ,liver & muscles, mostly depends on physical training, basal metabolic rate and eating habits. Small amounts of glycogen are found in the kidneys, and even smaller amounts in certain glial

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cells in the brain and white blood cells. The uterus also stores glycogen during pregnancy to nourish the embryo.

Function and regulation glycogen

In liver

As a meal containing carbohydrates is eaten and digested, blood glucose levels rise, and the pancreas secretes insulin. Glucose from the hepatic portal vein enters the liver cells (hepatocytes). Insulin acts on the hepatocytes to stimulate the action of several enzymes, including glycogen synthase. Glucose molecules are added to the chains of glycogen as long as both insulin and glucose remain plentiful. In this postprandial or "fed" state, the liver takes in more glucose from the blood than it releases.

After a meal has been digested and glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops. About four hours after a meal Glycogen begins to be broken down and converted again to glucose. Glycogen phosphorylase is the primary enzyme of glycogen breakdown. For the next 8–12 hours, glucose derived from liver glycogen will be the primary source of blood glucose to be used by the rest of the body for fuel.

Glucagon is another hormone produced by the pancreas, which in many respects serves as a counter-signal to insulin. When the blood sugar begins to fall below normal, glucagon is secreted in increasing amounts. It stimulates glycogen breakdown into glucose even when insulin levels are abnormally high.

In muscle and other cells

Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. Other cells that contain small amounts use it locally as well. Muscle cells lack glucose-6-phosphatase enzyme, so they lack the ability to pass glucose

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into the blood, so the glycogen they store internally is destined for internal use and is not shared with other cells, unlike liver cells.

Synthesis

Glycogen synthesis differs from glycogen breakdown. Unlike breakdown, synthesis is endergonic, meaning that glycogen is not synthesized without the input of energy. Energy for glycogen synthesis comes from UTP, which reacts with glucose-1-phosphate, forming UDP-glucose, in reaction catalysed by UDP-glucose pyrophosphorylase. Glycogen is synthesized from monomers of UDP-glucose by the enzyme glycogen synthase, which progressively lengthens the glycogen chain with (α1→4) bonded glucose. As glycogen synthase can only lengthen an existing chain, the protein glycogenin is needed to initiate the synthesis of glycogen. The glycogen-branching enzyme, amylo (α1→4) to (α1→6) transglycosylase, catalyzes the transfer of a terminal fragment of 6-7 glucose residues from a nonreducing end to the C-6 hydroxyl group of a glucose residue deeper into the interior of the glycogen molecule. The branching enzyme can only act upon a branch having at least 11 residues, and the enzyme may transfer to the same glucose chain or adjacent glucose chains.

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Breakdown

Glycogen is cleaved from the nonreducing ends of the chain by the enzyme glycogen phosphorylase to produce monomers of glucose-1-phosphate that is then converted to glucose 6-phosphate. A special debranching enzyme is needed to remove the alpha(1-6) branches in branched glycogen and reshape the chain into linear polymer. The G6P monomers produced have three possible fates:

• G6P can continue on the glycolysis pathway and be used as fuel. • G6P can enter the pentose phosphate pathway via the enzyme Glucose-6-

phosphate dehydrogenase to produce NADPH and 5-carbon sugars. • In the liver and kidney, G6P can be dephosphorylated back to Glucose by the

enzyme Glucose 6-phosphatase. This is the final step in the gluconeogenesis pathway.

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Glycogen debt and endurance exercise

Due to the body's inability to hold more than around 2,000 kcal of glycogen, long-distance athletes such as marathon runners, cross-country skiers, and cyclists go into glycogen debt, where almost all of the athlete's glycogen stores are depleted after long periods of exertion without enough energy consumption. This phenomenon is referred to as "hitting the wall". In marathon runners it normally happens around the 20 mile (32 km) point of a marathon, where around 100 kcal are spent per mile,[citation needed] depending on the size of the runner and the race course. However, it can be delayed by a carbohydrate loading before the task.

When experiencing glycogen debt, athletes often experience extreme fatigue to the point that it is difficult to move.

Disorders of glycogen metabolism

The most common disease in which glycogen metabolism becomes abnormal is diabetes, in which, because of abnormal amounts of insulin, liver glycogen can be abnormally accumulated or depleted. Restoration of normal glucose metabolism usually normalizes glycogen metabolism as well.

In hypoglycemia caused by excessive insulin, liver glycogen levels are high, but the high insulin level prevents the glycogenolysis necessary to maintain normal blood sugar levels. Glucagon is a common treatment for this type of hypoglycemia.

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Various inborn errors of metabolism are caused by deficiencies of enzymes necessary for glycogen synthesis or breakdown. These are collectively referred to as glycogen storage diseases.

Why Glycogen as an Energy Storage Molecule?1. Fat cannot be as rapidly mobilized in skeletal muscle.2. Fat cannot be oxidized to produce energy in the absence of oxygen.3. Energy input required to initiate fat oxidation.4. The carbon atoms of fat cannot be used by any pathway of the human body in order tomaintain blood glucose levels for use by other tissues such as the brain. (i.e. fat cannotbe converted to glucose)

Reference

1. P. C. Calder (1991) Glycogen structure and biogenesis Int. J. Biochem. 23, 1335-

1352

2. Z. Gunja-Smith, J. J. Marshall, C. Mercier, E. E. Smith and W. J. Whelan (1971)

A revision of the Meyer–Bernfeld model of glycogen and amylopectin, FEBS Lett.

12, 101–104

3. E. Meléndez-Hevia, R. Meléndez and E. I. Canela (2000) Glycogen Structure: an

Evolutionary View, pp. 319–326 in Technological and Medical Implications of

Metabolic Control Analysis (ed. A. Cornish-Bowden and M. L. Cárdenas), Kluwer

Academic Publishers, Dordrecht

4.Carbohydrate Chemistry (Oxford Chemistry Primers, 99)

5. The sweet science of glycobiology: complex carbohydrates, molecules that are particularly important for communication among cells, are coming under systematic ... An article from: American Scientist

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